Re: [electricboats] Paralleling LiFePO4 battery packs

Rob-

 

I'd like to hear more of your thoughts as regards the problems alluded to in regard to uneven cable lengths for paralleled series lithium modules (each w/BMS).  When I went down the path of paralleling up lithium modules (each 12S2P) on my boat, I planned to make all cable lengths equal to each module from common nodes, but the more I thought about it, it really made little sense to do so given the very low cell resistances, very low cable resistances, the maximum 200amps draw from the controller and with 20 modules in parallel (10amp max each).  Yes, at any throttle level, any imbalance will certainly see more current being drawn from the closer battery module, but that module has 19 others in parallel with it (varied cable lengths) and each will be doing its best to maintain the voltage should 1 module voltage dip.  I haven't done the detailed math or measured the current in each module under load (I should do this, for kicks at least) but it certainly does not come anywhere close to 2x---if it did, the fuses on the entire string would 'zipper' under full load (10a average causing 20a+ in closest battery).

 

My pack's 20 submodules are spread widely in my boat---8 forward (4 each side, 6' apart, 10' cable length) and 12 aft (6'apart, 10' cable length).  Separation between fore and aft subpack is nominally 10'.

 

Anyway, I do my best to keep local cable lengths equal, but as I recollect, I gave up on the thought of adding an extra 1/0 cable from fore to aft in order to accommodate balancing the resistance among all the modules.  I've never blown a fuse due to motor/controller loads.

 

-MT

 

From: electricboats@groups.io [mailto:electricboats@groups.io] On Behalf Of Rob Hepler
Sent: Sunday, January 3, 2021 8:55 AM
To: electricboats@groups.io
Subject: Re: [electricboats] Paralleling LiFePO4 battery packs

 

I don't think you need another bms, but make all your cables exactly the same length or you will have uneven discharge problems

On Jan 2, 2021, at 7:51 PM, mmiri@sfu.ca wrote:

Hello dear friends,

I have a 20kW electric motor for my 40ft sailboat and now I want to get the Lithium batteries (LiFePO4). The battery packs that I found are 48V200Ah each so I would be putting 2-3 of them in parallel (since the MAX discharge current is about 150A each). Note that Each pack has its own BMS. Now my question is that if I want yo parallel these individual modules (with each having their own BMS) do I need a master BMS for the whole pack (2-3 parallel modules)? If yes, how should I implement it? What do you recommend?
Thank you all in advance!

- Mohammad