There is such an animal and its known as a Battery Monitor. They start around $250 and go up to $800+ depending on the features. Or you could learn how to use a digital voltmeter for around $19.95. The reason I say "learn how to use" is that you would need to compensate for some inherent defficiencies in the readings. A lead acid battery is considered 100% charged when the voltage reads 12.67V. This reading needs to be taken with the battery at rest for at least two hours, preferably much longer. A battery being charged will show a higher voltage reading than actual and a battery under load will show a lower voltage reading than actual. We seldom allow a battery to be at rest so it is hard to get a meaningful reading but you can learn to observe, compare and estimate readings and those readings can then provide useful information about the state of your charger and batteries. I'm not knocking the monitors as they do serve a valuable function when running expensive battery banks and systems which require close monitoring but for the usual boater using 2 group 27 lead acid batteries a voltmeter can be of great help. It can easily tell you if your charger is working but not so good at estimating how much capacity may left at a particular moment.
