PIGGYBACK BATTERIES

[visionquest]

To add to that, now that I read the last part of your message. The fact is, it doesn't really matter what the rated capacity is, actual capacity is what matters. Consider two brand new batteries. We can use your 85 AH AGM that is considered at full charge at perhaps 12.75 Volts. If that battery is added in parallel to one that is 100 AH AGM that is considered fully charged at maybe 12.65 Volts, If the information given that says they would settle at the same voltage is true and both would level out at 12.65 Volts, the 85AH battery will have lost some of it's storage capacity. Batteries from different manufacturers can have different "full" voltage ratings. I can't say if two batteries of different capacity from the same manufacturer will have the same "full" voltage. Maybe, but if you want to combine different capacity batteries from the same company, might be interesting to check. It is definitely worth knowing what to expect your batteries should be at full charge anyway.

BTW, the above example just uses arbitrary voltage values for the example of full. The two different capacities if from the same manufacturer may in fact have the same voltage.

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

[Mike]

Here's my supplemental q and a from Trojan....

Sent: Tuesday, May 18, 2010 5:55 PM
To: Stacey Delzeit
Subject: Re: Trojan Battery Response
Hi Stacey,
So while it wouldn't be an ideal situation to combine 2 deep cycle AGM batteries, one new, one old,
with no other physical problems, in parallel, in a 12V DC RV coach/house application, and as long as they're
charged together, and at an appropriate level within the optimal recharge time frame, there should be no
rapid degradation or damage to either battery's performance or condition, as a result?
I understand that the ideal solution is to try to replace all the batteries at the same time,
but some people prefer not to, usually for cost efficiency.

Kindest regards,
Mike.
--------------------------
"Hi Mike,
You are correct. You should not have any terrible issues with mixing old and new batteries installed in parallel with each other.

Thanks,
Stacey Delzeit
Technical Support Engineer
Trojan Battery Company
125 East Church St. - Suite L
Sandersville, GA 31082
Tel: (67 518-7378
Fax: (47 553-9933
sdelzeit@trojanbattery.com
www.trojanbattery.com
Trojan Battery Company - Clean Energy for Life™

[markopolo]

There's some really good info here. Now we know what is best and also what is acceptable.

If your house battery bank is nearing end of expected life, or questionable or inadequate then replace it completely for optimal performance and capacity.

If your house battery is in good shape and you just need more capacity then, based on what we've read here, it is acceptable to add another similar battery in parallel.

It comes down to your priorities - cost - performance - capacity etc.................

[Mike]

So it would seem.
I've got one more reply from Battery Mart in Virginia. Here it is......
--------------------------------------------------------------------------------------------------------------
Sent: Saturday, May 15, 2010 6:36 PM
To: info@batterymart.com
Subject: Battery capacity/age mixing

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? I've also found blogs where people have said they've mixed batteries of different capacities and age and seen no problems.
Thanks for any explanation (preferably technical, if possible) of why this is an issue to some people.
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: Anthony Cresegiona
Sent: Tuesday, May 18, 2010 10:23 AM
Subject: FW: Battery capacity/age mixing
Mike,
The reason everyone says not to mix older batteries with newer ones is because the older battery can draw off the newer battery thus diminishing the life of the newer battery. Generally you can get away with doing this if the batteries are within a year of each other, but if there older then the general rule is not to mix the batteries. This doesn't mean they won't work it just means you probably won't get the full life out of the newer battery due to the old ones drawing off them. In some cases the older batteries are so bad the can kill the newer batteries. You never mix deep cycle/rechargeable batteries & starting batteries together. On deep cycle/rechargeable batteries you want to try & keep the batteries close to the same amp hour rating. Reason for that is the smaller battery will always discharge & charge sooner then the larger battery which can cook the smaller battery when charging.

Thank You,
Anthony Cresegiona
Web Sales/Call Center Mgr.
BatteryMart.com
1 Battery Dr.
Winchester, VA 22601
Ph: 800-405-2121 x201
Fx: 540-665-9623
E-Mail: anthony@BatteryMart.com
http://www.BatteryMart.com
--------------------------------------------------------------------------------
Sent: Tuesday, May 18, 2010 6:02 PM
To: Anthony Cresegiona
Subject: Re: Battery capacity/age mixing
Hi Anthony,
Is that always true when the batteries are connected, charged, and discharged in parallel?
I've received some indications that the "vampire" effect on the newer battery, by the older one,
is only the case when the batteries are connected in series, or when there's a physical problem with
the older battery. In my example, both batteries are fully functional, but one is older and weaker.
I just wanted to make sure that I had clarified that.
I thank you for your insights and response.
Kindest regards,
Mike.

--------------------------------------------------------------------------------------------------------------------------------------------

Mike,
It doesn't matter how the batteries are connected. The older battery will always draw off the newer batteries.

Thank You,
Anthony Cresegiona
Web Sales/Call Center Mgr.
BatteryMart.com
1 Battery Dr.
Winchester, VA 22601
Ph: 800-405-2121 x201
Fx: 540-665-9623
E-Mail: anthony@BatteryMart.com
http://www.BatteryMart.com
-----------------------------------------------------------------------------------------------------------------------------
Not exactly the techie answer I asked for. So he's in the "no old and new batteries together, period" camp.

I also started a thread at Discovery Channel's Myth Busters Science and Tech section to
see if I could get them to test these different theories and opinions, too.
So far, no interest in it for an upcoming episode, it seems.
I did get a handfull of postings to the thread, and, you guessed it,
got as many different answers as I've gotten sending out these emails.

I think markopolo's take is probably the best choice for now,
and I'm getting tired and confused and hungry, so I'm going
to make a sandwich, have a nap, and contemplate life......

[Mike]

Originally Posted by visionquest

To add to that, now that I read the last part of your message. The fact is, it doesn't really matter what the rated capacity is, actual capacity is what matters. Consider two brand new batteries. We can use your 85 AH AGM that is considered at full charge at perhaps 12.75 Volts. If that battery is added in parallel to one that is 100 AH AGM that is considered fully charged at maybe 12.65 Volts, If the information given that says they would settle at the same voltage is true and both would level out at 12.65 Volts, the 85AH battery will have lost some of it's storage capacity. Batteries from different manufacturers can have different "full" voltage ratings. I can't say if two batteries of different capacity from the same manufacturer will have the same "full" voltage. Maybe, but if you want to combine different capacity batteries from the same company, might be interesting to check. It is definitely worth knowing what to expect your batteries should be at full charge anyway.

BTW, the above example just uses arbitrary voltage values for the example of full. The two different capacities if from the same manufacturer may in fact have the same voltage.

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

I probably shouldn't do this, but I'd like to try to understand what you're saying.
So you are saying that rated capacity doesn't matter, but actual capacity does (see red highlighted
text above)?
Food for thought: How is an older battery whose "actual" capacity is now down to 85 ah fully charged from it's
brand new "actual" capacity of 100 ah fully charged, any different from a new battery whose "actual"
capacity is 85 ah fully charged? They should be equal capacity by your comments in red above in electrical terms,
shouldn't they?

Also, In your previous posting to that one you said

Originally Posted by visionquest

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.

So the capacities aren't affected, and fully charged equals fully charged?
Ok, I'm good with that, because that's all I really cared about from the start. Capacity, not voltage, or continuous charging, or anything. Just how much power do I have available, when all my batteries are off the chargers, and ready to run stuff.
And there seems to be enough opinions both ways on pairing old (not faulted) and new, that I feel comfortable not worrying about it. Others may not agree. That is their prerogative.

Thanks for going to all this trouble. It has been interesting, if not enlightening.

So, this'll be my last entry, barring some new and interesting development.
Thanks to any and all who chipped in.
Sorry to beat a horse as it were.

Best regards,
Mike.

[visionquest]

Food for thought: How is an older battery whose "actual" capacity is now down to 85 ah fully charged from it's
brand new "actual" capacity of 100 ah fully charged, any different from a new battery whose "actual"
capacity is 85 ah fully charged? They should be equal capacity by your comments in red above in electrical terms,
shouldn't they?

They should if "Fully Charged" equates to the same Voltage. Consider these points.

1) Depth of Discharge can be measured by resting voltage. Full charge "100%" at approximately 12.7V for and 12.2V approximately at 50% charge. (there is variance between types and manufacturers etc)

2) We have found reasonable information that two batteries in parallel will tend to settle at the lowest voltage in that circuit.

So, if either the newer battery or older battery in the same parallel circuit has a lower "Full Charge" Voltage the other one settling to that voltage would not actually be at full charge and capacity would be lost. At least it would seem logical for that to be the case. Probably not significant, but possible.

There are other things to consider with using different batteries(same type though) in the same bank. Some manufacturers use different specs to calculate AH Rating. For example 100AH @ 20 hours. is higher capacity really than one that is rated 100AH @ 100 hours for example. This has more to do with how the ratings are applied to different manufacturers batteries, some will use the 100 hour rate to make their batteries appear as higher capacity. So for the purposes of calculating total AH capacity based on what the manufacturer says, you should compare them all at the same testing rate. @20 hours is typical, but they don't all adhere to it.

Another consideration in using old batteries with new batteries is the number of cycles they have already been through. Consider this. You have a battery that is a year old and has seen average use. It still seems to hold a good charge and has good capacity, but it has seen a number of charge/discharge cycles so likely it's life is going to be shorter than the new battery added to the bank. Assuming the new battery will last longer in this circuit, how will you know when the older battery is failing enough to be a problem? It seems to me that you would have to monitor the voltage of the older battery separately to the new one to really have a handle on it's condition and how far down it is discharging compared to other batteries in the circuit. This may not matter to some folks but I also consider these types of things. It is just more simple for me to match them and I tend to give my advice based on that as well.

Hope that made sense

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