Nursing yet another set of batteries back to health.

 I recently aquired my aunt's Invacare wheelchair that she got many years ago
and used for a few months before abandoning it in her garage and allowing
the batteries to go completely dead.  I am hopeful that they will recover as
they seem to be charging well so far.  They are currently charging in
parallel at 13.7 volts and falling as the internal resistance drops with a
charge current of about 6 amps (3 each).  They started with OCVs of 1.4 and
.5 respectively.  The batteries are MK M22NF SLD G (gel cell) dated January
1999.  Other than the obvious rule of not exceeding 14.1V what can I do to
help them recover?  I understand that MK gel batteries are very good quality
(and a very high price to match) so I am hopeful that they may survive.

   "Daniel Who Wants to Know"


Highly recommend you recharge them separately, parallel opens the
door for a lot of trouble.

 

I found that out after I posted lol as 1 started hogging all the current and
the other was just sitting there with a high resistance still.  What I did
since the still weak one wouldn't even take 2.2 amps without going over 14.1
was I just charged the other up to 14.1 by itself and then took it off of
charge and connected them in parallel again to let the strong one trickle
current into the weak one and let them sit overnight with no charge current.
When I checked them before posting this and putting them back on charge
again they were at 12.25 and now on charge they are at 13.66 volts 8 amps.
I am wishing I had a proper battery charger but for now I am using laptop
power adaptors and just watching a DVOM and an analog 0-10 amp panel
ammeter.  BTW the internal switching adaptor out of a Toshiba Satellite Pro
420CDT and other similar models works great as a battery charger as it is
current limited to 2.8 amps for the first 5 seconds then drops to 2.2 amps
and is voltage limited to 15.0 I plan on adjusting the voltage set point on
it soon but for now a single diode drop knocks it down to ~14.3 for
absorption and 2 drops takes it to ~13.5 for float.

 

     The fact that at least one of them is "sitting up and taking nurishment" is
mildly encouraging.  That said, even if your batteries had not been abused, they
are quite old for gell cells and under the best of conditions would probably be
under 50% of their original capacity.  By all means, try whatever you want
and/or whatever anyone else here proposes.  Just don't spend any money in the
process because your chances for success are slim.

Honestly, I have never managed to bring back gell cells that were as far gone as
you describe.

Vaughn

 

As an update I have fully charged them individually and then used them in
the wheelchair but they appear to only have about a tenth of the rated
capacity. According to my DMM they are staying balanced when charged in
series by the wheelchair's 24V charger but I am fearful of using it to
charge them as them as the absorption setpoint on it is 28.4 not 28.2.  I
plan to contact MK and see what they say about it.