I know a "bad" starter can drain battery, but am unsure what constitutes "bad enough" to allow a starter to drain the battery. Suppose a hypothetical coolant leak has corroded one's starter. Would that cause a drain on the battery?
A 'bad' starter wont drain the battery while it is not being used to start the car, if that is what you mean. If the starter has a damaged commutator, it will not run at full power and so could mean that you have to turn the engine over longer to get the engine to start which will use more energy from the battery. Even if coolant is leaking on the starter, unless the starter is running there will be no electricity near the starter, so the water will not cause a short. The outward appearance of the starter should not be used to judge its internal condition. I would be surprised if you starter is draining your battery. If the starter starts the engine well, then it is more than likely not causing your problem.
Yes starter will drain battery n ruin battery n some time ruin alternator ..I have ran into this few times . Once cops mechanic kept replacing battery's n alternators when was starter frying them . I told parts store . They thanked me for clearing it up . With new starter 14 years later no more problem n few same many times. It will take out core in batterys first