[Oeva-list] over discharged lithium pack

phil hochstetler phil.hochstetler at gmail.com
Fri May 6 14:45:30 PDT 2011


Here is what I can add.

If an A123 M1 cell is run down to zero (but not under load), it can be
safely charged back up with no or little loss of capacity.  I have seen this
documented by several folks over many years.  If you run a M1 cell to zero
under load and this results in actually reversing it, it will become a dead
short.  You can take a bank of cells out this way and not notice it if you
have no BMS (until you figure out your pack voltage is down a bit on charge
and you have some reduced capacity).

Other technologies may not be as forgiving as A123 cells are.  What I would
do is get a fast single cell charger (or make one using a high capacity
DC/DC converter -> 48v to 3.3 or 5.0v trimmed to target voltage of the cell
in question) and bring each cell up to about 10% charge by hand.  Any cell
that can not hold that charge overnight is toast and you should not attempt
to charge the pack using the car charger.  In that case, you will have to
replace cells.  The quality and features of the BMS may prevent damage when
charging the pack but it may not so I personally would not chance it.

If all cells take about 10% of their stated capacity in watt-hours and hold
their voltage overnight, I would go ahead and carefully try to charge the
pack using the car charger.  Watch it carefully as this is a dangerous
activity (do it outside, not in a garage that might burn down if the pack
catches fire) and monitor very carefully on a regular basis during charge.

Good luck.

Phil H.

On Fri, May 6, 2011 at 11:15 AM, Ron Jayasuriya <evmail4ron at yahoo.com>wrote:

> I have a customer who accidentally left his car on and ran his 400V Kokum
> battery pack too low, and his charger won't try to charge them now.
>
> AC Propulsion kit, probably proprietary charger, do not have any specs.
> Car has BMS and balancing, so we will probably be ok once we get them
> charged up somehow.
>
> Ideas?
>
> I was thinking that if there were a Rudman PFC 20 out there somewhere I
> might be able to use it to give the pack little boost using the adjustable
> voltage/current feature of that charger.  Not sure for certain, but I think
> I should be able to monitor the batteries for voltage AND heat during the
> process.
>
> Does this sound reasonable?  Dangerous in any way?  I don't think so, but
> it's worth asking.
> Anyone with some experience with over-discharged lithium batteries, or a
> PFC-20 they'd let me use, feel free to chime in.
>
> Thanks!
>
> Ron
>
>
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