Monitoring battery capacity

The cheap and easy way is to get an inexpensive digital voltmeter (roughly $20 for a decent one), then occasionally measure the voltage at the battery or at one of the 12v receptacles. You can also look for a 12v voltmeter that plugs directly into a 12v receptacle.

Fully charged, you should be at about 12.6 volts. Don't let the battery get below about 12.0 volts and you'll be OK.

If you want your batteries to last longer, paying attention to how they are charged will make a difference. Occasionally checking specific gravity and water levels is important.

If you want, you can make this really complex and expensive, with hundreds of dollars in chargers, gauges and meters - but in reality, watching the voltage and specific gravity are the two most important things to pay attention to, and $30 will get you everything you need to do both.

Some of those battery monitors are really slick. Had one on my boat that registered amps use, as they were used. Simply by turning the stereo up you could see a difference. When a thin cloud went across the sun you could see a charge rate difference. Neat toy.

For the basics of a trailer an inexpensive but decent voltmeter is a whole lot better than the stupid guages they come with.

There is only one answer...Trimetric 2025RV.....Will give you way more info on your batteries than you might even want.

What you are asking for is something that acts like a fuel gauge or oil dip stick.

Perhaps you could look at these Trimetric Manuals.

We all worry about battery life. It is important to use good charging procedures because that will make a bigger impact on battery life.

HTH;
John

On my 'to buy' list is a Trimetric 2025 monitor. This unit is recommended by several solar experts as the best monitor available. Not really cheap, but will give you all the info you want. Several solar people think you should have one even if you've installed a solar charger as their monitoring capabilities aren't as good.