Actually, there is a method of determining the level of charge of a sealed-top battery by monitoring the open circuit voltage at the terminals. Since the battery is the final voltage control, even more so that the "voltage regulator", it is pretty important to know whether it's up to par. A slightly shorted battery with one cell unable to come to full charge can create all sorts of charging system gremlins, and that situation can be extremely hard to diagnose. I've seen plenty of examples of multiple generator/alternator/regulator changes being done before a vehicle got to my shop, when a simple hydrometer test of the battery was all that was necessary to find the problem- - - -one bad cell in the battery!
Jerry
