[Boatanchors] GEL CELLS

[Bill Cromwell]

Hi John,

I've had decent luck 'recovering' lead-acid batteries with some really 
spectacular successes and of course some that were just plain *gone*. 
The electric bill for the charging I use is pretty small so there is 
nothing much to lose trying.

My charge box starts with 50 volts DC (calm down) and feeds the battery 
through a solid state current limiter. The limiter is a pair of 
transistors with the main pass transistor a big honking TO-3 on a bigger 
honking heat sink. At full steam the transistors get warm and so does 
the heat sink but not *hot*. I have three charging outputs set for 50, 
100, and 400 mA. The voltage applied to the battery terminals is *only* 
that voltage that is required to force the selected current (start with 
50 mA). Of course the voltage can't rise higher than 50 volts and it 
will when there is almost no current flowing. I have left the battery 
being 'recovered' on - sometimes for weeks. When the battery terminal 
voltage comes on down near to what it should be step up the current - in 
my case to the 100 mA terminal. On my unit that is a different pair of 
transistors and no switch contacts to go bad.The voltage will be a 
little higher at the terminals and in a few hours should level off at 
typical voltage for the battery in question. 12 volt batteries at or 
near full charge are NOT 12.0000000000000 volts but range upward from 
12.5 to 14 volts. It certainly should NOT go down to 9 or 10 volts (or 
lower). That's a sign of a shorted cell (junk). When the battery has a 
decent charge let it sit a couple days (or even a couple hours and make 
note of how long it takes to sag a little and how much. If it gets real 
droopy real quick - recycle it. If the terminal stay up near 50 volts 
(in my example) it's not going to take any charge - recycle it. My 50 
volt, constant current charger has been in regular use with a wide 
selection of batteries since 1985 and there have been zero mishaps.

I use the batteries too and I don't need to swap them onto a maintenance 
charge because they are in use as the primary power for some of my gear. 
I swap them in and out as they get discharged in actual use. I use the 
constant current approach and usually just 50 mA for my 7.5 AH batteries 
(and leave them on for the needed time. In the past I used a charger for 
my AA NiCads that used both current limit AND low voltage to bring the 
cells up to full charge. The charge current would be about 15 mA until 
the battery terminal voltage approached the available voltage from the 
charger and then the current tapered off. It's all automatic thanks to 
Ohm's law. I had three AA packs on my bench and the cells lasted between 
seven and twelve years!

Probably leaving those batteries sitting around for several years (and 
how long did the former owner leave them sitting) has pretty much 
wrecked them. It's possible that battery that shows 12 volts at the 
terminals won't actually drive any real load. Connect an automotive tail 
light bulb to give some modest work to do and see what you get for 
voltage. If it looks good there then put it to work and see how long it 
lasts. Do the arithmetic and see how much capacity it actually has. When 
they are at end of life they don't drive a load very long.

You might recover some of those batteries so give it a try. Put the 
charge in a box that will contain and/or direct explosions and debris to 
a safe place and not all over your kitchen walls or you. I have never 
had a battery do that to me but I have seen fancy store bought chargers 
blow up batteries. Plain ones are even worse.

Good luck with your project.

73,

Bill KU8H

On 01/21/2015 02:57 PM, John via Boatanchors wrote:
> Greetings,
>   
> Several years ago, I bought a case of gell cells at a hamfest.   They  were
> rated at 12V, 7A.   They were supposedly "recent pulls"  having full
> capacity. Today I needed one as a replacement in a UPS unit and  found that only
> one was at full terminal voltage and would work in the UPS.  Several others
> were at 6V and the remainder essentially dead.
>   
> Does anyone know of a safe way to restore these batteries or should I take
> them to the recycle place?
>   
> Thanks and 73,
>   
>   John,  W4AWM
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