23 Feb 2003, 13:39, Wayne Chirnside (1:275/311), wrote to Roy J. Tellason:

Hi Wayne.

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

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

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

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

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

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


     Good luck...  M.

--- Msged/386 TE 06 (pre)