12v batteries - Allowable voltage deviation prior to balancing

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schmit01 asked Jan 8, '23

12v batteries - Allowable voltage deviation prior to balancing

Hey All

Learning here so go easy.

When purchasing 2x 12v Lithium batteries for a 24v phoenix inverter, what is an allowable voltage deviation between the batteries when fully charged and settled prior to balancing them?

Both accept the full AH rating charge and a little more when charged from 100% DOD but one settles at 13.6v and the other at 13.1v. I would think this will cause issues when charging in a 24v arrangement?

Also when I put a load on the 13.1v battery it jumps up to 13.35v, then back down to 13.1v when disconnected. Making me think it is faulty and enough for me to return the battery for a replacement?

Any thoughts would be appreciated.

Lithium Battery

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Answer 1 · 2023-01-08T09:49:47Z

kevgermany answered · Jan 08, 2023 at 09:49 AM

Could be an internal imbalance between the cells in the battery. I assume these are batteries with a built in BMS. If you can, try leaving them on low charge rate for longer. This will allow the BMS to complete internal balancing and the overall voltage will rise. Might take longer than you expect.

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schmit01 commented · Jan 08, 2023 at 10:57 AM

Yes, they have an internal BMS. I suppose the question is, is the 13.1v vs the 13.6 outside of the allowable voltage range and just return the battery. I shouldn't be havening to resolve this anyway.

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Answer 2 · 2023-01-08T13:34:08Z

Alexandra answered · Jan 08, 2023 at 01:34 PM

@Schmit01

Usually they would need to be both fully charged as individuals (when new) and balanced before putting them into the 24v system.

But still you will need a battery balancer. (Neat demo at the bottom) This is acommon situation in a built battery bank. Different internal resistances and a few other factors affect them, especially if they don't have comms between them.

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schmit01 commented · Jan 08, 2023 at 05:06 PM

I get that and is what I am trying to do, but that is already 0.5v difference between batteries to start with. The one battery once fully charged will be around 14.1v then slowly settle down to around 13.65v. The other battery regardless just reads 13.15v. right after I pull the charger off but still accepts the full AH charge rate based off my Victron smart charger.

I just tried to balance the batteries in parallel and then also top them up in parallel, they wont balance. One is now sitting at 13.22v and the other 13.65v. I've checked and there is no flow of power between the batteries.

Yes I have the battery balancer, but that only works when charging and starts at 27.3v.
So yes the battery balancer will bring down the voltage of the higher battery, but that's 0.43v difference ever charge cycle, not every few months.

So I am just going to try take the battery back for a replacement.

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Alexandra ♦ schmit01 commented · Jan 08, 2023 at 06:02 PM

@Schmit01

By fully charged are you referring to the dropped down to float voltage?

Most lithium 12v charge over 14v in bulk. What is the target or bulk/absorption voltage meant to be?

Like you said to trigger the balancer it must be over 13.6v both of them.

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kevgermany ♦♦ schmit01 commented · Jan 08, 2023 at 08:08 PM

You might also find it better to lower the charge rate. With a decent BMS, if one cell in a battery is over voltage the BMS will stop the charge. At this point you need to switch to less aggressive charging. Guessing this is where @Alexandra is heading.

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