Is it OK to charge batteries in series, individually?

[NVergunst]

I will have a string of batteries (72 batteries in series, ~245VDC) that require CC-CV charging (LiFePO4 chemistry). What I would like to do is make individual "little-packs" out of the "big-pack" and modularize it. I feel this may be a better solution albeit much more costly. I am envisioning each cell of each little-pack having a small FET to discharge the individual cell, and each little-pack having its own 110vac to 220vac to N*3.4vdc CC-CV charger. Right now I am thinking the little-packs will be 12-cells.

But my main question is, am I missing something or can I just connect like this:

**broken link removed**

So each of the chargers maintains its own "little-pack" and uses an isolated interface to deliver the charge like a flyback smps or something. Can you isolate the little-packs and charge them like that OK, or will something bad happen here?

 

[NVergunst]

I did find a battery charger PDF with the following diagram in it, suggesting it can work.

**broken link removed**

 

[FvM]

As you already mentioned, your battery type most likely requires a charge balancing circuit. But this point hasn't to do with the charger circuit. I think the only important property is sufficient output voltage isolation. It shouldn't be a problem for a SMPS charger, but I don't know if your intended device has it.

 

[NVergunst]

If I use a flyback or something like that with a full transformer for the charging system, that should be sufficient isolation.

I will have high side balancing so as I charge, I will also discharge some cells within the little pack as the whole little pack charges and eventually stop charging some little packs while others are charging to the target voltage. While in use, no active balancing will be done, it will just shut off when the lowest cell is lower then the threshold. That bit doesn't bother me, but I was having some reservations on what would happen between the + and - sides of 2 isolated chargers when they are both pumping in current. But it seems like it should work.

 

[FvM]

but I was having some reservations on what would happen between the + and - sides of 2 isolated chargers when they are both pumping in current. But it seems like it should work.

Yes. surely. As already, mentioned sufficient isolation against VAC input and possibly connected protective earth is the only essential. But what's the expected benefit of the modular charger concept? Most applications with HV batteries, e.g. large UPS systems in a hospital or a public building, or e-cars, have a high voltage charger. A failure of single modular charger would still stop battery operation. But you have at least a smaller replacement part.

 

[NVergunst]

Yes. surely. As already, mentioned sufficient isolation against VAC input and possibly connected protective earth is the only essential. But what's the expected benefit of the modular charger concept? Most applications with HV batteries, e.g. large UPS systems in a hospital or a public building, or e-cars, have a high voltage charger. A failure of single modular charger would still stop battery operation. But you have at least a smaller replacement part.

A single modular charger can be made smaller with smaller current components than one gigantic charger. Also I am thinking that if I sequence the phases of the smps in such a way, I wont be pulsing the vac line as hard. Also I am trying to make things modular, so that I only have $100 of batteries to replace if one dies versus $10k. My other reasoning is to do the project in stages, and I don't like designing for now just to design differently later.

Another reason is that I am not confident enough with AC to design a boost circuit for it. If I charge the "big-pack" all at once, I will need something above 245VDC, so my AC input would need to be more than doubled for 110vac and boosted slightly for 220vac. I have seen people boost the AC to 400-ish then convert it to the large DC they need. I find that to be a good solution, but like I said I don't feel comfortable enough with my AC skills for that.