I was charging my car battery for an hour almost and charger still blinks same light and it even discharged even more while charging.
I have a working generator, i plug the charger first, otherwise there is no option to plug in (with battery already attached). When i plug the charger, i place it on the ground by holding mouse one. Then i attach battery, then i start generator, switch on charger and its light start to blink. Then i though, maybe it can only charge complete empty battery's, cause mine was half empty at the start. Yes, it was half full or even more than half and during all this process (taking it out of the car, carrying it to the charger and during charge) it lost charges, i think it was discharging while it was in my inventory as-well. OK so I drained it completely with a car (was left only 7-10% of those 50%) and then it started to charge finally, but only till the level that was before putting it back into the car, charged to those 7-10% and stopped.

Car battery, spark plug, battery charger, power generator - all these items were in pristine condition.
Before this car battery was removed from front-crashed car but still in pristine condition.