>I've glossed over a whole bunch of issues (mostly temperature
>compensation) but, in general, a 12V gel cell battery charges very nicely
>from that voltage.  It is important to limit charge current and I'm not
>real happy about the ~0.18V drop of a large sized schottky diode
>(increases the top-off time) but it does work.

I think that temperature compensation is a non-issue if the two batteries
are essentially at the same temperature. I also think that using a PFET
and an opamp as current direction sense and limiter would solve the 0.18V
voltage drop problem. I do not know how the final voltage of the SLA
compares to that of the car battery (different electrolyte
concentration/plate surface/state/current density etc). I think that one
can hope that the car battery has more voltage for a given current at the
same temperature on account of the plate surface ratio. I think that this
is a good idea. I had thought of using a small boost converter for this.

Peter

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