Re: [Paddlewise] Gel Cells

At 07:32 PM 2/4/99 -0500, Larry C. wrote:
>What type of charger is used for Gel cell and would a solar charger work
>if it was mounted on rear deck?

Hi Larry, et al!
The purpose of any charger is to push them li'l electrons back into the
battery. If you push 'em in too fast, bad things can happen; sealed
"gell-cell" batteries get hot, and may go BANG!!! As a general rule of
thumb, you must limit the charging current to 1/10th of the battery's
ampere-hour rating. Thus for a gell-cell rated for 1000mAH (1000
milliamp-hour, or 1 ampere-hour)the maximum charging current would be 100mA
(0.1 Amp), and it would take about 10 hours to fully charge an exhausted
battery. Another thing to consider is the maximum voltage available from
the charging source. A nominal 12V gellcell will actually read about 13.6
volts when fully charged. You probably do not want to charge from a source
that produces significantly higher voltage than this, say 14 or 15 volts
max. This is one reason why your car has a voltage regulator. You can
charge from pretty much any DC source if you keep the above in mind. Get a
cheap voltmeter from radio shack, and as large a variable resistor as you
can find.  Somewhere around 50 ohm, 25 to 50 watts should be about right.
Connect the source + to one end of the variable resistor, which should be
set to maximum. Connect the other end to the battery +. Set the meter to
read DC current, and connect the battery - to the voltmeter +, and the
meter - to the source -. Now your meter is measuring the charging current.
Adjust the variable resistor until you read no more than 100mA (from the
above example). Remove the voltmeter from the circuit, set it to read DC
volts, and connect it to read the battery voltage (meter + to batt +, meter
- to batt -). Keep an eye on it, and stop charging when the voltage reaches
about 13.8 to 14 volts. The voltage will drop a bit after you stop
charging, but should settle in at about 13.6 volts. You can try a higher
charging current, but watch the temperature of both the battery and the
resistor. If either starts to get more than just slightly warm, back off!
If you have access to a decent variable DC power supply you can adjust both
the maximum DC voltage and the maximum current independently from the front
panel. Most will even have built in meters. If you plan on doing a lot of
charging different types of batteries, you might want to consider getting
one. A solar cell is just another current source as far as your battery is
concerned, and the same procedure applies. Many solar panels produce quite
high open circuit voltages (16 - 20V) in bright sun, but this is not
usually much of a problem as the voltage produced drops off sharply as you
begin to draw current. Be sure to disconnect the panel before the sun goes
down or it may discharge the battery. Some panels have a built-in charge
controller; if you have one of these, just hook it up and go.
Please note that the values I have provided are for a lead-acid chemistry,
and will not be suitable for nicad or lithium batteries. Somewhere I have a
more detailed discussion of this, if you're interested I will try to find
it and forward a copy. 
Hope this helps . . . S. 
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