Lane, a boosting switcher would be easiest to use working with the
battery voltage and raising it to 13.8. Anything linear won't boost, and
will be only 52% efficient dropping from two batteries to 13.8.
Maxim makes lots of chips just made for controlling such a small boost
regulator. Using a FET, the current can be most anything you want though
switching supplies tend to be picky about minimum loads, though you can
get to lower currents with regulation by increasing the size of the
inductor that's invariably involved.
Otherwise you could try to raise the full charged specific gravity of
the batteries to get their full charge voltage up to 13.5 or more.
Typically a full charged sg of 1280 corresponds to about 13.8 volts. Are
you sure its the battery that limits the full charge voltage? Otherwise
raising the full charge sg would take replacing some of the electrolyte
with pure acid... Or replacing it all with 1280 sg mix.
73, Jerry, K0CQ
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