LiFePO4 battery packs in series (Discharging and Charging)

Hi everyone,

I'm running a bit of an odd configuration for my battery setup currently and I'm hoping to run an even odder one!

What I currently have is 6 tenergy 12V 15AH packs (each with their own seperate 4S BMS), I connect 3 of these up in series at a time for discharging on my 36V controller/motor setup. I however charge them individually with three channels of the Q200 quad channel skyRC charger.

I haven't found much information on the disadvantages/dangers of doing things this way but I have heard a few things I'd like to verify with someone with more knowledge on the subject!

- Discharging the packs in series like I'm doing risks burning out a BMS on one of the packs if one of the BMSs cuts out due to it's LVC, and a BMS "sees" the full 36V due to this. (and diodes across each pack can prevent this from happening

- Charging the packs in series is not safe (For a reason I don't know or understand, I assume it has to do with cell balancing)

So, to complicate matters further, I am now considering a solar system to hook up on my bike trailer, extending 6 ft behind my bike. My thought, due to not being able to charge pack of packs with 36V directly, was to get three panels, 3 12V charge controllers from Genasun and hook each panel/controller up to one of the three battery packs (while they are still connected in series). My worry though is that the solar panels may not all output the same current (it should be close considering all the panels would be pointing in the same direction), and my packs will be unevenly charged so that when they're being simultaneously discharged and charged one pack might run out before the others.

Does anyone have any advice regarding this setup? One thought I had would be to charge up one 3 pack of packs while discharging the other and alternate them (This would avoid the issue with packs running out before others, since I could allow the off duty packs all charge up to full before putting a load on them)

Thanks!

Liam

That sounds like a good solution to me. (bearing in mind I have an agriculture degree, not EE)

While you run, it should not matter too much if they are at moderately different voltages,, Provided you are not getting low enough to have one bms shut off the packs while in series. I don't mean one at 11v and the others at 12.5v, I mean one at 13, the other at 12.9, better still 12.95v. that kind of difference.

When parked, and getting full, each section should reach full charge eventually, and balance up at full 3.5v per cell or more. (since lifepo4 tends to balance at more than 3.5v.)

I'd want three voltmeters, one for each 12v section on the handlebar display. That way if you did get one pack seriously lower than the others, you see that, and can stop before you hit lvc.

The simple solution though, is very obvious. Get a 36v bms, and have it run the whole battery rather than use the 12v bms's. Then you can set up solar 36v charging.

Thanks for the reply!

I actually did buy a 36V BMS for exactly this purpose but I later thought that putting cells with different internal resistances and potentially even different capacities under the same BMS was unwise.

I like the triple voltmeter idea (provides peace of mind), it'd be nice to set up ammeters on the outputs of the charge controllers to check how much charge they're each individually putting in to each pack. I'm not sure how that's done though, I've read you need a shunt because typical ammeters have fuses that blow if you run a significant current through them for anything more than a very short while.

*sigh* I wish my battery was just as simple as a 36V 12cell pack, but I got these 12V packs for a very good price.

How different your packs were I did not realize. But that's a good reason right there to monitor each packs voltage seperate. Or at least, figure out which pack drains first every time, and watch that one.

You need to get married and check up on her every like a jealous husband . Its way you know what she's upto. So to speak.

I tried to set up voltage monitoring through my Arduino and a voltage divider but realized I can only measure one batteries voltage at a time with it, since if I were to try and measure two batteries at the same time (which I definitely did not do) while they were connected in series, I might get a short . Any ideas for getting around this, besides using 3 different Arduinos? I've thought of using relays and switching them in my code to only have one battery connected to the Arduino at a time but I'm not sure that's reliable enough to ensure I will not have venting batteries under my legs.