Author Topic: Charging batteries connected in series-parallel  (Read 228 times)

Offline MRMILSTAR

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Charging batteries connected in series-parallel
« on: July 02, 2021, 04:26:17 PM »
I have a project that requires a 36 volt high current DC supply. My plan is to use six 12 volt car batteries connected in a series-parallel arrangement. There will be two banks of batteries. Each bank will consist of 3 batteries in series. The 2 banks will then be connected in parallel. My question involves charging. I have a commercial 36 volt battery charger. Can I just connect the charger to the 2 parallel banks? Will all of the batteries be properly charged by the single charger?
Steve White
Cedar Rapids, Iowa
Retired electrical engineer

Offline Twospoons

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Re: Charging batteries connected in series-parallel
« Reply #1 on: July 02, 2021, 11:09:10 PM »
If the batteries are all the same capacity you should be OK.  But you should definitely keep a close eye on the electrolyte levels and terminal voltage of each battery because it is likely there will be a weak one in the group that will get overcharged / overdischarged.  As I recall lead acid batteries are reasonably tolerant over overcharge - they just boil off electrolyte - but over discharge will lead to sulfation and further loss of capacity.

I would be inclined to connect them like a ladder -  connecting two batteries in parallel, then connecting the three pairs in series. Helps average out the capacity at each level.

Offline Da_Stier

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Re: Charging batteries connected in series-parallel
« Reply #2 on: July 03, 2021, 07:59:59 PM »
Hi,

as Twospoons said, it is possible in theory if you assume all your batteries are perfectly equal.
In practice you want to take a close look at the voltages.
It might also work a couple of times but they will propably drift apart further with each cycle.
To do it the right way you should build some kind of BMS (battery management system) with cell balancing.
The easiest thing would by far be to simple switch in a resistor, if one cell is full to keep it balanced.


Greetings,
Michael

Offline davekni

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Re: Charging batteries connected in series-parallel
« Reply #3 on: July 03, 2021, 08:14:08 PM »
Car batteries are 6-cell and have no active cell balancing (no BMS or other electronics).  They balance by slight over-charging.  I agree with Twospoons that over-discharge is the primary thing to avoid.  That's the same as for normal car battery use.  Complete discharging (such as forgetting to turn of lights in an older car or months of storage) is hard on a single car battery too.
David Knierim

Offline Da_Stier

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Re: Charging batteries connected in series-parallel
« Reply #4 on: July 03, 2021, 08:27:00 PM »
Car batteries are 6-cell and have no active cell balancing (no BMS or other electronics).  They balance by slight over-charging. 

That's an excellent point which I never really thought about.
However it makes total sense.
I guess I was to focussed on Lithium batteries and their problems.
Thanks for making that clear.  :)


Greetings,
Michael

High Voltage Forum

Re: Charging batteries connected in series-parallel
« Reply #4 on: July 03, 2021, 08:27:00 PM »

 


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