OK, I will try as you suggest. You said something about watching the
polarity across the output of the charger -- what exactly did you mean?

The plus output of the charger goes to the plus terminal of the battery.

I urge you NOT to do this. There is no guarantee this charger will correctly
charge your battery. Find someone knowledgeable to help you. Or pay a
service shop to charge the battery. Or buy a new, working charger.