Re: [electricboats] Thoughts on 12 volt Lifepo4 batteries

To adhere to that MinnKota 40.8V maximum voltage with LFP cells, one could use an 11S arrangement (maximum of 40.15V).

Of course this makes charging/balancing more of a challenge.

-Bobkart

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From: electricboats@groups.io <electricboats@groups.io> on behalf of Dave Yamakuchi via groups.io <dyamakuchi=yahoo.com@groups.io>
Sent: Tuesday, May 3, 2022 10:31 AM
To: electricboats@groups.io <electricboats@groups.io>
Subject: Re: [electricboats] Thoughts on 12 volt Lifepo4 batteries

 

Most lithium chemistries I've looked at had a significantly higher voltage at full charge than a typical lead acid.  If the cells are doing a straight swap of four Li = six PbSO4 cells (ie so-called '12V') it's important to remember that.

Where the lead-acid cells are, at full charge, at room temperature, at standard pressure, etc. maybe a bit over 13V (...for AGM, usually 12.6V for regular flooded cells) many Lithium cells can output significant current at over 16V!

Minnkota says absolutely no more than 13.6V MAX.  ...for their 12V models, and no more than 13.6x3=40.8V 'absolute maximum voltage' ...a very specific term... for their 36V devices.  If you're throwing a bunch of amp-hours at it for range, you might easily exceed the maximum rating for significant periods of time...if your motor was not designed for the higher voltage.

On Tue, May 3, 2022 at 11:50 AM, Myles Twete

<matwete@comcast.net> wrote:

Sounds complicated J.

I have some experience with these type (and size) of LiFePO4 batteries.

At first, the experience was not good: We had about 30 of these, new-in-box, which we took off the shelf to find them all dead, not taking a charge.  These were high quality NEC batteries (~$120ea as I recall).  It turns out that the internal BMS operates constantly, even before you ever use the battery, and within months it can bring the voltage down to the point that the BMS will not allow the battery to be charged again---i.e. it's bricked.  I spoke with the Mfr and told them that even the lowest cost "batteries included" items typically have a small plastic tab that needs to be removed before the battery cells can be drained and with such a high internal BMS usage coupled with shipping batteries at 50% capacity the risk of bricking these in the box was high.

 

Anyway, if this does happen to you, you can "kick start" the battery cells and get the batteries to work again, but that's another story.

 

Main Point: These batteries (the NEC ones anyway) do have internal bypassing such that you can even leave up to 16v on them indefinitely and they will internally bypass current and keep the cells topped off at their max.  So this should allow a lot of flexibility for charging them.

 

-MT

 

From: electricboats@groups.io [mailto:electricboats@groups.io] On Behalf Of HF via groups.io
Sent: Tuesday, May 3, 2022 9:25 AM
To: electricboats@groups.io
Subject: Re: [electricboats] Thoughts on 12 volt Lifepo4 batteries

 

Hello, Skray,
Here's an idea.  I'm not sure it's a good one, so hopefully others here will chime in about its risks and flaws.
I've bought several 12 V 6 Ah LiFePO4 batteries on Amazon when they come up for $18-$20 each, delivery included.  I've tested them for capacity.  They've always been at least 90% of the claim.  For 36V 100 Ah, you'd need an array of 51 of these, which should set you back about $1k.  You could charge them simultaneously using fully isolated power supplies for each set of 17 in parallel.   There would be a lot of wiring.  To make the pack more compact, you could bust open the cases to access each set of 4 cells strapped to their management board individually.  But take care to remount them with enough space and materials between them to prevent fire propagation if one of them spontaneously shorts itself.
I haven't done this yet, but I intend to so something like this for my 3 hp outboard motor conversion project.
Cheers,
Halden