Charging batteries in parallel

MM5AHO

MM5AHO

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On another thread (about idling diesels to charge batteries), it was asserted ...

"2. Never charge the batteries on "both" connected. They will NEVER fully charge and the connection process may damage them. The physics of this is explained elsewhere but I can confirm from experience that it is much better to charge one at a time and you will end up with 13 plus volts rather than about 12.7. Your batteries will also last longer."

and about charging 2 batteries in parallel versus one larger capacity one...

"Sorry but you are wrong the physics of charging two batteries in parallel is not the same as one large battery. "

What is the truth of this matter and what principles of physics confirm (or deny) it?
 
I would also be interested in understanding this theory; as I use this all the time and never had a problem.
 
I think there is a grain of truth sometimes, which has soemtimes been taken out of context.

Conventional wisdom is that lead-acid batteries are charged using a constant voltage.
If you parallel the two batteries and apply the float voltage, both will eventually be fully charged.

But, if the two batteries are in any way different, they will take a different length of time to reach full charge, so if you use any cyclic charge, or any voltage other than float, either one battery will be overcharged or the other will be undercharged. Usually minor overcharging within reason does not matter as the electrolyte which gases off can be replaced in a wet battery or re-combines in an AGM battery.
A smart charger may not correctly detect the state of charge of two batteries in parallel. It depends on the exact technique used. This may mean the charger carries on charging at a high rate when the first battery is fully charged, resulting in damage.
If the two batteries always live together, at the same temperature, they will probably share charge and discharge happily, it is different from taking tow random batteries and paralleling them to charge.

At the risk of opening another debate, voltage spikes on the circuit can (in some people's experience/opinion) help to 'desulphate' a used battery, two batteries in parallel probably won't see the same spikes even if the DC voltage is the same, so this effect may be reduced.

In my view, if you have two batteries that start off fully charged, then you use them as say house batteries, they will recharge together OK.
But using them heavily as engine batteries, the impedance will be critical so they won't share the load, and they won't always be at the same temperature. So they could end up at different states of charge. Maybe something like windlass use might be worst? Longer bursts of heavy current.

Also beware that 'lead acid batteries' vary, the lead can be alloyed e.g. with calcium, which shifts the voltage a bit. So, ideally paralleled batteries would all be the same flavour. But that may be a small difference in real-life use.

I think if you float charge e.g. with solar a fair % of the week, there may not be much for the average yot to worry about?
Any other opinions?
Any issues I've missed?
 
The original matter was raised in an engine charging context. Therefore float chargers, cyclic, etc are all not relevant.
This is a case of charging from an alternator running off an engine. So voltage control and rate of charge etc is controlled by the alternators normal system.
 
If you have a "Bank" of batteries in parallel they will have leveled out, equalized. The voltage, for illustrative purposes, will be the same where ever measured across the "Bank". Therefore the alternator sensing will see that voltage and charge accordingly. If you have two separate batteries, with one at 12 volts and one at 12.6 volts. If you then connect them in parallel with the "Both" switch and run the engine to charge they will not have had time to equalize and the alternator will sense the highest voltage and again charge accordingly. Therefore it will stop charging before one of the batteries is fully charged. You could continue to run the engine for a few hours until they equalize and then charge together. You could connect the batteries via the "Both" switch for a few hours before you start the engine and allow them to equalize before charging.
Hope the above helps. It is the non-technical simple description and I am aware that there are many other things that could be include to "Blind you with science"
The above assumes two batteries in all respects equal and normally used on separate systems. i.e. House and Engine.
 
Alex_Blackwood said:
If you have a "Bank" of batteries in parallel they will have leveled out, equalized. The voltage, for illustrative purposes, will be the same where ever measured across the "Bank". Therefore the alternator sensing will see that voltage and charge accordingly. If you have two separate batteries, with one at 12 volts and one at 12.6 volts. If you then connect them in parallel with the "Both" switch and run the engine to charge they will not have had time to equalize and the alternator will sense the highest voltage and again charge accordingly. Therefore it will stop charging before one of the batteries is fully charged. You could continue to run the engine for a few hours until they equalize and then charge together. You could connect the batteries via the "Both" switch for a few hours before you start the engine and allow them to equalize before charging.
Hope the above helps. It is the non-technical simple description and I am aware that there are many other things that could be include to "Blind you with science"
The above assumes two batteries in all respects equal and normally used on separate systems. i.e. House and Engine.
Click to expand...

I'm keen to get the "blinding science" explanation too!
 
MM5AHO said:
I'm keen to get the "blinding science" explanation too!
Click to expand...

You'll wait a long time, because there isn't one. The theory is deeply flawed.

Batteries accept charging current according to their state of charge, so a discharged battery will take more current from the alternator than a charged battery of the same type/size. Batteries in parallel automatically share the alternator's output according to their needs. Alternators are commonly set to limit charge voltage to around 14.4v, and will not "stop charging before one of the batteries is fully charged", as has been alleged.
 
Alex_Blackwood said:
I..... If you have two separate batteries, with one at 12 volts and one at 12.6 volts. If you then connect them in parallel with the "Both" switch and run the engine to charge they will not have had time to equalize and the alternator will sense the highest voltage and again charge accordingly. ..
Click to expand...

If you connect two batteries in parallel, they are at the same voltage, give or take the resistance drop in the connection. Current flows to make the voltage equal.
If the alternator forces a charging voltage, the current into each battery will be different.
Assuming we see an ideal, simple alternator which looks like a voltage source with some impedance.
In the absense of alternator current, current will flow from one to the other.
So if you stop charging early, one battery will charge the other to some extent, not 100% efficient, so some charge will be lost. This will be small though, unless they are left part charged a long time.
Which is never ideal anyway.
 
MM5AHO said:
"Sorry but you are wrong the physics of charging two batteries in parallel is not the same as one large battery. "

What is the truth of this matter and what principles of physics confirm (or deny) it?
Click to expand...

Charging two batteries in parallel is physically and chemically precisely the same as charging the left and right hand sides of the plates of a single battery in parallel.
 
Alex_Blackwood said:
If you have two separate batteries, with one at 12 volts and one at 12.6 volts. If you then connect them in parallel with the "Both" switch and run the engine to charge they will not have had time to equalize and the alternator will sense the highest voltage and again charge accordingly.
Click to expand...

There won't be a "highest voltage" because, as you said, when they are connected in parallel the voltage must be the same across both.
 
The panel seems to have decided that charging in parallel is OK. I would agree. Assuming that both batteries are similar design and both are healthy.
Indeed charging in parallel will take more current out of the alternator for a given regulator voltage so you will get more total charge in for a given length of engine run. Of course current will still tail off as batteries reach full charge. Actually fitting another battery can be a way of increasing charge rate for a nengine run in lieu of fitting a smart charge controller. But only if you charge batteries in parallel. good luck olewill
 
William_H said:
The panel seems to have decided that charging in parallel is OK. I would agree. Assuming that both batteries are similar design and both are healthy.
Indeed charging in parallel will take more current out of the alternator for a given regulator voltage so you will get more total charge in for a given length of engine run. Of course current will still tail off as batteries reach full charge. Actually fitting another battery can be a way of increasing charge rate for a nengine run in lieu of fitting a smart charge controller. But only if you charge batteries in parallel. good luck olewill
Click to expand...
Which is another good explanation of the reason why a large battery bank is beneficial even with a standard engine alternator as the alternator can be made to work harder putting back those precious amps.
 
Well I am here to learn so thanks for the replies. I changed to charging my batteries separately after seeing a technical explanation of why the level of charge would be higher if I did so and it does seem that I end up with a higher state of charge. However I cannot now find this info so will bow to the panels expertise pending practical tests which I will have time to conduct over the next winter.
Meanwhile can anyone confirm that it does 110 ah batteries no harm to be connected in parallel when they are at different states to charge?
 
Halo said:
Meanwhile can anyone confirm that it does 110 ah batteries no harm to be connected in parallel when they are at different states to charge?
Click to expand...
Usually not, but if one battery is duff it will pull current out of the others.
 
I'm going to swim against the tide here and offer an argument against, although my usual practice is to charge in parallel.

Much depends on how you use your batteries, how closely you monitor them and the charge they are getting and what facilities you have for doing that, but it strikes me that at that point when you decide to turn the switch to both, you might not know:

a) Anything about the state of the battery that didn't just start the engine.
b) How well the alternator and charging circuit is working. Yes the red light went out, but I don't believe that proves that a healthy charging current is being delivered.

I don't think I need to spell out the rest. The nightmare scenario is unlikely, but not impossible.
 
Alan ashore said:
I'm going to swim against the tide here and offer an argument against, although my usual practice is to charge in parallel.

Much depends on how you use your batteries, how closely you monitor them and the charge they are getting and what facilities you have for doing that, but it strikes me that at that point when you decide to turn the switch to both, you might not know:

a) Anything about the state of the battery that didn't just start the engine.
b) How well the alternator and charging circuit is working. Yes the red light went out, but I don't believe that proves that a healthy charging current is being delivered.

I don't think I need to spell out the rest. The nightmare scenario is unlikely, but not impossible.
Click to expand...

Fit an Adverc & battery monitor to give a good idea of whats going in & coming out
 
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