On Mon, Feb 23, 2004 at 03:01:52PM -0700, Dwayne Reid wrote:
> At 09:33 PM 2/22/2004, Adi Linden wrote:
> >I am just looking for some feedback on a circuit design idea I am playing
> >with.
> >
> >I am thinking about building a gel cell charger using the UC3906 charger
> >IC. While the design is straight forward, it requires about 16V input
> >voltage to charge a 12V gel cell with dual step mode.
> >
> >I hope to use this charger in a motorvehicle or camper. So my power source
> >is a lot closer to 12V then 16V.
>
> The simplest (and one of the most reliable) charger for charging 12V gel
> cell battery from a 12V automobile is to use a schottky diode in series
> with a current limit resistor.  The 12V supply is already at the correct
> voltage.

No it isn't. Lead Acid requires a minimum of 2.3V/cell to charge. That means
you need 13.8V. Now usually this isn't a problem because the car alternator
will produce somewhere in the ballpark of 14V. But if it's a true 12V it won't
properly charge.

This is a continual problem on my to do list and I still haven't taken the time
to build a good charger. But here are the elements:

1. (Optional) current limited bulk charge until battery terminal voltage is
   14.4V. You can use any voltage and it can be unregulated. But the current
   has to be limited to the inrush current on the battery. This is usually
   something like C/3 where C is the Ahr charge of the battery. For example
   on my 33 Ahr gel cell the max inrush is 10A. My charger is going to be
   powered by a 18V 4A transformer, so in this phase I'll throw in a 5 to 8
   ohm resistor to limit current to 3.5A or so.

2. Voltage controlled main charge at 2.4V/cell. This mainline charge will
   top the battery off. However at 14.4V the battery will gas with continual
   exposure. This is real bad for sealed gel cells. Common wisdom states to
   continue this charge until the battery is drawing less than C/100 current
   which for my battery is 330 mA. One task I need to work out is how to
   measure the amount of current drawn. I figure I can amply the voltage
   drop across the resistor in step 1.

3. Trickle charge can be applied indefinitly at 2.3V/cell.

Note that point 1 isn't required but will get the bulk part of the charge
done faster.

Hope this helps,

BAJ

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