Re: [electricboats] Parallel LiFePO4 Packs

Or you can go with 2 strings, 2 BMS and combine the strings like you originally talked about.  But you don't have to make these formally "switchable" strings---they can be segregated for emergency or other reasons (e.g. separate charging) very easily with appropriate connector choices.  Fuses are always a good idea.

My pack is effectively composed of 20 separate nom. 42v strings all combined in parallel thru 20amp fuses.

That would ordinarily be a very expensive and complicated way to go (especially given also the 20 remote BMS cards I use) but the strings are comprised of modules including BMS cards that are ex-EV and relatively inexpensive.

So not counting cabling and fuses, one can pick up modules like this (including BMS cards) for $50/kwh or even less if you cut out the retailer.  I paid about this for the latest modules I picked up and could have gotten them for about half that.

So in my case, if I suddenly had a shorted cell (internal or external short), that string's fuse would blow and it would be some time before I'd discover it---eventually I'd discover it, but in the meantime that string would be isolated.  In several years of use (lithium ion) I have yet to see a shorted cell pair, though I do know there are several weaker or leaky cell pairs out of the 240 cell pairs in the pack.  Also, I can quickly isolate any or many strings by simply slipping off any particular "Radsok" connectors at the top of each stack.  The Radsok connectors also makes it quick and easy to remove one of the modules.

 

If these USED batteries were not available cheap, I'd probably still be running with flooded PbA or would have gone with a single- or dual-string LiFePo large-capacity pack as you are planning.

 

-MT

 

 

 

 

From: electricboats@groups.io [mailto:electricboats@groups.io] On Behalf Of ChristopherH via groups.io
Sent: Wednesday, November 8, 2023 2:26 PM
To: electricboats@groups.io
Subject: Re: [electricboats] Parallel LiFePO4 Packs

 

I found this great article by Orion BMS that discusses parallel strings. Reading this really makes me lean toward parallel cells as you wise gents previously suggested.

 

Thanks again for all the valuable input.

 

Chris 

parallel_strings PDF Document · 408 KB

 

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On Nov 8, 2023, at 16:58, ChristopherH via groups.io <clh5_98=yahoo.com@groups.io> wrote:



Reuben,

From what I've seen, Victron only have BMS for their line of packaged batteries in the 12 and 24 volt ranges.

 

Chris 

 

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On Nov 8, 2023, at 16:55, Chris Hudson <clh5_98@yahoo.com> wrote:

John,

Thanks for your input and good point about having an ace in the hole!

Chris 

 

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On Nov 8, 2023, at 16:22, john via groups.io <oak_box=yahoo.com@groups.io> wrote:



I don't have a LOT of experience with this, but am now running my electric dinghy with two separate 48V ebike batteries.

 

I like keeping them separate so that if one gets accidentally discharged (or any other failure), I still have a second battery to get home on.

 

On Wednesday, November 8, 2023 at 01:59:35 PM CST, Myles Twete <matwete@comcast.net> wrote:

 

 

Thought: Dangerous for the switch unless it's a break-before-make or other exclusive-or arrangement.

 

Why: You've been running for hours on one string, then decide to switch to the other.  If you thought that you'd just use a standard marine battery combiner switch, you could accidentally switch to "ALL" position, which would then put both strings in parallel thru the switch, quickly shorting it and possibly melting the housing.  Might also have a problem with that type of switch switching between each string.

 

With Lithium-Ion there's something to be said for having separate strings---ability to switch and get a higher pack voltage again for a time giving higher power/speed than if the strings were paralleled and slowly going down together.  But really, a slowly dropping voltage is better than one that drops much faster due to not paralleling.  Not as much of a big deal with LiFePo, but still, I'd tend to think paralleling is better.

 

Just a thought.

-MT

 

From: electricboats@groups.io [mailto:electricboats@groups.io] On Behalf Of ChristopherH via groups.io
Sent: Wednesday, November 8, 2023 11:49 AM
To: electricboats@groups.io
Subject: Re: [electricboats] Parallel LiFePO4 Packs

 

Bobkart,

Thanks for the reply. Actually I'm proposing to have 2 independent 16 cell strings with the ability to parallel them at the 48V level with switches, not at the cell level. Thoughts on that?

 

Chris 

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On Nov 8, 2023, at 14:40, bobkart <couch45@msn.com> wrote:



Connecting two 230Ah cells together in parallel (sixteen times) would allow you to treat the 32 cells as one large 16S 48V x 460Ah battery

BMS, load, charger, capacity monitoring would be none-the-wiser.

 

One concern of that configuration that wouldn't otherwise come up is if one cell in a two-cell pair were to fail.

If the failure is a short, it would bring the other cell with it.

 

---

From: electricboats@groups.io <electricboats@groups.io> on behalf of ChristopherH via groups.io <clh5_98=yahoo.com@groups.io>

Sent: Wednesday, November 8, 2023 10:34 AM
To: electricboats@groups.io <electricboats@groups.io>
Subject: [electricboats] Parallel LiFePO4 Packs

 

Hi Group,
My Morgan Out Island 41 is ready for new batteries. I'm ready to embrace LiFePO4. When I originally converted to electric I built and glassed in 2 battery boxes to hold 2 banks of 8 (16 batteries) golf cart batteries. I always run with both banks in parallel, but have the ability to run on just 1 and this has worked well. Unfortunately this size battery box doesn't make efficient use of most individual LiFePO4 cells. I would however be able to use 32 X 230Ah EVE cells in 1 battery box, leaving the other box for some other purpose. My questions: I'd like to be able to run the 2 X 16S strings in parallel. Is this possible with LiFePO4? Is it safe to do this? Any precautions? Would I have 2 separate BMS units? How about capacity monitoring for the 2 banks in parallel? I have a Victron 48V/5KVA Victron Quattro charger inverter. 
Thanks in advance for any assistance/suggestions.

Regards,
Chris Hudson

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