The part(s) or condition(s) listed below for the symptom Small engine battery keeps draining are ordered from most likely to least likely to occur. Check or test each item, starting with the items at the top of the page.
While the engine is running, the voltage regulator sends the proper amount of voltage from the alternator to the battery to keep the battery charged. If the voltage regulator is defective, the battery won't receive enough voltage, causing the battery to drain quickly.
To determine if the alternator is defective, use a multimeter to test the alternator output.