Running BMS-equipped batteries in parallel

I'm pretty knowledgeable with things electrical and a little less knowledgeable with things electronic although I have been successful at diagnosing and repairing non-functioning switching power supplies. I'd like to run my two Prodeco 51.2 Nominal/12 Ah LiFeP04 batteries in parallel but am unsure of the reaction of the individual battery BMS's in trying to work them together. I am familiar with equalizing battery Voltage prior to hooking batteries in parallel and run and charge non BMS equipped batteries in parallel all the time. I would imagine it might depend upon the individual brand and type of BMS as to whether or not they can run together. Can someone offer their advice and knowledge on this issue? The batteries are under warranty hence I don't want to open them and break the seal to find out what BMS they use.

can you post up a picture of them? if they have a separate charging mosfet they will have to be separated at the P- spot when charging. but you can charge them both in parallel. if there is no separate charging lead and it is charged through the same P- leads then you do not have to disconnect them when charging in parallel.

I don't want to void the two-year warranty so I can't provide any pics. Each battery has a dedicated charge port and they discharge/feed the controller through different ports. That's probably not going to help though. I don't mind separate charging, it's not a big deal. But do you think they can be paralleled for discharging?

of course they can discharge in parallel connected at the P- spot and B+. thought that was clear already.

Sorry, this is where my electronics knowledge is a little light. If I understand correctly - right now with the two batts sitting on the bike, I could connect both battery outputs in parallel Pos to Pos and Neg to Neg (assuming they are equalized) and discharge them together during my 50-60 mile ride. When I get back, do I have to disconnect the paralleled output connection and then charge separately thru each battery charging port or can I always leave the outputs parallel connected and charge separately thru each battery charging port?

I am not sure what you mean by P- ?

P- is the negative output of the BMS that connects to the controller.

Well if I had done a search at first, I would have found a few or your posts with your answer - especially the "Parallel battery packs incl BMS" thread where you posted twice - Jan 24 and Jan 25, 2012. Can't be any more clear than that! Thank you very much dnmum!

One last question. Can the same electrical disconnect for charging be done with both P+ wires instead of the P- wires? Reason I ask is that each battery P+ wire feeds directly to its' own key switch so it would be simple to achieve disconnect by simply turning each of the two key switches to their off position.

my explanation was based on the assumption that one charger, or two chargers wired in parallel, would be used to charge both batteries at the same time. so a switch on B+ would mandate using a second charger.

the P- output is what you think of as the negative terminal of the battery but it is really the P- output of the BMS.

the reason the two batteries need to be isolated at P- during charging is to prevent overcharging of one battery that has shut off the charging mosfet for HVC and the current can continue to flow around that turned off mosfet by going through the BMS of the other battery when they are connected at the drains of the output mosfets. which is P-.

OK, I got it now. Again, thanks very much!

Not to beat a dead horse....but I guess I will. I have one of the new Grin Satiator chargers so I'm able to choose my termination Voltage in hundredths of a Volt. If I choose to terminate the charge at 3.45 Volts per cell, well below BMS pack HVC, then I'm guessing that parallel charging with one charger and a "Y" harness would not be an issue any longer since neither BMS will trip off on HVC?

I've charged each pack to 3.65 VPC before while monitoring with a watts-Up meter in-Series to determine that normal CC to CV transition takes place and that the BMS's are not tripping on HVC.

I'd say that's a bad idea. Firstly, the balancing normally happens at the end of the charge cycle when the cells near their maximum (3.65v), so you will have compromised that function. Secondly, you'll be bypassing the charge control on the second battery, so if out of balance, which is more likely than normal, the charging won't be switched off by the BMS. If a cell reached HVC in one battery, it's BMS would switch off the charge FET/s, but the other battery would continue to rise in voltage, which would then continue to charge the first one through its discharge wires.

The problem is theoretical. It depends on cells going out of balance. The more charge cycles that you complete, the more likely you'll realise its effect. I ran two batteries in parallel like you propose for about a year with no problems, but I might have just been lucky.

what does it mean when you say 'tripping on HVC"? i don't think you know how a BMS operates and i certainly do not understand why someone would charge to 3.4V/cell if the BMS is set to balance at 3.60V/cell.