1991 Questions for all other cars electrical problem

Electrical problem
1991 Questions for all other cars 4 cyl Front Wheel Drive Manual 50 000 miles

hi there,
it's about yugo 45 (900cc) based on fiat 127. Here's the problem: when car is at idle 850 rpm, and whenever I put on brakes or the radiator fan starts blowing, the idle decreases to 600 rpm and it starts to misfire. There are 4 x 21 watt bulbs for brake lights. Also while I cruise, and turn on the headlights the car immediately loses a slight power I can feel it. Can it be due to a bad battery? Because in summer after a long journey it doesn't hold the charge and it won't start the engine (no noise coming from the starter but the control lights fade). I don't face these problems in winter. The battery voltage reads 12.45 all the time and it starts to drop gradually as I turn on any car accessory. Can this affect the ignition coil? Sometimes car runs smooth as silk. I'll go crazy. Thnx a lot.