Re: 12v Lead Acid Battery Charger aka I need to stop killing my batteries!

Kevin Walton formulated on Monday : 
Hi

I have killed a 12v lead acid battery, and would like some help
understanding why, and how to stop it:

I currently have a 'plug in the wall' style 12v lead acid battery
charger that I use to recharge a sealed 7ah 12v gel cell.

The cell is generally used one afternoon a week to start model
aircraft engines (i.e. high discharge rates for short periods) and is
left in a cold garage the rest of the time, usually on charge.  After
less than a years use the capacity of the cell is less than half that
of new.

The charger I have just researched a little more, no load it outputs
12v, loaded (ie charging the battery) the voltage is 15v.  The current
flow is a constant 400ma (all measured using a electric flight watt
meter) and does not change between a discharged battery and a fully
charged battery.  I opened up the wall charger and it is simply a
transformer and a rectifier - nothing else.

I don't understand the theory of above, how come a current is flowing
when attaching the battery 'appears' to raise the voltage across the
charger and the rest voltage of the cell is higher than the rest
voltage of the charger?

So, the reason I am killing the battery is that I routinely over
charge it for long period of time. Once I understand the theory above
I hope to add something to the output of the existing charger to stop
me killing the battery - be that turning it into a float charger by
limiting the voltage (so all I would need to do here is to add a 13.x
v regulator to the output?) or by something more sophisticated. Options welcomed.

Thanks in advance for the help :)

Cheers
Kev

I keep an automatic motoecycle battery charger in my flite box. never killed a battery that way

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