So basically a battery can be "charged" meaning full and it is temperature dependent and a few twists but basically to determine the state of charge - battery voltage can be used. See the attached chart for a sample.
To get amps into a battery any charging
source must raise voltage above the current
battery voltage level. So let's say that's 14 volts. This energy transfer creates heat due to resistance and that heat creates more resistance. Therein lies the need to taper off the charge to get the last amps in.
When removed from charge the battery takes awhile for the voltage to stabilize and drop down to what is considered a "full" charge.
Now here is the confusing part. Over repeated cycles the battery plate material falls off and collects on the bottom of the battery and the battery just can't hold as many amps (it is wearing out) - the voltage still comes up to "full charge" but if we use the water
jug analogy - the jug has sediment in the bottom and can't hold as much water
as it used to.
A battery monitor
will measure voltage and "state of charge" but there are some very complex things going on inside the battery which in essence is a little chemical reactor. So the battery monitor needs to be "programmed" and if not programmed and periodically calibrated it can have great error.
There is a new monitor called SmartGauge - our resident and respected expert MaineSail has independently tested the SmartGauge and it basically handles all the errors correctly and automatically and can tell you the state of charge!
I would readily recommend you get this gauge. What it does not do unfortunately is measure amps in and amps out like a "traditional" battery monitor. I think it is a big miss on this monitor. While essentially amps in and out don't matter in this new gauge (prior monitors used amp counting as the SOC measure) I like to know what's going in or out to monitor the charging
What no monitor yet made can really tell you is how much of your capacity has been lost
. As you live with a bank you will get a sense of how long it should last before charging. Eventually you will notice they "don't hold a charge" or "discharge quickly" - this is just saying that the capacity is worn out.
Batteries can last anywhere from a year (poorly treated batteries) to 10 years plus.