Charging parallel packs

OK, I've got two LiPo packs (each with BMS) that will have their output leads connected in parallel. Each pack has it's own charging leads. With the way the BMS is set up, connecting a charger to one of the packs will charge both (via the connected output leads), but at half the speed. So, here's the dumb question: Can I connect both packs to their own chargers while they are still connected in parallel via their output leads, or is this a really bad thing (like billowing smoke and flames, and all kinds of havoc)?

Thanks!

Disconnect first, or isolate them with a schottky diode.

I don't have these, but I recall reading of problems with the bms when charging connected packs. Make a little diode and fuse board for it, and that will also keep the stronger pack from trying to charge the weaker.

Hi,

Charging through diodes doesn't always work; it depends on the battery & charger technology. I have had a NiMH charger refuse to co-operate, as one of the things it does is to check for the right battery voltage before turning on. The diode of course stops it seeing that.

Lithium chargers may well be ok, since in that case the chargers are more like simple power supplies and the intelligence is in the battery pack. In that case isolating diodes would be a good thing.

There are also potential problems with the trickle charge phase when parallel charging NiCds or NiMH. Again, not a problem with Li if the charge termination is controlled by the battery pack.

Nick

There have been so many warnings about the LiFePO4 batteries and their BMS's, that I have set mine up easy to disconnect, plus an optional bypass for the BMS. I always charge thru the BMS with the two packs disconnected. I only discharge through the BMS when using one battery on the BOB trailer (see avatar) with a single BD36. Before hooking the batteries in parallel I always check that the voltage is the same on both. With the 20AH packs I may never approach low voltage problems, but if you might be in that position using diodes and the BMS would make sense.

Hi

No wouldnt do that! it depends on the charger of course but its not a good idea, most of the lipo chargers I have seen are intelligent and monitor the battery volts alongside the BMS, you can discharge them in paralell assuming they are both matched and charged to the same level, if you are ever unsure fit a diode, the Point-1 packs can be connected in parallel however series connection requires higher voltage rated fets to be fitted to the BMS board.

Knoxie

Thanks for all the advice. Since I'll eventually have four, 36V 8.2AH packs, what I'll probably do is make two, double packs directly wired in parallel. Then I'll connect these two packs via a 40 Amp Schottky Diode Parallel Battery Joiner from ebikes.ca. I'll charge each pair separately with a 3A smart charger that I'll order from batteryspace.com (I'll be getting two, one for each pair).

This set up will keep me from having to carry four chargers in order to charge all of them at once if I used schottky diodes on all four packs. I can deal with two chargers, but four would be just too much for a single system.

Thanks again for all the advice!