math formula?

[4E fam]

Anyone know the formula to figure out how long it will take to charge your coach batteries by driving around? Basically, is there a way to figure out how much power your alternator generates/distributes to the coach batteries over time driven?

I know I can just drive around and get an estimate, but I didn't know if there was a more "exact" method.

Thanks in advance!

[MsNomer]

Too case-specific. Depends on the vehicle, the alternator, the house battery and its state of charge. You will have to make your own measurements.

[booster]

Originally Posted by 4E fam

Anyone know the formula to figure out how long it will take to charge your coach batteries by driving around? Basically, is there a way to figure out how much power your alternator generates/distributes to the coach batteries over time driven?

I know I can just drive around and get an estimate, but I didn't know if there was a more "exact" method.

Thanks in advance!

You can do a calculation based on how much the battery will accept but you need to measure how much that is for your system and it would only apply up to about 80% full. After that charging slows down a huge amount for lead acid batteries, both AGM and wet cells. It is not uncommon to take 6 or more hours to get the last 20% full.


In general for a small to moderate system and charging off van alternator with good cabling size it would probably be in the 3-4 hours to get to 80% from an 80% discharge (deep discharge) 2-3 hours from 50% discharge.



Total time to true full 8+ hours.


Obviously, very few people drive that long on a regular basis so most batteries don't get full charges from the alternators. Solar can be used to do the final "top off" which is lower amps but longer time. Many of us use that way to get full charge.


There is another discussion going on now concerning battery monitor settings that gets into this also, so you may want to look at it for reference.



A battery monitor would show you what is going on for answering your question, but the news would probably be disappointing because you would rarely, if ever, actually get to 100% full unless you had very shallow discharge and drove a long time.


It is fairly quick to get back the power used from 20% to 80% full on the batteries, so for a quick recovery to that level to functional a moderate drive will do it. For best battery life, getting to true 100% every 10 cycles or so is needed, but rarely happens in RV use IMO.

[Doneworking]

Over many years of ownership of different Cs and Bs, experience tells me adequately what to expect as far as battery charging, tank levels, etc. is concerned. Now, many want to know more specifically the status of those components but intuitively from experience works fine for most users I suspect.

[4E fam]

Originally Posted by booster

You can do a calculation based on how much the battery will accept but you need to measure how much that is for your system and it would only apply up to about 80% full. After that charging slows down a huge amount for lead acid batteries, both AGM and wet cells. It is not uncommon to take 6 or more hours to get the last 20% full.


In general for a small to moderate system and charging off van alternator with good cabling size it would probably be in the 3-4 hours to get to 80% from an 80% discharge (deep discharge) 2-3 hours from 50% discharge.



Total time to true full 8+ hours.


Obviously, very few people drive that long on a regular basis so most batteries don't get full charges from the alternators. Solar can be used to do the final "top off" which is lower amps but longer time. Many of us use that way to get full charge.


There is another discussion going on now concerning battery monitor settings that gets into this also, so you may want to look at it for reference.



A battery monitor would show you what is going on for answering your question, but the news would probably be disappointing because you would rarely, if ever, actually get to 100% full unless you had very shallow discharge and drove a long time.


It is fairly quick to get back the power used from 20% to 80% full on the batteries, so for a quick recovery to that level to functional a moderate drive will do it. For best battery life, getting to true 100% every 10 cycles or so is needed, but rarely happens in RV use IMO.

Thanks for that great explanation! I appreciate it. I'm wondering if I was boondocking and needed to power up the batteries to basically keep my fridge running, how long I would have to drive. Your answer helped me figure out it is probably longer than I would want. I suppose my onboard generator would probably charge it faster, but I wouldn't want the noise.

So best bet is to see how long it takes with the fridge running for the batteries to get too low (in my driveway), then maybe consider solar to see about getting it recharged when stationary.

Thanks again!

[jrobe]

I have a Nations Starter 2nd alternator on my Sprinter Class B with 200 AmpHr AGM batteries and a Magnum Energy battery monitor. I usually have the battery monitor set to show my "State of Charge" (SOC) as a percentage of full charge. I think the monitor is very accurate. I have watched it closely as I am charging my batteries on shore power and it seems to gradually increase to 100% SOC exactly as I would expect with shore power charging from my Magnum Energy Inverter Charger.

My refrigerator is about 3 cu ft and depending on how warm it is outside, it usually requires 20-30 AmpHrs of my battery power per day to maintain perfect inside temperatures. If I just use the refrigerator and no other power (for example when it is sitting unused in my yard), my battery SOC usually goes down about 10% per day.

My 2nd alternator is rated at 270 amp as I recall and seems to measure about 150-170 amps at idle and about 220 amps at typical driving speeds. You can then do the math to calculate how long I need to drive to replace my battery amp hours. From my personal observation watching my battery monitor, it takes about 15 min of driving to return the monitor reading from 90% to 100% SOC (the same SOC reading is if it was plugged into shore power for long periods) which would be about the typical battery usage for the refrigerator alone.

I used my van this weekend and ran the batteries down to 60% SOC which would have been about 80 amp hrs of battery power used. I then drove for about 80-90 min and when I stopped the reading was back to 100% SOC. I am not sure yesterday how much of that time it took to get to 100% but it was there when I arrived at my destination. For our typical daily battery usage, we can almost always replace our battery power with 30-60 min of driving per day which we almost always do anyway.

Since we usually drive at least a short distance each day (or every other day), solar panels would be a waste of money for us. Even on a sunny day with the van sitting directly in the sun, the output of the solar panels would only require minimal driving time.

[booster]

Originally Posted by jrobe

I have a Nations Starter 2nd alternator on my Sprinter Class B with 200 AmpHr AGM batteries and a Magnum Energy battery monitor. I usually have the battery monitor set to show my "State of Charge" (SOC) as a percentage of full charge. I think the monitor is very accurate. I have watched it closely as I am charging my batteries on shore power and it seems to gradually increase to 100% SOC exactly as I would expect with shore power charging from my Magnum Energy Inverter Charger.

My refrigerator is about 3 cu ft and depending on how warm it is outside, it usually requires 20-30 AmpHrs of my battery power per day to maintain perfect inside temperatures. If I just use the refrigerator and no other power (for example when it is sitting unused in my yard), my battery SOC usually goes down about 10% per day.

My 2nd alternator is rated at 270 amp as I recall and seems to measure about 150-170 amps at idle and about 220 amps at typical driving speeds. You can then do the math to calculate how long I need to drive to replace my battery amp hours. From my personal observation watching my battery monitor, it takes about 15 min of driving to return the monitor reading from 90% to 100% SOC (the same SOC reading is if it was plugged into shore power for long periods) which would be about the typical battery usage for the refrigerator alone.

I used my van this weekend and ran the batteries down to 60% SOC which would have been about 80 amp hrs of battery power used. I then drove for about 80-90 min and when I stopped the reading was back to 100% SOC. I am not sure yesterday how much of that time it took to get to 100% but it was there when I arrived at my destination. For our typical daily battery usage, we can almost always replace our battery power with 30-60 min of driving per day which we almost always do anyway.

It would be very interesting to see what the settings are in the Magnum (ARC-50?) remote, as those charge times are not typical for AGM batteries getting totally full. The size of the charger or alternator gets to be essentially irrelevant at about 80% full, as the batteries won't accept many amps. What Magnum charger do you have, and how long does it take to charge on that? 200ah of AGM would be able to accept about 80 amps average over time without getting too hot for most brands of batteries. 200 amps for about 15 minutes would probably be about max time before heat would be an issue with them.



The data we have seen on the Nations alternators, which we also have, would indicate that in many installations they will also get hot and thermal cycle on the heat sensor in the Baldar that turns them down to half output. Net average with cycling is about 165amps so still too much for putting into 200ah bank for very long. Do you have the Magnum battery temp sensor hooked up so you can see it?

[jrobe]

Originally Posted by booster

YIn general for a small to moderate system and charging off van alternator with good cabling size it would probably be in the 3-4 hours to get to 80% from an 80% discharge (deep discharge) 2-3 hours from 50% discharge.

Total time to true full 8+ hours.
.

I have read your comments like this before but this is completely inaccurate with my 2nd alternator system based on my real world experience monitoring my battery system. My AGM batteries are also now 4 years old and seem to work just as well as when they were new. For the last 4 years I have recharged and maintained my battery system with minimal daily driving (usually well under 1 hour).

[MsNomer]

I have noticed this dichotomy on other forums as well. My system, measured and controlled by a TriMetric, behaves as booster describes. I wonder if "it depends on what your definition of 100% is."

[booster]

Originally Posted by MsNomer

I have noticed this dichotomy on other forums as well. My system, measured and controlled by a TriMetric, behaves as booster describes. I wonder if "it depends on what your definition of 100% is."

That is exactly correct. With any monitor you can make it say you are getting to 100% full, but according to folks that made the batteries, they are not totally full.


It is an easy test to see if you are full if you can force the stages like you can with Magnum monitor and charge system of certain level. After it switches to 100% and says you are full force it back into absorption voltage and see what the amps are and if they are dropping slowly over time. If they are still dropping, you weren't full.


I have no issue if folks are setting their meters that way to match their use and system, as it is their choice, but it is not accurate to tell people you "know" that the batteries are really full unless they meet the criteria given by the manufacturer defining full for their batteries.

[MsNomer]

According to my manufacturer, if my 250Ah AGM’s can take in more than 1.25A, they are not full. That can take quite a while.

[Bud]

Originally Posted by booster

That is exactly correct. With any monitor you can make it say you are getting to 100% full, but according to folks that made the batteries, they are not totally full.


It is an easy test to see if you are full if you can force the stages like you can with Magnum monitor and charge system of certain level. After it switches to 100% and says you are full force it back into absorption voltage and see what the amps are and if they are dropping slowly over time. If they are still dropping, you weren't full.


I have no issue if folks are setting their meters that way to match their use and system, as it is their choice, but it is not accurate to tell people you "know" that the batteries are really full unless they meet the criteria given by the manufacturer defining full for their batteries.

Booster, if it hadn't been for that Whistle, I might not be responding. I have not spoken up in quite awhile about state of charge.......

With my 2 very inexpensive battery voltage meters, one in thousandths of a volt and the other in hundredths of a volt (tenths being of little value). I KNOW 100% state of charge! Granted, I would to like have a chance to compete with you and your stuff, fun for me. I'll loose determining 40 or 50 or 60 percentage charge, but how far off will I be with battery voltage meters? Battery Sag is everything and learning it with your appliances, etc. Very doable, just requires the motivation to learn it.

Still, I would like a Beyond to become available in my area with everything, extra lithium too. I don't know why I check everyday as it not going to happen and I'm not willing to order one.

Enjoyed the whistle, thanks. imo, You make times better, more fun here with those.

Bud

[booster]

Originally Posted by Bud

Booster, if it hadn't been for that Whistle, I might not be responding. I have not spoken up in quite awhile about state of charge.......

With my 2 very inexpensive battery voltage meters, one in thousandths of a volt and the other in hundredths of a volt (tenths being of little value). I KNOW 100% state of charge! Granted, I would to like have a chance to compete with you and your stuff, fun for me. I'll loose determining 40 or 50 or 60 percentage charge, but how far off will I be with battery voltage meters? Battery Sag is everything and learning it with your appliances, etc. Very doable, just requires the motivation to learn it.

Still, I would like a Beyond to become available in my area with everything, extra lithium too. I don't know why I check everyday as it not going to happen and I'm not willing to order one.

Enjoyed the whistle, thanks. imo, You make times better, more fun here with those.

Bud

The problem with voltage, no matter how accurately you measure it, it just isn't a great indicator of SOC accuracy wise. It is accurate if you look at repeatability if under the same conditions and that if what you are really doing. You know that you have about so much power left when the voltage reads "X" with some known stuff connected. This is the way it was for many years before monitors were popular and people survived fine. After you run low on power a couple of times you get a feel for the voltage vs time left before charging needed, etc. I have no problem with running that way if you are comfortable doing it. Your chargers may or may not be getting the batteries full, most aren't able to so likely not full, but your observational history has taken that into account. This is just setting the 100% full on a monitor at whatever point your chargers get to, which is necessary if you can't get the batteries more full on a regular basis. With a monitor, you should run a capacity test from that new 100% full point to adjust the bank size to match, but you have done that with experience, probably by running low a couple of times until you got the timing right.



Bottom line is you don't need to know state of charge, be it accurate or not, because you can prevent yourself from running out of power, and get acceptable, if not perfect, battery life by watching the voltages.



Monitors probably work better for those that aren't interested in the learning curve stuff, if the monitor is really telling them what they think it is. The fact that many of the monitors we hear about and see the settings for aren't telling the user what they think it is. This is where I would like to see better understanding. I hear pretty regularly that people don't understand how they ran out of power when the monitor said they had some left. Usually the batteries get blamed for not making rated capacity, but my bet is that it was not the batteries at all. As I mentioned, the information with the meter set for 100% at less than full can still be just as useful as the one that does get to full all the time, IMO. The only caveat is knowing and understanding that you don't have the full rated capacity of the batteries to use because of the non full thing. The amp hour counting can be good for variable amounts of power users as they can look at see how many AH they have left and know if they can run this or that for whatever time. Big use stuff like a microwave can chew up a lot power in a little time compared to the old school very low power use vans.


In general your method is probably more useful in not running out of power than a poorly set/misunderstood monitor and that is why I try to explain as best I can what the monitor is capable of doing, and not doing, so users can get useful information, not just what they want to see.


So we can officially say that (Bud+voltmeter) X ten years experience = walking/talking battery monitor???? And you don't do it by just believing because the little light on the charger or a monitor says your batteries are 100% full, because you know they are as full as they always have in the past. + for Bud.


Does a Beyond with extra lithium come with a battery monitor for them?

[eric1514]

A few days ago I used 92Ah from a 420Ah battery bank. After, I left the batteries to rest overnight and the next morning the voltage of the bank was 12.6+, which all those colorful charts will tell you is 100% full. The idiot lights in the coach said they were full and the PD charger thought they were full and didn't start in bulk mode when I plugged in. But they were far from full. That convinced me testing voltage is worthless if I want to know how full my batteries are.

[booster]

Originally Posted by eric1514

A few days ago I used 92Ah from a 420Ah battery bank. After, I left the batteries to rest overnight and the next morning the voltage of the bank was 12.6+, which all those colorful charts will tell you is 100% full. The idiot lights in the coach said they were full and the PD charger thought they were full and didn't start in bulk mode when I plugged in. But they were far from full. That convinced me testing voltage is worthless if I want to know how full my batteries are.

We see similar with our Lifelines at that level IIRC. It takes a long time to come off of the very conservative battery full voltage they give in the charts. When totally full rested and surface charge off, our batteries are generally over 13v. The inability of most charging systems to get the batteries totally full is losing a lot of user's capacity, that they think they have per the ratings, IMO. Most don't know they don't have it, though because of charts like you mention or monitors that are telling them they do have it.

[MsNomer]

Booster, what AC charger are you using on your lifelines? My NOCO doesn’t start charging until voltage drops to 12.6, which presents an obvious problem, as I also have Lifelines.

[avanti]

I've posted this before, but its been awhile, and it is quite informative.

Below is a chart that I compiled using data from my Trimetric SOC meter soon after I installed my Nation's 270Amp Second Alternator. It shows the system charging my 440 amp 4 AGM battery bank from a fairly deep discharge during a long highway drive on a hot summer day:

charge data.jpg

Kind of brings the equations to life.

The bouncing around on the low-side of the "amps" chart shows the Balmar alternator throttling charging to prevent the alternator from overheating.

[booster]

Originally Posted by MsNomer

Booster, what AC charger are you using on your lifelines? My NOCO doesn’t start charging until voltage drops to 12.6, which presents an obvious problem, as I also have Lifelines.

We have a Magnum MS2000 with an ARC-50 remote, temp sensor, and battery monitor kit BMK on it. It was pretty much the only shore charger I could find that would do charge control off the internal battery monitor and shunt so we get true amp controlled charging every time.


What is unfortunate with Magnum is the same as you see with the NOCO, a it won't go into a full charge cycle if the voltage is in the 12.7v range. I talked to Magnum about it because the unit does charge ever time in CC/CV mode, and they said it is an error on their part as it is a carryover for previous units that didn't have the better control through the monitor and could overcharge running a full timed cycle every time. They said they are going to try to correct it at an update if they have enough capacity to do so in the hardware. It would only run a full cycle when turned on if you had the BMK kit on it and was set in "amp control" mode. I don't know if the newer ones have changed, though.


In real world use, it is only a minor inconvenience for us as the ARC-50 allows manual charge stage control forcing. If we stop driving for the day with some surface charge, or the solar is holding up the voltage and Magnum goes to float when we plug in it is just a couple of button pushes on the remote to start a full cycle. There is no risk of overcharge because it is battery charging amp controlled so as soon as the battery is full or even if it starts full, it will go to float at the correct return amps for 100% full.


I just took a quick look at the NOCO site and it appears that they are algorithm/timer based chargers so don't monitor battery charging amps unless they do it internally. With no settings for actual battery capacity it is unlikely they do any other than volts and time algorithm control, so if your charger went in to full charge every time, it might overcharge or just go to float right away before doing anything. This is the reason that many/most smart chargers won't do a cycle on every plug in unless the voltage of the system is low enough to justify a charge. They know that chronic overcharging is very bad for the batteries and worse than the lesser damaging undercharging. Sometimes, you can fool a charger to start by turning on some high load items on the 12v system to drop the voltage. Once it starts charging, you may or may not have to leave some of them running to keep it charging. Some chargers, once started have a hold time at absorption voltage that is timer based so they will charge for that long once the stuff is shut off.


It does not look like the NOCO has any way to force the stages so you wouldn't be able to use it like we do with the Magnum. Progressive Dynamics chargers with the Charge Wizard can be run manually and force stages, but they are one of the few.


Of the available smart chargers, the only one I know of that will run a charge cycle without worrying about voltage is the Blue Sea Systems unit, which is a very interesting charger. We used one for a few years successfully and it always ran a cycle when turned on. It has 3 isolated outputs and measures the output amps internally. The amps can be set to control the charging so you can get very good charging with them, but you have to put your loads on one bank and the batteries on the another so the loads don't mess up the amp readings to batteries during charging. I rigged a simple relay setup to switch a connection between the two banks together when the charger was off and open when the charger was running and it worked well. I still have that charger and use it for a bench charger sometimes as I can set the amps way down to do small batteries. It is 40 amps max. We went to the Magnum when we went to the 440ah battery bank of Lifelines from our previous 260ah of Trojan GC2 wet cells to get the higher output of 100 amps of the Magnum. Most places get about $700 for the Blue Sea 40 amp version which is the largest they make. Biggest downside is the charger needs to be easily accessible as all the settings and such are done directly onto it, not on the remote, as is any monitoring on the unit screen.


There are a few other smart chargers around that measure amps internally and are settable, kind of, in large steps but they have no way to isolate the loads so not as good for accuracy of charging as the charger thinks the loads amps are going to the batteries. Some of them might start each time turned on as they woudn't be so likely to overcharge. There are a lot of specs on chargers around this forum, but probably hard to find. It is best to look at the manual for chargers and see what they let you set, and if there is a amp setting other than maximum is it worth investigating what it does. Those are also the chargers most likely to run a full cycle every turn on.

[Bud]

"Does a Beyond with extra lithium come with a battery monitor for them?"

Sorry about the delay booster, I thought that I had responded. Guess I did not make the final click.

The answer is heck if I know. Why? I thought the odds that it would be a deciding factor was virtually zero. Heck, I don't know if it even has a monitor, 99% chance though. I would guess the added Lithionics/Zantrex stuff (extra amps) uses the same monitor. I'll learn whatever then, not enough curiosity and excess laziness prevents it. I like many are just avoiding a conventional generator. I might live with the newer onan?

Next, see if I can live with the seating/steering wheel ...... Promaster. I'm guessing after driving one, it will be about the same - No. But I also know some that felt the same, but purchased the Promaster because the rv part offset the driving position downside. Then overtime/miles they liked driving the Promaster, right decision. Might happen with me. IF I knew that, then I don't want 6 wheels, rather 4. I need to drive one.

Bud

[booster]

Avanti's log is very informative, and I would ask those that look at it to put aside what they see from a monitor they may have currently which is probably set very conservatively. Avanti's monitor, AFAIK, is set to battery manufacturer's recommendations for full batteries and the log reinforces that assumption for me.


Note the straight line for time vs SOC up until 60 minutes/75% SOC. This is the bulk charging area where the batteries are accepting as much as the alternator can put out. This is also the only section of the charging curve that will change with a change to a big alternator. Everything beyond this on his charts will not change.


This is also, unsurprisingly, about the point that the very conservative monitor settings will indicate a 100% full battery, basically at the bulk to absorption transition maybe plus a bit of hold time. If you think the battery is full at this point it is at 75-80% in reality.


Avanti's log goes to 200 minutes, nearly 3.5 hours and is still not to 100% full, which is surely correct from what I have seen in testing. My bet is that he would not show full until 6+ hours, but where the trigger is set can make a large difference in time in the tapering section of the charge curve. It could go to 8 hours or 12 depending on small changes in the transition amps. This is the area where solar top off excels because it is low output long time, for free, charging, no connection required.


As I have said many times in the past, it is not bad or wrong to set your monitor to show full before the batteries are 100% if your charging systems won't get you to actual 100%, as it is about the only way you can decently monitor your SOC (Bud does similar based on experience, same principle). But my gripe, if you want to call it that, is that users need to realize this so others aren't confused and misled by the often repeated, "my batteries get to 100% full in a 45 minute drive". That may get to the max attainable SOC in that system full point, but it is not the battery full point if you are using lead acid batteries, IMO. If someone carefully designs their system capacity to cover use based on rated AH and only get 80% because of the above issue, they are sure to be disappointed and unhappy.