Measuring battery condition

If you install a solar system with a display, during daylight hours you can tell the SOC by the amount of current the batteries are accepting.

It depends on the type of battery, and few methods work for all types of batteries.

You can't measure SOC with any accuracy using a voltage measurement on a Lithium-Ion battery.

You can't measure SOC using specific gravity on a sealed battery.

But if you start with what you know is a fully charged battery and use a permanently attached coulometer to measure the power in and power out, it will give you a pretty good idea of your battery's SOC at any point in time.

best SOC indicator is measuring specific gravity with a calibrated hydrometer with temperature compensation. Look for a Frease (sp?) brand hydrometer.

The cheap ones in the auto parts store are hit and miss

If you want to know battery condition then after fully charging the battery to a load test.

There is the right way (install a proper voltmeter), and then there is my way -- I disconnect the battery. Let it sit for 15 min. Take out my little multimeter and hook it up.

They say you need to leave the battery resting for quite a while to get an accurate reading. But I tried taking a resting reading after 5 min, 10 min, 15 min, 30 min., etc., and found that the reading was very close to correct after only 15 min.

If you try to take a reading when the battery is connected to your RV, you will probably get an artificially low reading. It'll say 12.2 volts when it is really 12.5, for example.