Charge battery bank by splitting it up with multiple chargers?

barclaybrown · February 2014

I need to know if I'm crazy. Say I have a bank of six large batteries (AGM, 250AH, 12v) all in parallel. That's a total of 1500AH. I have a 3-stage charger that is rated to charge at up to 150A DC. Now say the batteries are discharged by 40%. That means I need to put pack 600AH.

Of course, charge current varies during the charging process, but say it averages 100A. This recharge will take about 6 hours (with the bank getting to 80% more quickly and the last 20% taking longer).

Now, instead of charging them while they are all connected together, let's say I split up the discharged batteries into three pairs of batteries, each pair containing two batteries in parallel, and I put a separate charger on each pair. Say each charger is rated at the same max 150A. (The batteries can tolerate inrush charge current of at least 500A, so no worries about that.)

The question is, will I charge the batteries up faster?

To reduce it to math, what charge current will flow in each of the recharging pairs? Will it be 100A average, the same as the current that flowed when charging the entire set of six batteries in parallel? Or will it be 100/3 = 33A? If the latter, then of course splitting the batteries up and adding additional chargers will have no benefit. If the former, then I charge the entire set in 1/3 the time!

I think the heart of the question is, how do battery resistances during charging, combine in parallel? Do they combine just like resistors, or some other way because they are also "generating" voltage in the circuit?

Thoughts?

(Oh, and so people don't bother telling me, yes, I know the total energy to charge the batteries will be the same in each case. In this application, I'm looking to save running time on a generator by using more of the available power to supply multiple battery chargers.)

Cariboocoot · February 2014

Re: Charge battery bank by splitting it up with multiple chargers?

Welcome to the forum.

You're saying you have a 1500 Amp hour 12 Volt battery bank made up of six batteries in parallel?

Are you familiar with our oft-repeated chant of going higher in Voltage and staying away from more than two parallel strings of batteries? Would you like to consider going up to, say, 48 Volts? It would make things so much easier and more efficient for you.

That aside for the moment we have the issue of trying to recharge that massive bank. A 150 Amp charger certainly ought to do it, although I don't want to be around when it's being done.

Charging is not linear. You don't apply 'X' Amps for 'Y' hours to replace 'XY' Amp hours. The current will taper off as the batteries charge so it ends up taking longer to 'put back' that power than you might think. Usually about 20% longer.

If batteries were perfect you could apply 'X' Amps and each of the six parallel batteries would receive 1/6X Amps. But they aren't perfect so there will be some variation in current between them. This is where the multiple parallels cause problem, because every piece of wire and connection involved changes the resistance for any given battery in the bank.

As you probably know it would be desirable to have peak current of 25 Amps per battery, although you can have a bit less or a bit more. That would be the full 150 Amps of your charger. If you divide them in two then you have the potential of (150/3) 50 Amps max per battery which may be more than they should have. In this case since they are AGM's they'd probably take it.

There would not be any time advantage to doing this, but it may be better for the batteries because of the lower amount of resistance difference between fewer paralleled batteries. AGM's have lower resistance than FLA's so it matters even more.

Was that any help?

BB. · February 2014

Re: Charge battery bank by splitting it up with multiple chargers?

Probably not... More or less, flooded cell lead acid batteries "optimally" recharge from discharged to ~80-90% (most current goes to charging, not heat/gassing). Above ~80-90% more energy is converted to heat/gassing (at ~100%, probably ~1/2 of the energy goes to heat and the other 1/2 goes to gassing).

So, your best bet is to dump as much current as you can into the whole bank. That portion will simply be AH to refill / Amps available for charging.

Once the battery bank gets above ~85% SOC, you are looking more at "time" while holding a fixed charging voltage (absorb). That will probably run from 2 to 6 hours--The deeper the discharge, the longer the time frame.

Also, battery design has influence too... Batteries designed for industrial use (forklift, sweepers, etc.) tend to want high voltage held for many hours--Something that pure solar power systems are not great at.

Installing a tracking array or a "virtual tracker" (some panels facing South East, others facing South West) can give you more hours of charging per day--But when winter hits, your available sunlight can be much less.

There is a method you can use with Lead Acid Batteries... Basically cycle them from 50-80% daily state of charge 6 days a week and fully charge (over 90% SOC) on a Sunday (basically every 6-10 days or so). Some vendors have said you only need to recharge >90% once every 4 weeks). This is supposed to be very efficient and good for the batteries (gassing/equalization, charging to 100% SOC is actually "hard" on lead acid batteries).

Another is to use AGM--They can take high charging currents better--But you need to review the manuals for specific battery types... They are not all the same (nor are all flooded cell the same).

Sometimes it is difficult to get this type of information/charging details from the Manufacturer.

-Bill

barclaybrown · February 2014

Re: Charge battery bank by splitting it up with multiple chargers?

It's a 12volt bank in an RV with a 12volt inverter. Very standard set up. I understand about taper. I was using an average amp charge as an example. My central question is about using one charger vs. three.

Cariboocoot · February 2014

Re: Charge battery bank by splitting it up with multiple chargers?

barclaybrown wrote: »

It's a 12volt bank in an RV with a 12volt inverter. Very standard set up. I understand about taper. I was using an average amp charge as an example. My central question is about using one charger vs. three.

The fewer batteries in parallel you apply the charge current to the more even the and accurate the charging will be. Adding chargers to the parallel sets won't help, taking batteries away will.

vtmaps · February 2014

Re: Charge battery bank by splitting it up with multiple chargers?

barclaybrown wrote: »

let's say I split up the discharged batteries into three pairs of batteries, each pair containing two batteries in parallel, and I put a separate charger on each pair. Say each charger is rated at the same max 150A. (The batteries can tolerate inrush charge current of at least 500A, so no worries about that.)

The question is, will I charge the batteries up faster?

Yes. They will charge faster because each battery is getting three times the current. However, it is only the bulk phase of charging that will go faster... the absorb (tapering) time will not be reduced.

btw, six parallel batteries is very poor design, and especially so with AGMs. short discussion here.

--vtMaps

barclaybrown · February 2014

Re: Charge battery bank by splitting it up with multiple chargers?

vtmaps wrote: »

Yes. They will charge faster because each battery is getting three times the current. However, it is only the bulk phase of charging that will go faster... the absorb (tapering) time will not be reduced.

btw, six parallel batteries is very poor design, and especially so with AGMs. short discussion here.

--vtMaps

Pretty much an unavoidable design when each battery has the same voltage as the overall bank needs to be. You're the first to directly answer my question. Perhaps no one has ever thought about this or tried it. If anyone is in an experimenting mood and has an easy set up to try this, here's what I would LOVE to know:

Take two batteries and put them in parallel. Discharge them to some reasonable level, say 25% discharged. Attach a charger and measure the current flowing into the batteries (they will be bulk charging). Then, disconnect one of the batteries, leaving the other connected to the charger. Does the charge current change, and by how much?

If the charging current doesn't change or changes only a little (less than 50%) then my scheme of splitting the bank and using multiple chargers will have an advantage in charging time. If the current drops by half, then the scheme will have no benefit, and the batteries will take the same amount of time to charge whether they are all connected together or split up and on separate chargers.

Anyone have a set up where this is easy to try?

Barclay

Cariboocoot · February 2014

Re: Charge battery bank by splitting it up with multiple chargers?

barclaybrown wrote: »

Pretty much an unavoidable design when each battery has the same voltage as the overall bank needs to be. You're the first to directly answer my question. Perhaps no one has ever thought about this or tried it. If anyone is in an experimenting mood and has an easy set up to try this, here's what I would LOVE to know:

Take two batteries and put them in parallel. Discharge them to some reasonable level, say 25% discharged. Attach a charger and measure the current flowing into the batteries (they will be bulk charging). Then, disconnect one of the batteries, leaving the other connected to the charger. Does the charge current change, and by how much?

If the charging current doesn't change or changes only a little (less than 50%) then my scheme of splitting the bank and using multiple chargers will have an advantage in charging time. If the current drops by half, then the scheme will have no benefit, and the batteries will take the same amount of time to charge whether they are all connected together or split up and on separate chargers.

Anyone have a set up where this is easy to try?

Barclay

Well of course it changes.
If the two batteries are exactly the same and draw 'X' Amps when in parallel if you disconnect one it will draw '0.5X' Amps.

This is not the same as if you have a charger capable of 'Y' Amps but the batteries are only drawing '0.5Y' Amps and you add another charger capable of 'Y' Amps. In that instance both chargers will supply '0.25Y' Amps.

And that is not the same as a battery bank drawing 'Y' Amps from a charger capable of 'Y' Amps and you add another charger capable of 'Y' Amps in which case the batteries may draw >'Y' Amps since '2Y' Amps is now available.

One of the great bugaboos of paralleling batteries is trying to keep the 'R' value equal in all parallel paths even with "identical" batteries. Each wire and each connection is additional resistance and try as you might the two paths will never be identical, mainly because even "identical" batteries aren't.

This parallel problem is most prevalent with 12 Volt systems where the resistance differences have a greater affect, and of course the more parallel paths the worse the condition becomes (greater chances of variation in resistance).

vtmaps · February 2014

Re: Charge battery bank by splitting it up with multiple chargers?

barclaybrown wrote: »

Take two batteries and put them in parallel. Discharge them to some reasonable level, say 25% discharged. Attach a charger and measure the current flowing into the batteries (they will be bulk charging). Then, disconnect one of the batteries, leaving the other connected to the charger. Does the charge current change, and by how much?

It depends on the charger... if your charger has unlimited power, then the charging current to each battery will be limited by the resistance of the battery and the wiring. When you disconnect one battery the charging current will be cut in half. Do NOT try and test this... you will exceed the safe charging currents for the batteries.

If the charger's limits are less than one battery's safe charging limits, then disconnecting one battery during bulk charge will not have much effect on the charging current... the remaining battery will accept the charge. There will be a slight drop in the charging current because the remaining circuit (battery & wiring) will have a higher resistance than the two battery circuit.

Once you are in absorb charge, disconnecting one battery will cut the current in half.

--vtMaps

Cariboocoot · February 2014

Re: Charge battery bank by splitting it up with multiple chargers?

If the charging current for the two is less than the charging source can supply then disconnecting one battery will drop the current in half (providing both batteries truly are equal).
If the charging current for two could be greater than the charging source can supply then the source will be at its maximum and disconnecting one battery may result in the total current going down but the current for the one battery going up.

Example:
20 Amps put to two batteries from a source capable of 30 Amps. 10 Amps goes to each battery. Remove one battery; current drops to 10 Amps. No difference in charge rate.
20 Amps put to two batteries from a source capable of 20 Amps maximum. 10 Amps goes to each battery, but they could accept 5 Amps more current. Remove one battery; current drops to 15 Amps but all 15 Amps goes to one battery. Charge rate for the one battery increases, but over-all it will take the same time to do both batteries.

This is not exact because things are never exactly equal. One of the things we constantly warn about.

MGar · July 2014

Re: Charge battery bank by splitting it up with multiple chargers?

barclaybrown wrote: »

Pretty much an unavoidable design when each battery has the same voltage as the overall bank needs to be. You're the first to directly answer my question. Perhaps no one has ever thought about this or tried it. If anyone is in an experimenting mood and has an easy set up to try this, here's what I would LOVE to know:

Take two batteries and put them in parallel. Discharge them to some reasonable level, say 25% discharged. Attach a charger and measure the current flowing into the batteries (they will be bulk charging). Then, disconnect one of the batteries, leaving the other connected to the charger. Does the charge current change, and by how much?

If the charging current doesn't change or changes only a little (less than 50%) then my scheme of splitting the bank and using multiple chargers will have an advantage in charging time. If the current drops by half, then the scheme will have no benefit, and the batteries will take the same amount of time to charge whether they are all connected together or split up and on separate chargers.

Anyone have a set up where this is easy to try?

Barclay

OK I did that in 2011
I had wanted to simply power a desktop PC from solar power. ~230 watts
11 100 watt panels 12 volt mono's. (~1100 watts)
Inverter 3000w 12 volt DC to 120 AC pure sine.
And a battery ohm reader.

I started with two 12 volt marine battery's, 2 hours later the battery almost at 12 v. Got 2 more for 4 marine battery's
With 4 marine battery's it ran PC ok but I was not happy with the SOC.

Added pair's of golf cart battery's until I was happy with SOC or DOD. I wanted to use only ~10% per day.
there where 16 6 volt golf cart 232 amp hours and the 4 marine 100 Ah battery's. (20 battery's) ~4k amp.

I had I think about 6 MPPT chargers all 12 volt it worked OK. I think I had 2 chargers for the marine's and 4 for the golf cart's.

Yes you can charge the battery with the isolated solar charge controller, but some pairs(6 volt x2) will/may not be fully charged per day. Then you will have to monitor them.
You will find that some 12 volt (6volt x2) will start to work harder than other pair, that's what I saw. After many cloudy day's the will be out of balance. But a few sunny days fix's that. It would/could be worst with a singe charger as the character is magnified?

Well I learn a lesson you need about 5 times the battery than you first think, due to cloud, and battery life. Energy in energy out, the slower it happens the better...
BTW: those battery's are still working away. @ 48 volts, and yes have a 12 volt isolated charger balance. But it's now AC units @3 amps ea.

Charge battery bank by splitting it up with multiple chargers?

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