Balancing 12 volt battery banks

[toddbailey]

How do I balance 4 12 volt battery packs? these are sealed LiFePO4 batteries in a 4s config with their own internal BMS, similar to a drop in 12 volt replacement battery
I'm using a charger designed for 54 volt LiFe batteries but it doesn't offer balancing electronics like a separate 14s BMS would offer.
Right now I use a DVM and a 12 volt LiFe charger and charge the lowest bat to equal the others. But what would be desired is having a equalizer/balancer rated for 4 12 volt batteries instead of 4 3.2 volt cells

 

[john61ct]

Balancing multi-cell units against each other is of limited use, you just want them well within a half volt before combining.

Actual balancing is at the per-1S group or per-cell level within those 4S units

and would usually be the job of their BMS.

Do you get to see those cell/group level voltages, on a display or your phone or?

Or do you have access to the balance leads for a voltmeter, or additional balancer, or hobby charger, or adjustable PSU?

Many ways to skin that cat. . .

 

[Balmorhea]

Charge them in parallel with a 12V charger. (Only connect them in parallel if they’re very close in voltage already.).

 

[John in CR]

First, like Balmorhea said, you need to separate the series connections and charge the 4 batteries in parallel with you 12V charger. Be sure leave it charging for long enough for the BMS's in each battery to do it's job, so your starting point is 4 fully charged and balanced 12V batteries.

Then you can connect them in series and use them with the understanding that the total pack is only as good as its weakest battery, and that weakest 12V battery is only as good as its weakest cell (or weakest string of parallel cells).

During use you will be drawing exactly the same amp-hours out of each pack, so when you charge them back up they should be in balance despite being out of balance due to differences in capacity while partially charged. As long as each 12V battery is healthy and each BMS is working, you should only rarely need to do the disconnect and charge in parallel procedure.

I worry, however, about whether you have the correct charger to charge the whole pack in series. What do you mean by 54V LiFe charger? What is the voltage at full charge? It needs to be high enough for each battery's BMS to do it's charge. ie 4 times whatever your 12V LiFe charger cutoff point is, which is a 16s LiFe charger, about 58V.

If you're charging to the right total voltage and the 4 batteries are quickly ending up out of whack at full charge, then there's something wrong with one of the 12V batteries.

The only convenient way to balance charge every time without hacking into the 12V batteries and connecting a 16s LiFe BMS is to charge the pack with 4 12V LiFe chargers, one connected to each 12V battery. If using RC type chargers, which wouldn't be very convenient anyway with all the button pushing needed, you'd have to connect each to it's own power supply. Otherwise you'd have to disconnect the series connections of the 4 batteries, so you don't blow the chargers.

 

[john61ct]

There are many dozens of hobby chargers designed to remember your configured profiles that do not require much mucking about, just plug in and go.

Charging the 4S sub-packs in parallel is fine without balancing for normal cycling, so long as your stop point is well below the specd max

3.45Vpc for example will get to within 95% full and avoid getting up into the shoulder area where imbalance conditions arise.

But with access to the balance wires, properly fused paralleling boards will safely allow fully automated balanced charging every cycle using inexpensive hobby chargers

or sophisticated ones that will also test for increases in resistance over time, discharge or charge to a given storage SoC, automate capacity testing etc

These become very rare and pricey once you get over 8S, but choices ae egion if you retain th ability to deal with sub-packs in 12V units.

But whatever you do, you must have access to the per-cell balance leads in order to balance effectively without reducing pack longevity through overcharging

If nothing else just to get a per-cell/group voltage reading.

______

I'm using a charger designed for 54 volt LiFe batteries

Maybe you mean 48V nominal, 4x those 4S sub-packs, aka 16S?

55V would be a good normal-cycling termination voltage for that layout, but as you say not when balancing is required.

If that is required frequently, I would not use a balancer that required holding a 56+V charge for any long period of time.

> but it doesn't offer balancing electronics like a separate 14s BMS would offer.

Where are you getting 4S? That appears to be irrelevant to your use case.

 

[John in CR]

john61ct,

Why are you insisting that the OP abandon the BMS's aready in the the 4 12V batteries? Those take care of the cell level balancing. While yes he could charge them in parallel every time but the disconnecting and reconnecting every time is inconvenient and introduces significant potential for plasma displays and varporized terminal. For me anything more than connecting one charge plug and turning on the charger is too much.

 

[john61ct]

I do not know if the OP's existing BMS will do the balancing well or in fact at all.

I was not specifically saying they couldn't, and have asked several questions along those lines that are waiting for OP to answer.

The question actually asked was an entirely different moot one anyway.

Meantime I was correcting some false or at least overly general statements along the way.