There are plenty of posts on measuring voltage after the batteries sit for hours or overnight to determine charge. Better is to buy a glass hydrometer and check each cell for charge and variation. Excessive variation indicates the need for better charging or cell deterioration.
You make good usage of your batteries. Recommend you buy a digital voltmeter so that you can accurately check the battery voltage at their terminals. $10, sometimes free at Harbor Freight. A ammeter would also be a good investment as you could then measure the amp draw/charge. There have been posts on inexpensive battery monitors that you could consider.
