Tjaart;

I didn't read the whole thing, but got the gist.  I am usually a 'put a PIC
in anything' guy, but my current limit was real simple.  A TIP family part
regulated the high side voltage into the battery, and in the low side of the
battery you put a 10 ohm resistor, prepare to try different values.  Then a
small NPN is driven across the 10 ohmer with the base on the high side of the
10 ohm, the emitter going to ground like the low side of the reisistor, and
the collector going to the base of the high side TIP device.  When the
current goes too high, the NPN turns on, and the TIP rolls off.  You get a
close but imperfect box on the VI curve.  Then, the op amp that drives the
base of the TIP device must have some resistance inserted to protect the
opamp.  THEN you have to do one more thing.  When the battery is relied upon
for full current, the 10 ohm resistor will toast.  Place a 1n4001 in reverse
across the 10 ohm, and it will pass the battery load when it is necessary.
Um, that's 3 parts, and it works fairly well, limitting the fast charge then
letting the voltage regulation come into play for float.

Chris Eddy
Pioneer Microsystems, Inc.

Tjaart van der Walt wrote:

> Hi there
>
> I've been playing with all kinds of circuits to limit the current when
> charging a flat lead acid battery. I've been able to construct a 'dumb'