Best way to tell how much life a battery has left?

[Kevin and Sara]

I'm referring to a 12V battery and Goal Zero. Is there a way to test each of these and see how much life they have left?

 

[smritte]

The easy answer is yes.

The long answer is,
To test either battery accurately, you need to charge them properly and test each cell. I use to be able to do that with a hydometer on a lead acid. Sealed and AGM,s made that test go away. Now we do a capacitance tester. The real nice ones are fairly accurate, the cheap ones....ehh
On the lithium, "if" the BMS has Bluetooth, you can get a reading off each cell. Otherwise you need to take them out of the case and test individually.

 

[JohnSil]