Dead LiFePO4 chemistry - what's happened

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davewrathsharman
Posts: 7
Joined: Sun Aug 03, 2008 6:38 pm

Dead LiFePO4 chemistry - what's happened

Postby davewrathsharman » Mon Jan 18, 2010 7:54 pm

Hello everyone,

In my LWB recumbent I have two Valence Saphion LiFePO4 batteries of 60 cells each configured 5s 12p - 16V 14Ah. I haven’t got the batteries wired for a balancer – I was told by the supplier it wasn’t necessary with this chemistry. And five years ago when I first put this machine together, I couldn’t find a balance charger for LiFePO4 batteries as big as these. The two batteries are always charged separately, and when fully charged, have always been within .02V of each other - 16.65 -16.63V

I used to run one after the other, until I discovered I could get quite a few more miles by using them in parallel - less voltage sag - they are separate until I key switch the two 70amp relays.

Recently, while 'exercising' the machine on its stand, several things conspired to destroy one of the 12cell parallel strings. It was very cold - I didn't check that the two batteries were still matched - for some inexplicable reason the obviously weren't - and the LVC on the ESC failed to do it's job. Just before I switched off the combined voltage was 9.8V - should have cut-off at 11.5V. One battery recharged as normal to 16.65V. The other battery recharged as though it were a 4s battery i.e. 13.33V The dead parallel string being very dead indeed

The original supplier still has some new Saphion LiFePO4 cells and I'm minded to use these to replace the damaged cells. At the same time I will wire them so I can balance them in future. So, am I likely to have problems wiring new cells to my old cells?

But what I really want to know is what has happened to the dead cells chemically and why is it so irrevocable? I know it's not going to help me with this problem - I'm just interested.

I'd be grateful for any help and information that anyone can give.

Cheers now.

Dave

asumner
Posts: 1
Joined: Sun Feb 14, 2010 7:13 pm

Re: Dead LiFePO4 chemistry - what's happened

Postby asumner » Tue Feb 16, 2010 10:35 am

Hi,

Take a look at http://batteryuniversity.com/ its excellent, once you have read that you'll probably understand a whole lot more about how to choose and look after the battery types for EV's!

Anthony

User avatar
timpootle
Posts: 1361
Joined: Wed Mar 14, 2007 12:13 pm
Location: Chorlton-cum-Hardy, UK

Re: Dead LiFePO4 chemistry - what's happened

Postby timpootle » Tue Feb 16, 2010 10:37 am

Anthony, you made it!

Glad to see you able to post.
Tim Crumpton

davewrathsharman
Posts: 7
Joined: Sun Aug 03, 2008 6:38 pm

Re: Dead LiFePO4 chemistry - what's happened

Postby davewrathsharman » Tue Feb 16, 2010 9:21 pm

Hi Anthony, thanks for the link, very interesting but very little about LiFePO4. I've emailed a question direct to batteryuniversity.com

Dave


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