Dissimilar Batteries

DiploStrat

Expedition Leader
May be time for a new thread on the eternal question of charging auxiliary battery banks. The equation is rather simple if you have two batteries of the same size and the same construction (i.e., FLA, AGM, Gel) connected by a short cable of adequate size - slap a solenoid between them and be happy. (And avoid most diode based systems.)

But most of us want a system with some of these characteristics:

-- Starter battery optimized for vehicle start - lots of cranking amps for a short time.
-- Camper battery optimized for long term discharge - relatively low amperage discharge for a long time, a so called "deep cycle" battery.
-- For various reasons, vibration, ease of mounting, etc., most of us are using AGM batteries for the deep cycle camper side.

So now we have a problem:

-- Modern vehicles seem to be moving to calcium/FLA batteries with charging circuits that commonly run as high as 15.5v (E.g. 2013 Chevrolet 3500HD)
-- Common AGM batteries are limited to a charging voltage of 14.4v.

Feeding a voltage over 14.4v to an AGM battery will dry it out quickly and, as they cannot be topped up, kill them. (Lifeline sent me a sharp warning!)

The 15.5v, confirmed by a voltage meter and a review of the GM manual has caused me to toss my solution out the window. The controller that I was going to use took one look at the 15.5v, declared an over voltage, and shut down. Bummer as I was getting a nice 190Ah at the camper batteries. ;-(

Soooooo,

-- Any one dealt with this problem?
-- And if you are simply using a solenoid: 1.) What voltage are you seeing at the AGM batteries, and 2.) What amperage are you seeing?
 
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DiploStrat

Expedition Leader
Interesting; Mark is one of my contacts as well. But if both of your batteries are AGM, do they not have the same voltage requirements? Or does your alternator not go low enough?

It is ironic that, after years of trying to find a way to get a high enough voltage at the camper battery terminal, we now have to deal with having too much! :(

In the end, I think that I will end up with an isolated, parallel system; one alternator for the start battery and the other, with its own regulator, dedicated to the camper AGM batteries. The biggest question, at this point, is what happens when the truck discovers that the second alternator is no longer there. If it merely throws an internal code, then there is no problem, but if it throws the "Check Engine" light, thinks about limp mode, etc., then there is.

I might have done better to have ordered my truck with a single, HD alternator, and then bought a second alternator, brackets, etc. separately. (There is a gentleman over on the RV.net who did this quite successfully on a Ford.)

Courtesy of user "dwh" I have also found a company in UK that offers an interesting twist on a simple solenoid system. Here is how they approach the need to charge at two voltages from a single voltage source: http://www.smartgauge.co.uk/diff_batts.html

Might work, but I would have to test in real life. In the case of my Tiger:

-- The Chevrolet seems to stay at 15.5v for a LONG time; like 500 miles of driving. If that were the case, then the camper battery might never charge as the controller would see the higher voltage and disconnect.

-- Things might not be so bad if you make your connection at the battery terminal (as opposed to the alternator terminal). That is, even if the alternator is churning out 15v, the actual voltage at the battery may be lower, thus allowing a connection. Especially if the starter system actually went to float.

-- I fear, however, that as soon as the camper batteries are added to the equation, then the voltage at the starter battery will rise as the alternator takes the load and thus the system will disconnect again.

All of this argues for a parallel system as being the most elegant, if slightly Marxist, solution. You may not have "From each according to his ability" but you do get the "to each according to his needs."
 
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DiploStrat

Expedition Leader
Rob Blackwell of White Acorn (http://www.whiteacorn.com) tipped me to Sterling Products. He is running two B2B on his Unimog after an exhaustive analysis of why a solenoid wasn't working for him. (http://www.whiteacorn.com/vehicle/rtw/electricalpart1) He has gotten really good results and, based on his experience, I bought and installed a Sterling A2B. Initial tests were VERY positive as we achieved charge rates of 170A at idle and 190A on the road. But, after driving the truck for about an hour, the Sterling threw a high voltage alarm and shut down. As far as I can tell, the problem is this. When we tested, the truck had been sitting for months in the workshop as the camper was being built and thus the truck batteries were highly discharged. Once they recharged, then the voltage down the wire to the Sterling rose to its full 15.5 volts and the system shut down, as it is designed.

Basically, the Sterling products are designed to take the weakness of a typical 13-14 volt diode isolation system, voltage drop, and make it into a virtue. The voltage drop causes the alternator to think that the battery is discharged and increase its output current. The Sterling products take that current and then boost the voltage to the level required by FLA/Gel/AGM batteries and provide a proper charge, even to compensating for voltage drop in your wiring. The Sterling has no real way to drop the alternator voltage.

These products work great until faced with a truck whose alternator routinely produces over 15 volts. Hence my problem and my post. I was so sure that I had found the holy grail of charing dissimilar batteries! :mad:

When Sterling reopens for the new year, I may explore using two B2B. The only drawback is that the maximum charge that I can get would be around 80Ah and it is not clear that the voltage would ever drop low enough to achieve a real float.
 

Stuartag

Always Ready
These products work great until faced with a truck whose alternator routinely produces over 15 volts. Hence my problem and my post. I was so sure that I had found the holy grail of charing dissimilar batteries! :mad: .[/QUOTE said:
I'm faced with the age-old problem with the alternator not producing enough voltage for the AGM house batteries which are also different than the starting batteries. I think if the Sterling BtoB charger works as advertized, it should be the solution best suited for me. I need a 29 volt charge (for a 24 volt system).
 

DiploStrat

Expedition Leader
The only problem with the Redarc unit is that it is more expensive than the Sterling equivalent (http://sterling-power-usa.com/12volt-12volt45ampbatterytobatterycharger.aspx), and has lower output. (20A vs. 50A) That said, they are functionally the same. While the Redarc is fused at 30A, it does offer the possible advantage of being able to accept an input voltage over 15v. Trick is that even with two, I would only be getting a charge of 60A.

The really cool product in this area is still vaporware, but it offers interesting possibilities. http://www.sterling-power.com/images/downloads/Wildside.pdf I really like the option of one stop shopping, both solar and engine charging coordinated. But to be really effective, it has to reach a real world charge rate of 100A.
 

Stuartag

Always Ready
You can get a full charge with 60 amps, or even 30 amps....You just have to drive longer.:ylsmoke:
 

DiploStrat

Expedition Leader
Roger, but my design criteria are: 300Ah total usable capacity, 100Ah per day consumption, drive no more than 3 hours. That means I have to bulk charge at over 100A for a hour to allow two more hours of absorption and float. And then then it is tight, but AGM batteries LUV high current. They just don't like high voltage. I was there until I discovered that my Chevrolet runs about one volt too high. :mad:
 

mog

Kodiak Buckaroo
The only problem with the Redarc unit is that it is more expensive than the Sterling equivalent (http://sterling-power-usa.com/12volt-12volt45ampbatterytobatterycharger.aspx), and has lower output. (20A vs. 50A) That said, they are functionally the same. While the Redarc is fused at 30A, it does offer the possible advantage of being able to accept an input voltage over 15v. Trick is that even with two, I would only be getting a charge of 60A.

The really cool product in this area is still vaporware, but it offers interesting possibilities. http://www.sterling-power.com/images/downloads/Wildside.pdf I really like the option of one stop shopping, both solar and engine charging coordinated. But to be really effective, it has to reach a real world charge rate of 100A.

It looks like Redarc has one that is 40A (60 amp fused) 9 to 32 vdc input and includes solar control:
battery-charger-40a/
 
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Stuartag

Always Ready
Roger, but my design criteria are: 300Ah total usable capacity, 100Ah per day consumption, drive no more than 3 hours. That means I have to bulk charge at over 100A for a hour to allow two more hours of absorption and float. And then then it is tight, but AGM batteries LUV high current. They just don't like high voltage. I was there until I discovered that my Chevrolet runs about one volt too high. :mad:

I wonder if you could put some sort of diode unit between your starting battery and the charger, so it only sees about 14 or 14.5 volts?
 

DiploStrat

Expedition Leader
Diodes?

I am not wild about the idea of a diode just sitting there turning voltage into heat. (If nothing else, it will hurt fuel mileage.)

The Chevrolet factory set up is two alternators, each connected to a battery. One idea that occurs is to use a pair of B2B, each one connected to one battery. The question here is, again, high voltage trip - the B2B, like the A2B, is designed to shut down at 15.5v and, eventually, the question of whether it will every actually float. I am also mildly invested with Sterling Products as I have one of their remotes on my control panel.

A really exciting option might be a B2B on one leg and a Wildside on the other. This would theoretically allow me to remove my Blue Sky solar controller.

I think, however, that the right answer is to separate the two alternators, upgrading the second one, if necessary.

I was trying to pulse the combined experience of this forum as I wait for my various contacts to return from the Christmas/New Year holidays.
 

lysol

Explorer
My requirements is to just charge a single 60Ah battery as well as a separate 100Ah battery. The 60Ah battery will be the "second" battery under the hood to run all of my accessories. The 100Ah battery will be the rear cargo mounted battery for my fridge. The Redarc 25A (alternator + solar) would be perfect for my 100Ah battery and the 20A (alternator only) would be perfect for my 60Ah battery under the hood.

With your setup of 300Ah worth of batteries, I would definitely go with something larger. I believe the largest charger that Redarc has is the 40A (BCDC1240). That would take at least an entire day's worth of driving to charge that bank from completely drained. With that said, how often would you drain the entire battery bank?
 

DiploStrat

Expedition Leader
The rule of thumb is that FLA batteries should not be discharged more than 50% and then recharged, as soon as possible, with something like 110% of the current lost. Soooo, 100Ah AGM, you can discharge 50Ah and need to recharge, say 60Ah. At 50Ah, you will need about one hour of bulk, two hours of absorb, and then float. Drop to 25A and you will probably double the bulk stage. AGM's want the biggest charge you can come up with, up to C5, so I would go with the largest B2B you can find.

The only thing you need to check is that the voltage of your starter battery does not exceed any input limit of your B2B. The Redarc has the potential advantage here as it says that it can take up to 32v. That implies that it has a buck capability, as well as a boost capability.

If your alternator does not exceed 15 volts and you only need to recoup less than 100Ah, then the problem becomes a lot easier - either the Readarc or the Sterling product should work perfectly. Just remember to place the unit as close to the camper battery as possible, that way it compensates for any voltage loss in the wire from the starter battery to the B2B.

You shouldn't need to do anything for the battery under the hood, modern engine and body control modules should do all you need. (The GM modules compensate for weather, headlights, heater, sulfating, etc.)
 

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