PIGGYBACK BATTERIES

[markopolo]

re: 12v cigarette lighter socket to cigarette lighter socket chargers. I have an older one. The electronics in the middle actually ups the output voltage to around 17 volts! That high level voltage would be bad for my RV I've already lost a few expensive LED lights because my Magnum inverter/charger will output 14.9 volts when bulk charging. Also, as Vern pointed out, the 12v cigarette lighter socket to cigarette lighter socket chargers are not adequate for charging. At best they would trickle charge because of the low amperage capacity. I think they're used for emergency jump starting.

[Mike]

Originally Posted by markopolo

A few notes:

Multiple 12 volt batteries are connected in parallel ( + to + ) and ( - to - ) as already mentioned in this topic.

Two 6 volt batteries would be connected in series ( + to - ).

Chris - Check for a blown fuse or circuit breaker between the isolator and the house battery. My Roadtrek had a 30 amp breaker on that line.

Mike - re: matching batteries - I think it just something we all "know" without most of us knowing why. It is readily apparent on smaller batteries. Take a flashlight that takes two D cell batteries for instance. I think D cells are 1.5 volts. If you measure a new one it will probably be 1.6 volts. So your flashlight stops working and you take out the two batteries and they measure 1.1 volts each. Sure you can probably get your flashlight working again by adding one new battery but it won't last as long as it would if you replaced both batteries. I suspect that your flashlight would be basically "running" mostly off of the new battery until it was used up. In the case of new and old 12v batteries in parallel then I'd guess that the new one would really doing most of the work until it becomes prematurely old.

I think you will find out the answer and I'd like to know as well.

Hi,
I understand the analogy of the flashlight, and it's the method of connection tha makes the difference.
Flashlight batteries are connected in series, and I think most RV coach batteries are connected
in parallel, aren't they? So, the coach batteries will be seen as "one battery" electrically.If they
are connected in parallel, they will appear to be one larger battery, although I believe there is
some power loss in the connection itself, so a new battery with 100ah capacity and an older one that
can only hold 50ah will look like a 150ah (minus some trickle loss) battery to the rest of the system.
I don't see how they can damage each other, as was suggested. They won't be as powerful as 2 new
ones, but they'll still work.
Also, flashlight batteries aren't over $200 to replace!!! Deep cycle RV batteries are, so I'd like to get
as much extra mileage out of mine as possible, if they're still functional.
Doesn't this make sense?
How does a deep cycle battery become prematurely old? By excess usage, I'd guess. If it was by itself, it would
wear out at a finite rate, and if it's paired with another weaker battery, it will actually wear more slowly,
assuming the workload is constant (whatever normal charge/drain usage is) and the lower capacity battery
will handle some smaller part of the load, when they're connected in parallel. 2 brand new batteries paired
in parallel will last longer by definition. The lower capacity one won't damage the other, it will just not handle
an equal share of the load. I could see a bottleneck with a lower capacity battery in the series wiring, where
the current can't get through it as fast and the higher capacity batteries would have to "push harder" possibly
causing damage.

I have questions out to some battery sales and support websites and am waiting for some clarification.
If I get anything back, I'll post it up.
I hope I find the answer, too. Seems like it might depend largely on who you ask?

[markopolo]

Just thinking aloud here:

A 12v battery consists of 6 cells connected in series. Each cell, when fully charged, will be approx 2.12 volts. The "struggle" between weaker and stronger cells will exist within a single 12v battery.

On my RV, I'd mix batteries within parameters that I consider to be reasonable. I'm not saying I recommend this - just saying it is something I'd do on my RV. For instance, I'd add a new 12v battery in parallel to my year old series connected dual 6 volt batteries if I needed capacity. For me it is all about capacity and cost. I'd gain capacity for minimal cost with the risk of having to replace the whole bank maybe one year sooner than if they all matched perfectly. I know that is not the "best practice".

With an A, B & Both battery switch you can keep dissimilar batteries separated or combined as you wish.

[Mike]

Hi Mark,
I fully agree about price vs. performance being pretty important. I use it for just about
everything I buy. You all know that. I'm probably related to Cornbread Red.

Here's the back and forth between me and a techie person in the marine side of the
battery biz. I believe the principles are the same. I got this last night and was just too
tired to cut it up and paste it into this thread, in a sensible, readable, chronological order.
As I said, I've got several notes/questions out to various places regarding the subject,
and I will post up whatever else I get back from all sources.
Please don't shoot me, I'm just the messenger on this.


On 5/15/2010 6:59 PM,
has filled out a form with this content:

from: Mike

combinerQuestion: Hi,
I realize you're a Marine equipment company. My question is regarding RV deep cycle batteries and I've sent it to some other companies as well to try and understand combining old and new deep cycle batteries. I think marine and RV applications are similar, if not synonymous. Here's my question.
I've been told, and read, that you can't parallel connect an older deep cycle battery with a newer one in a 2 coach battery system. My class B van was built with 2 batteries and has a built in converter/charger designed to charge 2 deep cycle batteries. I can't find anything resembling a technical explanation of why older and newer batteries can't be used together. Both batteries still hold a charge, although one may hold less because of age, and both are charged to capacity before a load is applied, and there are no other problems with the older one. I've also found blogs where people have said they've mixed batteries of different capacities and age and seen no problems. If this issue is as important as it sounds, why don't battery retailers warn people not to mix old and new batteries when you buy a new battery?
Thanks for any explanation (preferably technical, if possible) of why this is an issue.
Mike.
p.s. The techier the better, as your website implies that pairing batteries of different capacities for whatever the reason,
brings the performance down to the level of the weakest member of the group. When a class A RV'er needs to replace
one deep cycle battery, does he need to replace the entire battery farm, just because one has a leak, or some other
failure that renders it useless? When a person goes and buys a deep cycle RV battery, the retailer doesn't mention this
caveat. I've never known anyone who has been warned about it. Again, thanks for any explanation of why this occurs.

-----Original Message -----
From: tech@yandina.com
Subject: Re: Form Submitted: Combiner Technical Enquiry


There is a very common confusion between series and parallel applications which causes frequent mis-quotes of good information and frequent mis-understanding of good quotes.

If batteries are connected in series you can think of it like the links in a chain so the total is only as strong as the weakest link. If the batteries are not matched that weakest link will discharge first and the remaining cells will FORCE current through the discharged one which rapidly destroys it. By matching cell size, age, chemistry, etc., the aim is to have them all discharge as near to uniformly as possible so when the first one becomes discharged there is not enough charge left in the others to damage it.

BUT

You are connecting batteries in parallel, like tying your boat to the dock with more than one dock line. So it makes no difference if they are not matched. You can tie the boat to the dock with a thick heavy line and a thinner line and both will hold it. In fact the stronger one will take exactly its share of the load and the thinner one will bear a smaller load but all contribute according to their capacity.

So the main reason for lack of warning by battery vendors is nearly all applications put batteries in parallel where it doesn't matter.

Regards,

Ann-Marie Foster,
tech@yandina.com

Hi Ann-Marie,
First, let me say thanks for the lightning fast response. Pleasant surprise. Especially late on a Saturday.
Second, I found your analogies easily understandable, and they would seem to reinforce my position.
I just didn't have the supporting "pretty pictures" which you've provided.

When the original question was asked on an RV forum, and the statement that "if mixing different ages or
capacities, in a parallel configuration, the weaker would ultimately damage the stronger". After some
extensive pondering and some internet research, I came up with a scenario involving 2 deep cycle batteries
of unequal capacity, due to the difference in age and usage, that are paralleled to a common source of
charge/discharge. I figured that no matter how much charge you supply, when they're charged, they're
charged. The assertion of my associates, suggested that during charge/discharge, the weaker battery
would vampire the power from the stronger and cause damage to it, simply because of the different
charge capacities/age. I suggested that there should be no discernable difference in overall performance,
because when paralleled, the 2 batteries act like or appear electrically to be 1 battery. Is that about right?
I would agree that 2 brand new batteries might be a preferred situation, but isn't a requirement for a 12V
house/coach deep cycle battery system to function, or won't cause any more damage, than what would occur
due to normal usage and wear.

If I've sort of "got it", I thank you for your reply, and no further contact is necessary.
If I need another comprehension "tweak" , please fire away. I am not easily offended,
and am very interested in learning about this, only if you have the time.

Kindest regards,
Mike.

You are correct. That is another MAJOR misconception, that a charger forces equal current into two batteries so you will overcharge the weaker one in order to fully charge the stronger one.
This is male bovine excrement.

If you have two columns of water connected by a pipe at the bottom, one 10 feet in diameter and one 10 inches in diameter, the weight of the large one doesn't force water out of the little one, they settle to the same height.
As you add water to either pipe the level of charge rises in proportion to their diameter, the water doesn't over-fill the small tube.

In order to overcharge a battery you would have to raise its voltage above the fully charged level.
If the batteries are in parallel, then they are all at the SAME voltage. (Same water height.)
If they are at the same voltage you can't overcharge one without overcharging the other and the regulator will prevent this.
You can put a small lawnmower battery in parallel with a 250 Amp-hour battery and each will get the full charge with the large one accepting nearly all the current (due to much lower internal resistance) and the small one taking very little (due to higher internal resistance).
It is self regulating.
Current does not flow from battery to battery as many will try to tell you.
If a cell fails in one of the batteries, then it is true that under fault conditions, current will flow from the good battery to the faulty one until they both end up at the same voltage, probably about 11 volts.

Regards,

Ann-Marie Foster,
tech@yandina.com

So, this is one take on it, for what it's worth. I think her explanation does cover the older
damaged cell battery scenario, and it supports not mixing them in that case, but I
wasn't concerned about that. That makes sense. If my questions were worded such that
the original point of Mike's post was missed or misepresented, I do apologize. I just
asked it the way I understood the battery setup to be, and how I didn't see it as a problem,
while some others did.
I'll post up any other replies I get.

[chris]

MY EYES ARE BLEEDING ,,LOL IT'S ALL GOOD THOUGH,I'M LEARNING DIFFERENT THINGS FROM ALL DIFFERENT DIRECTIONS...AND I FEEL GOOD ABOUT MY DECISIONS I WILL BE MAKING BASED ON THE INFO COLLECTED THANKS TO ALL,CHRIS IN PHX.

[visionquest]

Okay, Here is what I have found and it does not concur with "Ann".

1) She says quote: "Current does not flow from battery to battery as many will try to tell you.
If a cell fails in one of the batteries, then it is true that under fault conditions, current will flow from the good battery to the faulty one until they both end up at the same voltage, probably about 11 volts."

Her first sentence is in opposition to the rest of that comment. Current does in fact flow from a good battery to one with a shallower charge or a fault. That is one of the reasons combiners are sold. In the case of a weak starting battery, an isolator/combiner from some manufacturers has a switch to allow manual combining. The purpose being to allow the stronger coach battery to be connected in parallel with the starting battery so current can flow and bring the starting battery up to state hopefully decent enough to start the vehicle after about 15 minutes. The batteries are again separated before the starting attempt takes place to keep the cable back to the coach battery from seeing the high amp draw from the starter since typically they are only large enough to deal with charging currents.

She also uses the familiar water filling example but here is what I have also read on many other sites from other people with a background in batteries. She states they will equalize at about "probably" 11 volts. This isn't right. The good battery will continue to receive a charge in attempt to bring it up to full while it still continues to pass current to the faulted battery resulting in continuous charging and current flow to the new battery which shortens it's life greatly. The water flow example in this case can be thought of as two tanks connected by a tube one with a hole in the side at an arbitrary level. You can keep pouring water into either without raising the level of the tank with no hole above the other. They will continually equalize. The tank or in our case battery will not stop needing a charge so the alternator will not stop charging it. Overuse=premature failure.

The fact is, in all of my searching, I found no "scientific" explanation in the sense of "electron A then flows to" etc. But, observed results through repeated testing are considered valid in most cases that I know of. Medical, scientific etc given enough data. That is how most of the information is collected apparently in this case. I do know a few engineering sites I will attempt to get a little more "specific theory" from that describes the exact reason for the observed activity as time permits but for now, I do trust the information I have been reading.

Hope this helps a bit. I will query the nerdier characters on the engineering sites before too long here just to add that aspect of things.


-Mike and Heidi
97 Roadtrek 170P "Taj Ma Trek"
http://WWW.VanTramps.Com

[markopolo]

Hey Chris - I know we're really digging deep into this topic. I hope you don't mind us straying a bit. It is interesting.

[Mike]

markopolo: It is interesting, isn't it? Curious guy that I am, I googled the company, and did a bit
of backgrounding including her name, and found a few hits on different forums and blogs and
almost all are about batteries and electrical systems, primarily marine. To me (as stated previously,
not an expert) she sounds pretty knowledgable. And has a simple, no nonsense way of stating her position.
I remember some talk way back about the differences and pros and cons of switching to an AGM type
battery for your house batteries. This link has some interesting info on it, if you have time.
http://www.yandina.com/mixedbattery.htm

visionquest(Mike): I think the sentence starting "Current does..." was supposed to be the final
thought on the preceding comments, at least the way I read it. Her last sentence was a standalone
answer to "what if" a cell in one of the batteries fails, and she agreed that in that case, current
will tend to flow towards the failing battery.

[chris]

WELL NOW THERE'S A BIT OF INFO ABOUT THE AGM BATTS.AS I WAS CONTEMPLATING A COUPLE OF THOSE ,BUT IT SOUNDS AS IF THOSE HAVE ISSUES OF THEIR OWN ...AS I READ ON THE LINK YOU SENT HMMM,WHAT ELSE LOL CHRIS IN PHX.

[visionquest]

Okay, I emailed the creators of the HellRoaring Isolator/combiner and asked them the question of if and why it is a problem and here is there response.
************************************************** ***
It can be a problem, and sometimes it may not. It depends on what that particular battery will rest at in terms of voltage... Here is the basic problem. Batteries of a different type or condition tend to settle or "rest" at different voltage levels. For example, you may have a new AGM battery that after full charge will tend to settle at say 13.05Vdc. Then, you may have a wet cell battery that is old and no matter how much you charge it, it may "rest" at say 12.45Vdc. Now, if you connect these two batteries together, then a while after "resting", then the battery with the lower rest voltage will tend to draw down the other battery and the pair will tend to become at best, like the lower charged one. So, the entire battery bank is degraded from optimum performance or capacity.

It is not really a problem during charging (because they will tend to charge at their own independent rate), nor during a typical discharge, except that during discharge, the higher charged battery will see more of the exercise (i.e. less equal sharing of the load and charge rates). When they are isolated for independent banks, then they will tend to rest at a level that is optimum for that particular battery (i.e. one can rest at 13.05V and the other can rest at 12.45V). So, in an isolated environment, they can be a different type and be fine. But, when you desire to hard wire batteries in parallel for additional capacity, then it is best to try to match them.
************************************************** *******

I also emailed the Tech/engineers at Trojan Battery Company and will email Interstate batteries as well. I have not heard back from Trojan yet, but I expect they will answer, they are usually pretty good about it. I will keep digging at this. I have decided drill through the confusion out there and see if I can't find legitimate and specific data that the major players agree with. Once I have done adequate research on it, I will hope to write a very detailed blog entry and site sources for each bit of information etc. I plan to do this on a bigger scale than just the battery bank question too. I plan to cover the reasons why isolators make sense, what types there are, and a number of other basic common questions that I keep seeing misinformation on.

Notice the above info doesn't cover faulted batteries rather just type/condition/capacity. The shorted or other faulted battery scenario is pretty widely covered out there and definitely shows a good reason to avoid old batteries with new in the same parallel circuit.

More to come

And as mentioned before, I hope this is okay Chris, it seems this info is all pretty relative to your original post so hopefully it will be useful to you also.

-Mike & Heidi
97 Roadtrek 170P "Taj Ma Trek"
http://WWW.VanTramps.Com

[Mike]

That reply you got seems to support not mixing different types, wet cell, gels, or AGMs, and that they will
exhibit that kind of behaviour if you do. I wasn't asking that, the original question was based on not mixing
old and new. Your advice to Chris was
"One thing that is important to remember when hooking batteries in parallel is that they should be the same age,
capacity and condition. Adding a new battery to an older weaker one will shorten it's life and usefulness significantly."
I simply asked why? I wasn't questioning pairing different types, or functional with damaged. I just wanted to know
how the life and usefulness would be shortened.
I agree that if I keep a weaker (older?) battery with a stronger (newer?) one, the newer one will carry more of the load
by definition. I just didn't think it would harm either battery to do this. That's all.

I also thought about your rebuttal of Ann-Marie's analogy using the water filled columns with a tube connecting them.
In it, you mention a "faulted battery". Nowhere in her water column analogy does she include a faulted battery. That's
because it's the one that's been removed and replaced. The capacities (columns of water diameters) are different due to their respective ages, but the older (smaller diameter column of water) battery still has all it's cells working, just not as
hard. So the "hole in the tank" doesn't exist (I was going to say doesn't hold water ), in the context of the
example she uses. There isn't a good and bad battery, just an older (smaller diameter) and a newer (large diameter)
one. She agrees that a fault in either battery changes everything, at the end of her note, well past the columns of
water analogy part.

I'm also waiting for Trojan to reply.

I got another reply from OasisMontana as follows...
The original question was the same as the one to Yandina....
[color=#8040BF]
First reply.....
----- Original Message -----
From: Chris @ the Oasis
Sent: Monday, May 17, 2010 12:40 PM
Subject: RE: Mixing batteries of different age/capacity


I do not know why retailers don't advise folks not to mix old and new batteries (we always do). It's the same as adding a low output solar module mixed with a new one--it will pull down the system's performance.

If you have two or more batteries in your RV, and if they are being charged separately (like, starting battery and coach battery), they would not have to be replaced at the same time. But it would be a good idea of have even a cheapo analog meter on both of them so you could monitor each's cycling performance and voltage. Not having a meter on your battery is sort of like not having a gas guage on your car! But if you have several batteries and only one is bad, you can quite possibly integrate a new battery of equal capacity in with the others (but of course, it will operate at the efficiency of your poorest cells).

If you stick 'deep cycle battery maintenance' into your favorite search engine, you will get all the techie you can stand!

Regards,

Chris Daum
Oasis Montana Inc.
406-777-4309
406-777-0830 fax
http://www.oasismontana.com
http://www.grid-tie.com
http://www.PVsolarpumps.com
http://www.eco-fridge.com
http://www.LPappliances.com

To which I replied....
Sent: Monday, May 17, 2010 11:12 AM
To: Chris @ the Oasis
Subject: Re: Mixing batteries of different age/capacity

Hi Chris,
Thanks for getting back to me. Much appreciated. The vehicle starting
battery isn't involved, except that it gets charged by the onboard converter/charger,
from the engine alternator, shore power, or the gas generator built into the RV.
It is also isolated from the coach batteries by an RV battery isolator.
So it's out of the loop.
The scenario I was considering, was in an RV house/coach battery system,
with 2 deep cycle batteries, connected in parallel. One develops a physical
problem due to age and starts leaking, and has to be replaced.
Lets suppose they were identical when purchased and both were rated at 100 ah.
If I add a newer, same type, deep cycle battery to the system rated at 100 ah, but
my remaining battery is now only capable of 75ah at max charge, how will that
affect my overall power system? Won't I have around 175 ah (minus some amp bleed)
in total, because batteries connected in parallel resemble one bigger battery to the
rest of the system?

That's sort of where I was going with my initial question. I should have thought harder
about the example before asking.
How would that work?
Thanks,
Mike.

Chris replied....
----- Original Message -----
From: Chris @ the Oasis
Sent: Monday, May 17, 2010 2:54 PM
Subject: FW: Mixing batteries of different age/capacity

"If I add a newer, same type, deep cycle battery to the system rated at 100 ah, but
my remaining battery is now only capable of 75ah at max charge, how will that
affect my overall power system?"

Soooo.....say you have two 12V 100AH batteries, parallel wired for 12V. Or 200Ah of storage at 12V. After a year or so one gets injured or leaks or something, and their rates amphours at that time was 75 each. Then you add another new 100ah battery.... it's as if you have two 75Ah batteries now, or 150 ah total, not 75 + 100. Having a bad cell in an otherwise good battery system-- one bad cell or battery will pull the rest down.

My note: Notice he includes the bad cell in the mix, which has been removed and replaced. It seems some people are equating weaker (older, lessened capacity) with damaged (bad cell), which is not the case here, and does make a lot of difference.

To which I replied one last time.....
Sent: Monday, May 17, 2010 1:41 PM
To: Chris @ the Oasis
Subject: Re: Mixing batteries of different age/capacity

Hi again,
Maybe my scenario wasn't entirely clear last time.
Yes, I understand that one bad cell affects the group, but that's the one I'm removing, and replacing with a new 100ah battery. The second (old) battery is still in perfect condition, but is unable to hold 100 ah due to age. All it's cells still work, but it's older and weaker and can only hold 75 ah now. So I add a new 100 ah replacement for the bad cell battery, and I should now have slightly less than 175 ah. Any bad cells are out of the group now.
Does that make sense?

Thanks,
Mike.

To which he reiterated......
If you add a 75ah battery to a 100 ah battery, it's going to work at the capacity of two 75ah batteries. Your new battery is not going to be able to 'bring up' the older battery.

End of file.
So that's their take on it. Seems to disagree with Ann-Marie, and agree with Mike (visionquest), unless I'm
misinterpreting his position.

Next reply just in from Absolute Battery in Ontario, Canada.....
Again, same wording of the original question (cut and pasted it into every one I sent)
Their answer......
----- Original Message -----
From: Sales
Sent: Monday, May 17, 2010 3:12 PM
Subject: RE: Question about parallel connectiing 2 different types of battery

Hi Mike,

We do not recommend that an older battery and a newer battery be used together but we have customers do it all of the time. It is more cost efficient that way. Also many people simply cannot afford two new batteries at one time. The reason you have to charge them separately is that the chemistry of an AGM battery is different then that of a lead acid. I hope this does not come off as rude but your best option is to call our number and speak with Harold. He is my father and has been in the industry for so long and knows everything about batteries. This way you can have all of your questions answered and it is easier to speak it then it is to write it out. Plus I do not want to answer a question incorrectly. IT is also hard to get him to stop for 5 minutes to discuss and answer your e-mail with me. If you do not have the time let me know and I will print it off and put it on his desk but I cannot guarantee a response for at least a few days.

Thank you
Allessa Fisher
GREAT NORTHERN BATTERY


So it doesn't sound like they have a problem with people mixing old and new batteries. And $$$ seems to be the main
reason some of their customers do it.
There seem to be a lot of answers out there, and no two agree 100%.

If it can be reasonably proven that it doesn't do any damage,
I think it's looking like the answer may be just personal choice and money (cost efficiency, as it was put).

As visionquest (Mike) put it,
"more to come"?

[chris]

Thanks alot Mike ,keep me posted CHRIS IN PHX.

[Mike]

Chris, if you mean me, I'm still not an expert, and it appears that it depends on who you ask,
for any kind of answers to your questions. I'd still consult a specialist if you have any doubts about
anything to do with your van, as VernM suggested some time back. We have gotten a
bit off track, but it's all good info, and that's what classBforum is all about. We exchange
information, experience, and opinions on all sorts of stuff, but the final decision always
has to be made by, and at the discretion of, the user. That's us, you, and me, and everyone else on here.

Besides, as you said, I'm learning about batteries too. I'm actually in the market for
a new AGM, and I've learned many do's and don't and gotten some really good feedback
from markopolo, and visionquest, and VernM, all guys who have a lot more experience than I do.
Doesn't mean I won't ask questions, or challenge something that doesn't quite sound right to me.
That's why I asked for any scientific or techie reason to back up anyone's opinion or experience,
if it isn't obvious to me.
I always like to try to understand the how and why of these things. Makes me a better consumer.

Mike.

[chris]

IN ANY CASE THANKS FOR YOUR HELP MIKE ...........CHRIS

[visionquest]

Hi Mike, I will take these one at a time and try and make sense of what is getting very confused. Just to be clear, I don't mean this to be argumentative, rather just trying to get all our facts straight and the information correct and back on track. The end goal is the same as yours. We want the right info to be available to the forum members. Compiling it in a useful way here is a good thing we all benefit from. I appreciate all your info and effort.

In your response to me you said.

That reply you got seems to support not mixing different types, wet cell, gels, or AGMs, and that they will
exhibit that kind of behaviour if you do. I wasn't asking that, the original question was based on not mixing
old and new.

Actually. that was just an example of two different voltages, the different "type" example was just that, an example of one reason battery voltages might be different. A highly sulfated battery due to age would also have a lower voltage and if connected to another battery of the same type in parallel, the same conditions will still exist. If you read the response from him again, he says different type OR condition. Condition can refer to old and new.

In the second part of your response, you said.

In it, you mention a "faulted battery". Nowhere in her water column analogy does she include a faulted battery. That's
because it's the one that's been removed and replaced. The capacities (columns of water diameters) are different due to their respective ages, but the older (smaller diameter column of water) battery still has all it's cells working, just not as
hard. So the "hole in the tank" doesn't exist (I was going to say doesn't hold water ), in the context of the
example she uses. There isn't a good and bad battery, just an older (smaller diameter) and a newer (large diameter)
one. She agrees that a fault in either battery changes everything, at the end of her note, well past the columns of
water analogy part.

I wasn't actually describing her analogy. Just the fact that she used water to describe how current flows and that is a common way to describe it online. I applied it to the faulted battery by using the example of a hole in the side of one tank. If I use her 11 volt number in my example. The hole in the tank would represent the 11 volt limit of the faulted cell. As she mentioned, the good battery would flow current to the faulted one which would never rise above 11 volts. Dump water in it flows out the hole. with the tanks connected, dump water in the tank without a hole and it still balances with the hole in the other tank no matter how much water you dump in.(continual charging and discharging as the better battery will not get to full charge) I mentioned this because we were discussing adding a new battery to an existing one. If a battery is older, it is likely to develop a short sooner and is another reason not to add new to old.

Now that we are back to Ann's comments, I want to bring one up that she has backwards. She said :

You are correct. That is another MAJOR misconception, that a charger forces equal current into two batteries so you will overcharge the weaker one in order to fully charge the stronger one.
This is male bovine excrement.

The concern isn't that a charger will overcharge the weak battery, it is that it will continually charge the stronger battery as it is passing it attempts to bring the weak battery to full charge. . Calling it male bovine excrement doesn't give me any more faith in her opinion.

She also says.

If you have two columns of water connected by a pipe at the bottom, one 10 feet in diameter and one 10 inches in diameter, the weight of the large one doesn't force water out of the little one, they settle to the same height.
As you add water to either pipe the level of charge rises in proportion to their diameter, the water doesn't over-fill the small tube.

I agree with this actually. It is in reference to capacity in AH specifically. There is still some question in my mind as to combining say two batteries from different manufacturers as a fully charged battery from one can have a slightly different voltage than another even if both new and from the info that seems to be in consensus the slightly higher voltage battery should settle at the others lower level. Probably very negligible and probably not a common scenario, but it seems that it would be the case as well. Still makes sense to me to purchase batteries at the same time from the same company or at least with the same expected full voltage. I amend my original suggestion that people buy batteries that are the same type/condition/manufacturer rather than type/condition/capacity though I think most people generally do purchase them with the same capacity anyway.

So on to what she said and what I responded with in my other post.

She says quote: "Current does not flow from battery to battery as many will try to tell you.
If a cell fails in one of the batteries, then it is true that under fault conditions, current will flow from the good battery to the faulty one until they both end up at the same voltage, probably about 11 volts."

Her first sentence is in opposition to the rest of that comment. Current does in fact flow from a good battery to one with a shallower charge or a fault.

I still stand by this. Even if she didn't mean it to be grouped together in one paragraph, the first sentence is very specific and allows for no exception the way she says it. Then right after, she gives an exception. Wouldn't matter to me if the exception was at the beginning or end. I notice things like that when people speak and it does effect how much I trust what they say. She probably is very good at her job and does fine with the things she deals with. I do think there are much better sources of info than her. My opinion has no consideration for her gender just to be clear. Same would hold true if it were my own Dad giving me info. Just thought it wise to mention that

Sorry for making this post so long. It took me a while to sort through it all. I will still give the info from any more sources as they respond.

Hope everyone is doing well

-Mike and Heidi
97 Roadtrek 170P "Taj Ma Trek"
http://WWW.VanTramps.Com

[Mike]

Hi,
You're right I did miss "type or condition". Do you know if he was talking about 2 batteries connected,
charged, and discharged in parallel? The answers I've received aren't always specific in terms of series vs.
parallel, when they answer. If I have doubts as to the clarity of my question and model, I send a reply and
reiterate, and wait for a second reply, just to make sure we're comparing apples to apples. I also reiterate
the model and stress that the faulted battery is being removed from the system, and replaced with the
new one, so again, apples/apples.
Maybe we're losing sight of the original issue, which was why will pairing 2 batteries of similar types (AGM,
wet cell, gel cell) in a parallel charging, discharging environment, when one is older than the other, cause
problems for both, which should be considered prohibitive for doing it in the first place?

In your replies, you keep referring to the fault scenario to the columns of water analogy she used, not her.
"I wasn't actually describing her analogy. Just the fact that she used water to describe how current flows and that is a common way to describe it online. I applied it to the faulted battery by using the example of a hole in the side of one tank. If I use her 11 volt number in my example. The hole in the tank would represent the 11 volt limit of the faulted cell. As she mentioned, the good battery would flow current to the faulted one which would never rise above 11 volts. Dump water in it flows out the hole."
Try to keep in mind, there is no faulted battery in the columns scenario. The faulted battery
isn't the weaker one, it's the one that has been removed. You keep putting a hole in one of the columns,
where none should exist. The way I understand her analogy, is that the water in the columns isn't current,
it's capacity in amp hours. It won't flow anywhere until there's a legitimate draw like you turn on a 12VDC light.
She left her final thought in that note to the end, and admitted, with no contextual connection to the preceeding
comments she had already made, that if one battery had a "fault" (bad cell?) it would lower that battery's
capacity to around 11V (which has been echoed in another opinion I'l post up shortly) as you pointed out. True.
Like you, I'm not trying to be argumentative, just trying to keep the scenarios/models constant.

As far as charging the batteries, again, I'm not sure we have or need a model with continuous charging at all.
"The concern isn't that a charger will overcharge the weak battery, it is that it will continually charge the stronger battery as it is passing it attempts to bring the weak battery to full charge. . Calling it male bovine excrement doesn't give me any more faith in her opinion."
What happens when the charging stops, turning off the engine, genset, or unplugging from shore power,
is what I'm trying to figure out. What happens to the 2 batteries capacity, or what is their final capacity once
charging stops and they're just sitting there waiting for an electrical draw from something in the RV? What
damage or shortening the usefulness, or life, would you cause by pairing the new with the old?
That's why I asked "why?".

Maybe we need to modify the model. What if I paralleled/paired a brand new deep cycle 12V AGM with a stated capacity of 85 amp hours, with a brand new deep cycle 12V AGM with a stated capacity of 100 amp hours. Forget old batteries for a moment.
What would I get as my fully charged, at rest, parallel connected, storage capacity then? I think 185 amp hours,
but based on discussions in this topic, I believe you'd think 170 amp hours.
Would the retailer warn me not to do it, because the lower capacity battery will ruin the higher?
I doubt it.
I actually thought the opposite about the BS comment she made. I found it amusing. Sorry, my bad.

Don't apologize to me for a long post. Have you noticed HOW LONG MINE ARE?????

OK, speaking of Trojan, here's my q and a with them
and I just relaized I used "can't" in my question. However,
I think they asnwered it as if I said "shouldn't" ......

Trojan Battery Response

Dear Mike:
Thank you for submitting your question regarding Other.
Based on your comments below:

I've been told that you can't parallel connect an older battery with a newer one in a dual battery RV coach system. My class B van was built with 2 batteries and has a built in converter/charger designed to charge 2 batteries. I can't find anything resembling a technical explanation of why older and newer batteries can't be used together. Both batteries still hold a charge, although one may hold less because of age, and both are charged to capacity before a load is applied, and there are no other problems with the older one. I've also found blogs where people have said they've mixed batteries of different capacities and age and seen no problems. If this issue is as important as it sounds, why don't battery retailers warn people not to mix old and new batteries when you buy a new battery? Thanks for any explanation (preferably technical, if possible) of why this is an issue to some people. Mike.

Below is Trojan Battery's response:

As a battery manufacturer, we cannot comment directly on the installation for a specific vehicle. We can make the following general comments. 1. Adding a new battery in parallel with existing battery would not normally be an issue - as long as the charger settings are appropriate for the battery type and size (total capacity of the parallel batteries). It is always recommended that batteries be matched to the same type in parallel connections in order to avoid any potential issues - but the charger (voltage-current) still needs to be appropriate to the battery type and application to make sure the battery will deliver the required energy for the desired time and the battery will be sufficiently recharged in the time available. 2. The main reason for "isolating" is to keep the starting battery from discharging more than the normal engine-starting use. A starting battery will wear-out rapidly if subjected to regular discharge cycling conditions. 3. Any custom wiring done by the user should make sure that there is not a continuous load placed on the coach battery that would cause a deep discharge during non-use times. Thanks.
If you have additional comments or questions, please contact me at the information below.

Sincerely,
Stacey Delzeit
Product Engineer
Trojan Battery Company
sdelzeit@trojanbattery.com
678-518-7378
© Copyright Trojan Battery Company 2008 Trojan Battery Company
12380 Clark Street
Santa Fe Springs, CA 90670

So that's their take on it. Unless I'm misreading the answer, it sounds like they have no issues
adding a new to an old battery as long as there are no other issues with the batteries, and the
environment is designed for 2 batteries of the same type. I have sent them a clarification note
just to double check the model. I'll add it when I get it. He does mention "total capacity of the
parallel batteries" which I think implies the amp hours are added together, and that's the capacity,
as I said somewhere in the dim, and now distant, past. The rest of the electrical system sees the
paralleled batteries as one big happy battery.

I have a few more irons in the fire out there.
"More to Come!!!" seems to be the trend here

Personally nothing but problems with the house, but I'm hoping to get it all sorted and get
back out there sometime later this week, fingers crossed. Hope all is also well, with all the other
little forumites.
btw, I've edited this post several times and added some after thoughts to it, as well as corrected many
spelling/tense errors.

[chris]

SOUNDS GOOD

[visionquest]

Hi Mike,

Concerning the answers I received, the questions were all asked based on parallel circuits so the answers are as well.

Just respond to the column, faulted thing again, re-read my last response. The faulted battery column example is mine independent of hers. I know her example didn't put it that way and I wasn't saying it did. Using water flow and columns is a typical way of describing batteries and energy flow. It can be applied to a number of different scenarios.

I also realize you keep talking about removing the faulted battery. Again, different scenario from what I was referring to. What I was trying to get across was that by adding a new battery to an old bank (one of the questions we have seen asked) you are more likely to deal with that potential situation in the future. At that point it will be more difficult to detect as it happens until it may be too late and the damage is done.

Your answer from Trojan was the exact same canned answer they gave me. I don't believe they gave any consideration to what I asked them at all. I won't bother using them again. I bought an AGM from them last year (almost $300) and am giving it to a friend of mine since I bought two newer ones(Full River AGM) to expand my capacity to 220 AH.

Given that the new batteries ran close to $250 each and I don't particularly enjoy pulling the bed area apart to install them, I want the best possible scenario for both longevity and performance. I am a bit picky about the way I do things but it works well for me. Hopefully the information that has been shared in this thread will help others decide what route will work best for them.

Hope you are all enjoying your day

-Mike and Heidi
97 Roadtrek 170P "Taj Ma Trek"
http://WWW.VanTramps.Com

[Mike]

Hi again Mike,
Did you see the revamped question I asked in my last posting? It eliminates the bad/older
battery, and just asks the question about 2 brand new batteries, of identical type and condition,
but of unequal stated maximum storage capacity in amp hours.
Here it is again....copied from my last posting.

Maybe we need to modify the model. What if I paralleled/paired a brand new deep cycle 12V AGM with a stated capacity of 85 amp hours, with a brand new deep cycle 12V AGM with a stated capacity of 100 amp hours. Forget old batteries for a moment.
What would I get as my fully charged, at rest, parallel connected, storage capacity then? I think 185 amp hours,
but based on all our discussions, I believe you'd say 170 amp hours.
Would the retailer warn me not to do it, because the lower capacity battery will ruin the higher?
I doubt it.

So, what would you say about this one? 185ah or 170ah?
If your answer is 185ah, then aren't you agreeing that the combined capacity of two batteries
of equal type and condiditon, is the sum of their individual maximum capacities. And if that's
true, then the same applies for 2 batteries of equal type and condition, where the only other
difference is their relative age.
If you say 170ah, then any time 2 batteries of equal type and condition stray from identical
storage capacity in amp hours, aren't you saying they should be replaced with 2 new batteries
of identical storage capacity so as not to cause the stronger one to be vampired down to the
level of the weaker one, even if they're only around 1 year old (for example)?

Food for thought, perhaps?

[visionquest]

Nope, your missing a few things. Re-read my 2nd post back with the many quoted sections. I amended my original suggestion and said capacity was no issue. I think you are getting confused. When I am talking about things equalizing, I am only referring to voltage, not capacity. Never said capacity would equalize, only voltage. 185AH is correct.

-Mike and Heidi
97 Roadtrek 170P "Taj Ma Trek"
http://WWW.VanTramps.Com