Re: [electricboats] Paralleling LiFePO4 battery packs

Great questions Carsten.

Yes, current draw and resistance are key (as is Peukert effect, which for lithium batteries is largely insignificant).

Cable length/gauge/type, fuse and connector resistances and local cable resistances to bundle points factor in.

How local modules/submodule are interconnected matters.

I have no doubt that my back 12 sub-modules bear the brunt of the load (when loaded) compared to the forward 8.  Simple calculation suggests this: Additional 2milliohms of cable resistance to that subpack vs the rear pack probably makes the forward half-pack have 2x the resistance as the aft half-pack.  Assuming so, under load we would expect 2/3 of load current to come from the aft half-pack.  Maybe it's worse…I'll have to measure it.  But it matters little in my case.  Consider that max. current draw is 200amps and that I have 20amp fuses on each half-module.  With 12 of these aft in parallel, it would take more than 240amps load to blow fuses even if the forward pack did not help at all with the load.  Now, if those 12 are also not closely balanced in cable length, a "zipper" fuse failure could occur---closest battery module delivers more than 20amps, blows its fuse, then the full load applies to 1 less module, more current/module and all the fuses quickly blow in sequence---closest to furthest.  I've never had that happen.

As I configured my subpacks, I balanced the cables.  But between fore and aft subpacks, I chose not to---I could always add more 1/0 cable to make it balanced, but til now have not seen justification.

 

Oh, I am a career EE (40yrs now). J

 

-MT

 

From: electricboats@groups.io [mailto:electricboats@groups.io] On Behalf Of Carsten via groups.io
Sent: Sunday, January 3, 2021 9:41 AM
To: electricboats@groups.io
Subject: Re: [electricboats] Paralleling LiFePO4 battery packs

 

Interesting, Rob and Myles !!

 

Any eletrical engineers out there to tell us, what is best ?

Cable Ohms, Amps draw ? At what Amp power draw might this be an issue ?

Can the BMS('es) handle this separately, or MUST the cables be matched to length and resistance ?

 

Looking forward to be enlightened

 

 

On Monday, 4 January 2021, 01:29:10 GMT+8, Myles Twete <matwete@comcast.net> wrote:

 

 

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