Battery Chargers: why do they support/need multiple banks?

[sfluke]

I've been looking at different battery chargers and most of them seem to have support for more than one battery bank. Sometimes 2, 3 or 4. Some of the chargers not only support multiple banks but actually have a limited max current draw per bank so you're able to charge two separate banks faster than one large bank. Which then is a reason for splitting up the batteries. But once you've split them into two separate banks (say twice two 110Ah batteries in parallel) you now have two different circuits or do you then connect them again once you're done charging if you want to avoid two completely separated circuits.

Aside from being able to charge your starter battery and a second house battery bank separately is there an advantage to splitting the house battery into two separate banks? If so how would you then connect them again when actually using the batteries?

[chev49]

Originally Posted by sfluke

I've been looking at different battery chargers and most of them seem to have support for more than one battery bank. Sometimes 2, 3 or 4. Some of the chargers not only support multiple banks but actually have a limited max current draw per bank so you're able to charge two separate banks faster than one large bank. Which then is a reason for splitting up the batteries. But once you've split them into two separate banks (say twice two 110Ah batteries in parallel) you now have two different circuits or do you then connect them again once you're done charging if you want to avoid two completely separated circuits.

Aside from being able to charge your starter battery and a second house battery bank separately is there an advantage to splitting the house battery into two separate banks? If so how would you then connect them again when actually using the batteries?

One should have several rv deep cycle or 8d type batteries for the house. You separate by using isolators so at least you have some battery power left in one bank for your inverter. The battery wholesale places can steer you correctly on what to use, and they usually always have good ones such as the Trojan wet, and so forth. However, the good ones will cost 400 or so each so keep that in mind when buying 8 for a good system. The battery guys will also have all the connectors and cables that one would need. Also, is best to have the high power expensive inverters, and they aint cheap...

[sfluke]

Technically there is no advantage of not putting all batteries in series as long as you have a voltmeter you have a pretty good idea how much battery capacity is left. So isolating the different banks will just mean you have more complicated circuit because you then also need to be able to switch from one bank to the other. It also means that you won't be using all batteries in an equal way and will need to possibly replace one bank before the other. Are there any big advantages that I am missing?

[Robin97396]

The way I understood it was one battery can go bad and ruin other batteries. If they're in smaller groups they're somewhat isolated from each other and if the worst happens somehow you don't loose all your batteries.

That expensive controller/charger regulates everything the way you want it electronically so the batteries get charged equally and without your constant attention. Without the controller if you make a mistake of overcharging you again ruin your batteries.

[chev49]

you are right of course.
not to mention you put in paralell groups as you suggested

[cadillackid]

a car battery is effectively a group of 6 smaller batteries.. or "cells" each cell is 2.1 volts and they are all connected in series intewrnally to give you a 12.6 volt battery.. thats a bit simplified but when you used to hear people talk of a dead "cell" that one cell could run down the rest of the batteries asit carried an internal resistance high enough to actually draw power from the others..

when you place two car batteries in parallel you effectively have one of those "groups" mentioned.. however the issue still remains.. that one dead cell in one battery can still drain all the batteries in the group.. it might take longer to do so.. so you wont notice the dieing cell for longer until it may go to a near short and cause bigger issues like heat swelling etc..
-Christopher

[Iceni John]

I have effectively two separate house systems running side-by-side. Each side of four solar panels feeds its own charge controller that then charges its own bank of (eventually) four golfcart batteries in series and parallel. To prevent one bank of batteries from back-feeding into the other, each bank's output goes through a 250 amp Schottky diode before being combined at the DC load center. This makes it impossible for a serious failure in one battery or one bank to kill the entire system. I use the Cole-Hersee 48161 diodes, and any slight voltage loss through them is inconsequential because they are not on the charging side. So far, so good.

John

[sfluke]

John, you're the first person to mention the use of diodes to connect multiple banks. I'm familiar with diodes and have used them to power smaller devices both from the engine battery & house battery.

If a really big diode does the trick that would be very convenient, because my main concern would be the inconvenience of having to then manually switch between the two house banks.

I assume you're fine accepting the voltage drop across the diode? Seems like a realistic value is 8W loss for a 250W load at 12V.

[TJones]

Hey I'm bringing this thread back to life on multiple battery banks. Would an advantage of multiple battery banks be that one bank could be supplying power while the other is being fully charged? If there is only one bank and a continous load it may never get fully charged which shortens the life of the batteries.

Ted

[JDOnTheGo]

Old thread but Ted revived it not me!!

To the best of my knowledge, most RV battery chargers are single bank chargers (such as https://www.progressivedyn.com/rv/po...vertercharger/). However; there are some that do have a second circuit for the chassis battery. I've not seen one intended for the RV market that has more than two circuits - but would not be surprised that they exist.

There is some information here that is worth talking about. All of the following assumes we are talking about flooded lead acid (FLA) batteries.

1. Voltage is a terrible way to measure state of charge. It only sorta works after the battery has been resting for many hours (like 24). That never happens in typical use. Using a battery hydrometer to measure the specific gravity of the electrolyte is the only accurate means to measure state of charge of a battery. Since that is horribly inconvenient, the second best way is a quality battery monitor with a shunt that measures all the electrons flowing into/out of the battery. The better ones allow some configuration to fine tune. If you are operating your system using only a voltmeter, you fall into the category of people that are abusing your batteries. Sorry...

2. For typical use, more than one battery bank is just wasted effort. I agree, the concept sounds nice - backup and all that. However; if you properly care for your battery bank, it will take care of you. They do not fail without warning. If you ignore them, don't water them, discharge too deeply, overcharge, etc..., yes, it may fail when you least expect it. Don't do that. You wouldn't do that to your engine - why do it to your batteries? I would argue that properly maintaining your battery bank is time/money better spent than adding redundancy so you can ignore them. That said, if you have the knowledge and budget to do that sort of thing and it makes you feel better, by all means - do it. I just hate to see people with little electrical knowledge thinking this is the way to go, fumbling with how to do it, and then using it as an excuse to omit proper maintenance.

Assuming you have a battery bank with several batteries (more than two six volts), you already have a backup. If one battery does go bad unexpectedly (it shouldn't - you need to do more maintenance) then you pull it out and reconfigure the bank. Four 6V batteries would become a bank of two 6V's, two 12V batteries would become a single 12V, etc.

3. The notion of 'charging one battery while using another' is not useful. There is no such thing as charging a battery while consuming power from it. It is either being charged or discharged. For our type of use, a battery may be under charge while house systems are in use. When that happens (common), the charging source is providing the power to both. For example, a charger from shore power may be sending 20 amps at 13.6 volts down the line. Ten of those amps (for example) may be used by the house and ten of them may get into the battery (ignoring losses and all that).

[TJones]

JD,

I was think along the lines of a two banks system with a switch that would isolate one bank at a time from load so that it could go through the complete charging stages bulk, absorb, float. This would allow the correct charging currants as there would be no load on bank. Would a three stage charger go through all the stages if a load were on the bank?

[JDOnTheGo]

Yes, a good multi-stage charger/converter does.

[ol trunt]

But it will take a long time

[JDOnTheGo]

That would depend on the house load, battery charger output, type, size, and starting SOC of the battery bank, right?!

[ol trunt]

Exactly