Hi David etc,How many miles are you driving a week/month? I saw this out of
balance condition after 7k miles or 7 months (350 miles per week). My BMS
leakage is about 3ma & I'll check the low one's #12 & 13 that show up on the
battery scanner as lower than the rest at EOC and EOD. The BMS tested ok on
the bench at min (2.6V) and max (3.6V) operation. the cell that went down to
.9V now appears OK after charging back up with a Triton charger and measuring
the discharge Ah at 125ah on the 130ah cell.Have a renewable energy New
Year,Mark www.REEVA.info Subject: [EVDL] LiFePO4 batts drifting out of balance
*with* balancer
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Hi Cor etc,
I increased the final taper to 2A. (They {36ea CALB 130ah} drifted out in 7k
miles in 5 months.) I checked the cell that sagged to .9V on the Triton
charger, by charging to 3.6V then discharging and got 125ah out of it so I
charged it up again, seams fine.
With the final taper set to 2A all the cells appear to be coming up when two
hours match at dv/dt=0 (.o1V per cell per hour = stop), like previous EV's,
lead, ni-cad, now LiFePO4. The balancers (see www.evdl.org/lib/mh ) signal the
charger to drop from 10A charging current to 1 or 2 amps (selectable) when any
one cell hits 3.6V or bulk charge hits 127V on 36 cells. If any one cell hits
4.0V or drops below 2.6V the charger is shut off and displayed what the
malfunction is. Fortunately the battery scanner caught the low cell going up
the hill.
So having a stiffer final charge current during final taper seams to solve the
problem (in the 9 hours I'm at work or at home). Before I would unplug it
(120vac) before it was done tapering. I had a 4A taper when I had a ni-cad car
and previously lead cars back to the 70's. Due to all the balancers/with low
current shunting capacity I had to lower the final taper - but then it takes
*much* longer to taper. Anyway, 2A appears ok, I'll see in the next 7k miles in
a few months. (350 miles per week, 70 per day Mon-Fri). Usyually only do 30
miles per day on week-ends.
Have a renewable energy Christmas,
Mark in Roanoke, VA
www.reeva.info
..> Message: 2
> Date: Thu, 20 Dec 2012 13:26:33 -0800
> From: "Cor van de Water" <[email protected]>
> To: "Electric Vehicle Discussion List" <[email protected]>
> Subject: Re: [EVDL] FW: LiFePO4 Calb cells drifting out of balance
> *with* 1Abalancer
> Message-ID:
> <a73bc4b8b3218642a56a2c9eb01b44e001a36...@exchange.corp.proxim.com>
> Content-Type: text/plain; charset="us-ascii"
>
> Mark,
>
> It may not be the current, but the charge algorithm.
> What is the condition to stop charging?
> Only if *all* cells report that they are up to 3.6V?
> Or is the charger stopped at a certain voltage level?
> If one cell is self-discharging more than most other cells,
> the pack voltage when all other cells except this one is
> 3.6 might be enough to kick the charger off, even though this
> one lagging cell is still around 3.0V - it could be almost empty
> while the charging thinks everything is OK.
> This is not a problem of the BMS, unless the BMS is controlling the
> charger to do this, but I expect that the charger continues to
> charge at below 1A until the *last* cell starts shunting.
> Anything short of that (and it sounds that either the charger
> voltage was set too low or the charger switch off voltage threshold
> was too low) and you will find the highest self-discharging cell
> the hard way - when it suddenly hits zero because it is not fully
> topped up every cycle....
>
> Regards,
>
> Cor van de Water
> Chief Scientist
> Proxim Wireless Corporation http://www.proxim.com
> Email: [email protected] Private: http://www.cvandewater.info
> Skype: cor_van_de_water Tel: +1 408 383 7626
>
Mark,
I'm seeing nothing of the sort you are seeing with LiFePO4 cells going
out of balance. I'm also not the only one who isn't seeing any drift
with the prismatic LiFePO4 cells (TS or CALB). Either you have a bad
cell or something external to the cell is draining it more than the
rest. There is no internal self-discharge mechanism in a LiFePO4 cell
so if it is self discharging then it has a problem.
Do you have any way to measure the drain your BMS is causing on each
individual cell to see if that one is getting drained more than the
rest? I'm afraid that changing the tapering current is just a band-aid
covering up an underlying problem.
--
David D. Nelson
http://evalbum.com/1328
http://2003gizmo.blogspot.com
http://www.levforum.com
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