Battery Chargers in Parallel for a Lead Acid Battery Bank

In theory both chargers will charge the batteries in parallel, according their charging outputs. Personally I have not seen more lies on electronic equipment as is stated on chargers, especially when it comes to cheap ones.

Each charger will try to deliver current by filling up the difference in voltage between what it supplies and the battery voltage, depending on the battery charge present and the way the current limitation is provided by its design.

It might happen that e.g. the 6 A one reads the supplied voltage of the 30 A one and just finds it enough to not charge or only a little bit. It can just be opposite too.

To be sure of this, you need to know more about your chargers. With a solar charger goes the same thinking. Normally you can put everything in parallel, but measuring it will make you smarter.

Thank you for your comment. It would seem that in fact I should expect some interaction between 2 chargers connected to the same battery at the same time.

The way I have been using the battery system is to use power while charging. This would be similar to a car battery being charged by an car alternator while driving with the head lights on. I have a DC to AC Inverter to supply AC power to the house wiring and find the surge load required to start the deep freezer (about 2000 watts) is easier to get than if I plug in the deep freezer directly to the AC generator (which would then need to lug a bit while it tries to supply the surge load. Once running the deep freezer only needs about 300 watts. Thus I want to use a second charger to supply extra power for house hold use in addition to keeping the batteries charged.

The 6 amp charger recently quit (It looks like some rectifier? SB658 burned out) so I got a newer 25 amp "smart" charger that automatically changes amp rate. I don't know the programming of it but it seems slow to respond to changes in load. I think it is not intended to be used while the batteries are being used.

That sounds about normal for a smart charger. I think their primary function is to keep the manufactures from getting sued if the operator does something stupid with them or the battery they are connected to.

It's only as a last resort that they actually charge something.

I would have suspicions as well that it does not like having a variable load attached to the batteries.

The best battery charges I have found come farm auctions. Big heavy old ones with worn out cables are usually the best! They didn't get to looking that way by not being used a lot!

Some cleanup, new wires and a bit of paint and you usually have a good heavy duty reliable 'dumb charger' that will do what it's supposed to do for years and years.

That's the only type of chargers I own and use. Big heavy reliable dumb ones.

re: big heavy reliable dumb chargers

OK here is my dumb question about these.

It would seem from other comments in this forum thread that I should have one charger and not several small ones hooked up in parallel. If I had one charger then I would probably want one big enough to provide the maximum charge rate I might need. i.e get a big heavy duty one as you suggested.

Since I am limited by the cost of producing my own electricity, I would want to minimize the power consumption. Large chargers usually have switches where you can select the charge rate, e.g. trickle charge (2 amps), 30 amps, 50 amps, 225 amps (to start an engine).

Is the power consumption determined by the power into a charger or the selected power out?

I had one large charger and it consumed 30 amps even though it was on trickle charge and as it seemed to not charge batteries, I stopped using it. Perhaps this was because it got damaged as, yes, I was using it and another 30 amp charger and my 6 amp charger in parallel. This worked for a while, but I am beginning to suspect it has cause some kind of failure in my chargers.

Batteries being used should not be the issue. Batteries being abused might pose a problem. SB658 is probably 2SB658 and a power transistor, here used as series regulator. (your charger might be a CB radio power supply) They tend to go bad when trying to charge low state of charge SOG batteries, because the fuse in the system is too slow, or the transistor cannot handle the load.

The load on a battery, while an inverter is powered, is very strange. Almost no inverters succeed in NOT back feeding a part of their cycle (measure the amps with a AC and DC clamp meter on both settings). I had 20 Amps AC through my battery bank operating on 24 volts, coming from inverters.

Just get a old selenium rectifier charger and work in range of your battery specs.

Yes it is likely a 2SB658 and I will look more closely at the number when I can (it is at a supplier who was trying to find me a replacement part, the right part number might help).

Hmm. Well I did have a bad alternator in my car for about a month and carried around the 6 amp battery charger and an extension cord and plugged it in somewhere to charge the car battery after 4 or 5 car starts before the battery when completely dead.

Hmm I think this qualifies as "trying to charge low state of charge SOG batteries"

The item is in a small glass tube about 1 inch long and 1/4 inch diameter and seems to be a bit black on the inside on one of 2 items inside. This failure may not be related to using it in parallel to charge my house battery bank. The failure was intermittent for a while before it didn't charge at all.

Yes you can parallel chargers on battery bank systems to speed up the fast charging cycle.

If you need to do a lot of regular charging from a fuel based power source making your own charging system from a common small gas engine and an automotive alternator may be worth thinking about.

A small 3 - 4 hp gas engine will easily drive a 12 volt 100 amp automotive alternator.

Thank you for your suggestion about using a small gas engine and an automotive alternator. I haven't done the math, but it should be more efficient to convert mechanical energy directly to DC rather than to convert mechanical energy to AC and then to transform and rectify AC to DC.

If I did that, only generated DC, then I would need to run all AC items off a inverter connected to a battery bank. I haven't done the math here either but it would seem it would be better to run AC items directly off the AC generator that then to run an engine to run an alternator to charge a battery bank to power an inverter to get AC to run AC items. On the other hand, if my system plan is to run all electrical off a battery bank, then I would not need to shut things off when I stop or restart a generator. This would then make my variable the means of charging:

• gas powered AC generator and charger
• gas powered DC generator (alternator)
• solar panel to charge batteries with DC
• wind mill to power alternator to produce DC

Which then leads back to the original question about multiple chargers on a battery. Can I use all of these at the same time?

The other comments to this thread seems to suggest that it would be best to select a single charger to charge a battery or a battery bank and then switch the inverter between battery banks.

All chargers, whether of the dumb transformer/diode type or the intelligent type will react adversely to having another charging source in parallel with them.

The dumb type simply react to the opposing voltage of the battery getting closer to their own constant output voltage to limit their current output. So it follows that if another charger is outputting a higher voltage than the battery, then the first charger's output will be limited by that, and similarly, the second charger's output will also be limited by the first one.

Intelligent chargers work a bit differently.

Their bulk/boost mode supplies a set current until the battery terminal voltage reaches 14.7v (for a flooded 12v battery), and then goes to a constant voltage absorption mode. It determines the battery terminal voltage by periodically halting the charge current and testing the voltage. Unless both chargers happened to be in perfect sync. so that they both went to test at the same instant, which is highly unlikely, then the one that is testing will detect the output of the other charger rather than true battery voltage. This will cause it to go to absorption prematurely with a resultant reduction in current.

Next thing to happen is that (depending on the state of charge of the battery and the relative constant voltage set points of the two chargers)

1. When the other charger goes into its test cycle, the first charger senses a sudden drop in voltage and returns to boost, this then confuses the charger that is testing, which then goes to absorb. Or

2. The second charger goes to test and detects the constant absorption voltage of the first charger and also goes to absorption.

Either way, the charge time can be dramatically increased.

Better to only use the one charger.

Thank you for elaborating on post #1 and importing the smart charger that with all the smartness incorporated, will not be able to interpret the battery voltage from the charger that is also connected.

Before making any additional statement, I wait for more info from the OP. TCM's dumb selenium chargers will do the job without complaints and the smart ones will probably be too smart to even start when dumby is working.

Put a variable resistor in series with each charger, small value hi-Watts for big charger, high value lo-Watts for small charger. Adjust each resistor so the chargers give proportionate charges according to their charging capacity. Rough, but should stop chargers confusing one another.

Firstly, you need to know how your chargers are designed, are there blocking diodes in the cables to the battery, to stop any back feeding. If you do not know this at least 101%, then you cannot parallel chargers.....

Also, some intelligent chargers, even with the diodes, may react adversely (as already mentioned elsewhere here) and could possibly cause a fire in one or both of the units.

Parallel charging of batteries is also a really bad idea and should never be done. 80% of the time you will get away with it, but again, battery damage/overheating is still possible...

There are arguments for identical batteries (bought on the same day, from the same lot), used to feed a load or charge in parallel, e.g. always in parallel, but why did someone buy two batteries when one big one would have been better...????? ..and probably cheaper!!!!

If you need 24 or even 36 volts, then buy a good charger (identical ones with charge cutoff) for each 12 volt battery, switch them on and off together....

Car type batteries should never be used for Solar work, except maybe for short term experimenting, as car batteries are designed to deliver "Cranking" amps and then be recharged. For Solar and other uses there are deep discharge and leisure batteries that cannot deliver the cranking amps, but accept being "deeper" discharged with little or no damage.

The perfect charger will actually charge each cell separately in each battery, bit that is not easy to achieve, as EVERY cell in ANY Battery of any type is an individual.....

But charging each battery separately will at least extend the lifespan of those batteries substantially.....

High charging amps will not give the same capacity as lower......

Search around on the web for more details, but avoid charging voltages (12 volt Batteries) above 13.4 volts will promote gassing, water loss and shorter life spans, as will high charging amps at any voltage.

For Battery long life stay below C/10 amps and 13.4 volts....and if a car battery type, not below 12.6 volts, plus recharge immediately when below 12.6 volts.....not easy but thats why car batteries are cheaper!

There is possibly a case where an intelligent charger may have more intelligence than some posters here.....

For those people, when designing a good charger there is ALWAYS at least one blocking diode in one cable, usually the positive connection. Sometimes two.

Not having the wiring of every charger in the world, I had to make sure that it would be safe....I have never ever seen or built a charger without at least one, usually only one......

Secondly, when inserting diodes, at design time, you up the voltage to account for the losses.....DUUUUHHHHHH!!!!!! Simple enough!!!

Most chargers that I have built or looked at, actually could supply voltage at far too high a level to be safe for the long term battery life, that is so that as the battery voltage rises, you still have a large enough difference to allow current to flow. Which is why voltage sensing needs to be built in for safety. Having 2 extra volt drops of 0.6 volts can be considered immaterial......

I have only ever see a single charger, that had no intelligence and was set to only charge to around 13.2 volts. It was in my caravan when I bought it and the special transformer went wrong shortly afterward and it got booted out!!

There is also the possible error condition where someone connects a battery with reversed/incorrect polarity. I have seen it done many times by idiots and I have repaired the damage as well.....usually a fuse...

Luckily there is usually a fuse across the + to -, with a diode that allows current to flow through the fuse only when wrongly connected.....thats when the other "Blocking" diode(s) also prevent damage to other possibly sensitive parts of the charger.....

Here is a simple rendition of how many chargers are connected to a battery. I use a similar version myself:-

I just saw that the diode is inserted the wrong way round, sorry!!! Quick draw!!

Hmm...Is this an acceptable type of conduct on this forum?

A good indication of a man's intelligence is his propensity to denigrate others in an attempt to make his point.

Very poor form Andy.

The poor form was simply started by yourself, you opened the door, I simply walked through. Re-read your post.....you simply put egg all over your own face in a rude reply to me, where you THOUGHT you knew what you were talking about....but didn't....

I was simply reminded of the saying about "about people in glass houses........". I am sure you know the rest!!

You did not think before posting and your post demonstrated a singular lack of feeling for both myself and for the technical side of the subject, a serious lack of knowledge and a complete lack of forethought......what were you expecting for a reply from me? "Thanks Buddy!".....DUUUUHHHHHHH! I think not.....

An important thing to learn in this life is "look before you leap" and when you haven't done that by your age (25?), then you will get a few hard knocks back.....good manners are also important, but with modern families, they are getting fewer and farther between, sadly.

Its also important to recognize your own failings and not try to blame someone else first.......nor simply "pick" on them....

By the way, I was not/am not angry at you. You are simply one of many such unfeeling people around today.....I have simply got used to it, sadly.

One last thought, "air" your ideas in a more friendly and a less strident manner and you will find that when they are wrong, the "correction" will not be written so hard as mine was......but do remember that I always give at least as good as I got, sometimes more........both good and bad.....I simply try to be fair....

I wish you a pleasant weekend in spite of these "home-truths" that I have aired.....I hope that we can "spar" on the great pages of CR4 again soon.

You have already displayed one of those languages that you mention in your signature that you can make of a fool of yourself in...What are the other two?

Why should I tell you? Is there a "friendly" reason for the question? Or some dark thoughts?

Furthermore, I actually speak 4 languages. That was just a quote that I accurately typed in, it was not referring to me. I like the book so much I quoted from it. You should read it someday....

Most people find it funny......most!!

I can also translate several other languages into technical English relatively accurately I am told, as well.....if required.....

But enough about me, how many languages are you fluent in?

You need to provide a list of specifications for each of the 4 units you intend to use! Information like power capacity, output voltage, etc. for each device... having this information will help guide the proper way on how they can be connected together!

1) solar charger, 2) 6 Amp charger, 3) 30 Amp charger, and 4) gas powered generator.

Otherwise, it will just be a guessing game for everybody and you may end up with some defective unit(s)!

Good luck!