MM>  WC>> with the stuff all the time. Downloaded a 39.4 Meg. PDF of
MM>  WC>> Digikey's catalog last night to find some hefty stud type
MM>  WC>> rectifiers to isolate the UPS charger from external battery.

MM> I wouldn't go the "diode route", and have to put up with
the voltage drop.
MM> use a relay and wire it so energized is "open".

MM> When the power goes off it closes and connects both batteries to the UPS.

MM> When the power returns, the relay opens and isolates the 'spare' battery fr
MM> the UPS charger circuit.

MM> Additionally, to save the frazzles recharging the 'spare' battery, you coul
MM> connect a 50 Ohm 10W resistor across the _open_ contacts, and that would al
MM> ~240ma to flow from the UPS charger to the spare battery when the AC power
MM> up and the relay is open.  I don't think a ~240ma load would damage the UPS
MM> charging circuit, and it would eventually charge the 'spare' and keep it to
MM> off.  When the voltage on both batteries is near equal, there will no longe
MM> any current through the resistor.

MM> I=E/R, (.240)I=(12)E/(50)R

 Great ideas.

 For a faster charge, you could put a separate charger
on the extra batteries. When the power goes OFF, so
does the charger. When the charger comes back ON, the
relay is opened again.

 TTYL.

 Robert

 * SLMR 2.1a * My hard drive lost its FAT on a crash diet.
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