It could be a couple of factors. When a battery becomes sulfated, it becomes more and more difficult to charge. Also, any "extra", such as a blower motor for a/c, shares some of the recharging power. If your alternator isn't up to par, it could be barely charging the battery as well.

It would be a good idea to check charging voltages. With a meter across the battery terminals, check both DC and AC voltages. The DC voltage should be ~13V and AC should be <100mV. If DC is low, the voltage regulator is suspect. If AC is high, the rectifier diodes are suspect.

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )