Testing an AGM battery

take it into many auto elecs or battery distributers and they will fully charge the battery then put it onto a load tester probably a 20ah tester that simulates 20 amps till the battery reaches 9.00 volts then tells you haow many minutes the battery lasted should be about 250 for a 100 ah deep cycle.
where are you located?

Hello Michael,

since you already know that it doesn't store a lot of charge, you could try to make it perform better.

One way of checking its performance is to wire it in place of your cranking battery, and turn on the high beam.
If the battery is good and fully charged the lights should stay bright for 3~4 hours.

But before this, check the open circuit voltage 24hrs after charging. Voltmeter should read 12.8 to 13.2V.

If the voltage is ok, then you can try the high beam discharge test.
Leave the voltmeter connected during this and terminate the test when the voltage under load has come down to 10~10.5V.
Note the times.
Then, put the battery back on the charger for another 24 hours and repeat the test. You want to see an improvement. If there's none dispose of your battery in a responsible manner and get a new one.

Of course, there is a remote chance that your charger isn't up to the task, so check the voltages at some stages during charging and report back.
Recommended charger has the following specs: three stage, boost setting 14.4 to 14.8V, float setting 13.6 to 13.8V, max charging current between 10 and 25 amps.

Best regards, Peter