Mixing different ages of LiFePO4 batteries in the same pack

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Mixing different ages of LiFePO4 batteries in the same pack

Rob Trahms
Based on the high cost of LFPs relative to PbAs, I was considering replacing my pack in stages, i.e. take a hit on initial max voltage.  This would allow me to start with a pack at a more reasonable cost, and (hopefully) add cells to it later if I so choose.  

This approach is not advised for PbAs, because mixing old and new batteries has issues.  But since I would have a BMS anyway for LFPs that would be monitoring each cell, I can't see an issue here.  The only guidance I have found so far is that I should keep whatever cells I mix at the same Ah capacity, and this makes sense from a peak current draw perspective.

Is this approach viable, or am I setting myself up for some risk of damaged cells here?

Thanks!
Rob
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Re: Mixing different ages of LiFePO4 batteries in the same pack

Morgan LaMoore
It depends on your BMS. If it's a simple one that just makes sure you
don't discharge the weakest cell too far and dissipates extra energy
at the end of charge, it should be fine.

If it's got a fancy active balancing scheme that's active while
discharging, talk to the BMS maker to see how their algorithm will
react to varying capacities.

-Morgan LaMoore

On 5/13/09, Rob Trahms <[hidden email]> wrote:

>
> Based on the high cost of LFPs relative to PbAs, I was considering replacing
> my pack in stages, i.e. take a hit on initial max voltage.  This would allow
> me to start with a pack at a more reasonable cost, and (hopefully) add cells
> to it later if I so choose.
>
> This approach is not advised for PbAs, because mixing old and new batteries
> has issues.  But since I would have a BMS anyway for LFPs that would be
> monitoring each cell, I can't see an issue here.  The only guidance I have
> found so far is that I should keep whatever cells I mix at the same Ah
> capacity, and this makes sense from a peak current draw perspective.
>
> Is this approach viable, or am I setting myself up for some risk of damaged
> cells here?
>
> Thanks!
> Rob
>
> -----
> Rob Trahms
> [hidden email]
> Electro - the Cabby-EV
> http://chaosmgmt.blogspot.com chaosmgmt.blogspot.com
>
> --
> View this message in context:
> http://www.nabble.com/Mixing-different-ages-of-LiFePO4-batteries-in-the-same-pack-tp23521670p23521670.html
> Sent from the Electric Vehicle Discussion List mailing list archive at
> Nabble.com.
>
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Re: Mixing different ages of LiFePO4 batteries in the same pack

poweredbydc
In reply to this post by Rob Trahms
If you buy your cells at dirrerent times you can have the factory match the cells capacity and internal resistance to the cells you already have.  The LFP cells capacity changes very little from about 20 cycles to 500 cycles so if you are using a god BMS it should not be a problem.  You may have to watch them a little closer for the first 20 cycles or so though.

Dave Kois
Powered By DC, LLC
EV COmponents, LLC
www.evcomponents.com

Rob Trahms wrote
Based on the high cost of LFPs relative to PbAs, I was considering replacing my pack in stages, i.e. take a hit on initial max voltage.  This would allow me to start with a pack at a more reasonable cost, and (hopefully) add cells to it later if I so choose.  

This approach is not advised for PbAs, because mixing old and new batteries has issues.  But since I would have a BMS anyway for LFPs that would be monitoring each cell, I can't see an issue here.  The only guidance I have found so far is that I should keep whatever cells I mix at the same Ah capacity, and this makes sense from a peak current draw perspective.

Is this approach viable, or am I setting myself up for some risk of damaged cells here?

Thanks!
Rob
Dave Kois
Current EV Tech, LLC