APC UPS CS650. seeking help.

[flaotte]

APC UPS CS650. It was in working condition. I decided to add a connector to external battery.
During the process I connected battery wrong way. It stayed wrong polarity for 10-20sec, until I smelled something strange. The smell was going from the battery, not UPS board. No fuses blown.

On the board there is no visible damage. When I power it from mains it does not power up, does not charge battery.
On the other hand if it is powered from the battery, it works just fine. 

Connecting the battery wrong way or dis-reassembling process destroyed mains power circuit. It is not charging, also it has no idea there is power on mains and will not trigger bypass relay.

Does anyone has circuit diagram for this unit?
any ideas/tips how to proceed?
I am not very comfortable with high voltage, it would be nice to have a plan before I start poking random traces and points.

p.s. Active fan is(was) in my todo list too... Also it is 400w unit, my planned load is 30-50w, should not overhead with extra batteries.

[m k]

Charge is its own thing, but can have a protection something.

Measure with multimeter and start drawing.
Inverter parts have a heatsink, the separated and smaller one can be a charger.
Faulty charger circuit can pull the bypass switch power down.