> I've acquired, recently, about a dozen or so gel batteries...
> I'm told that the reason they were being disposed of is that
> they sat on the shelf for too long.  With only one exception,
> they seem to be showing me good voltage,  though.
> The part that bothers me about it is the idea of connecting all of these in
> parallel,  I keep thinking that there ought to be a better way.
I have been able to get some lead acid gel cells going that had never
been used and sat on a shelf for over 5 years.  The open circuit
voltage was low, and under load there was no voltage.  Hooking the
battery to a conventional charger showed no current flow.
Connecting a lifeless battery in parallel with good batteries for
charging will result in the good batteries being charged, and the
lifeless battery just hanging around not taking any current.  The
voltage across the lifeless battery will be good while connected to a
charger, but the battery will still not supply voltage to a load on
its own.
Caution -- this does not apply to Nicads.  Don't connect a bad Nicad
battery in parallel with good Nicads unless you want to melt your
wire and maybe have an explosion.
The solution that often works is to use a constant current charger.  I
use the same home brew charger for 2V and 12V batteries.  I set the
current to C/20 or C/40 and let it charge for a couple of days.  At
first there will be almost no current flow and the voltage across the
lifeless battery may be 3 to 30 times the rated voltage.
I can hear the controversy coming now :)  This does not cause the
battery to explode -- at C/40 it doesn't even get warm to the touch.
Only too much current will cause problems -- not too much voltage.
The constant current charger lowers the voltage when the battery
starts taking a charge so that the current remains at a safe level
that will not damage the battery.
After the first day the voltage across the battery should be slightly
above the rated voltage of the battery and the current should be up to
whatever you set as the limit.  Another day or two of charging and the
battery will be up to its full potential.  At C/40 you can charge the
battery for a week or two and not hurt it.
If the voltage goes below the rated battery voltage while charging,
you probably have a shorted cell and should discard the battery.
Depending on the maximum voltage your charger will supply, you can
connect several batteries in series.  Examples;
Let's assume;
- your charger can supply 48V
- you are trying to rejuvenate lifeless 12V 10 AH batteries
- you choose to charge at a rate of C/30
You can connect 1 or 2 batteries in series and set the current limit
to .33 amps.
You could charge 3 normal batteries at once using such a system.
This system can also be used to charge Nicads.  Actually the right
way to charge Nicads is to use a constant current.
These restored batteries may not have the same amp hours as
they did when they were new, but they should have many
charge / discharge cycles left in them.
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