Today we motored for 5 hours so going to pay close attention to see how it behaves. 30 minutes after stopping the motor we're at 12.97 volts. 45 minutes after shut down of motor we're at 12.9 v. After 2 hours, of motoring today the amp hour was at zero, meaning it had charged the 144 AH I had used over the prior day. After that, it was still charging at 30 amps for hours 3 thru 5 for the day. It could be like you suggest. Maybe I was assuming that the AH charged, when it zeroed out (not showing any negative number) was fully charged. But maybe zeroed out really isn't necessarily fully charged
. I hour after shut down volts are at 12.89. Maybe even though the BMV702 says 100%, perhaps the battery is not really fully charged, hence the lack of voltage holding longevity
I have a Victron BMV-712, but my understanding is the below detail applies to both - my manual includes both.
You have to watch the BMV-702 and what it says regarding state of charge. If you read close in the manual, the BMV-702 does not reset based on amp hours back into the batteries, but rather on the number of charging amps going into the batteries as a charge rate. There is a setting for this number, and what it will do is reset itself to 100% when the batteries still need charge, but the number of amps charging is at a set percentage. That set percentage is a function of the set battery bank size, so its important to ensure that setting is correct as well. I rely more on an actual reading of amps in (for example, when running my generator I look to see if my charger is bulk charging, absorb charging and what the number of amps are. I usually time it so I'm considering the batteries as charged as I can practically get around the time the AC charger is going to or switches to float). This could also interface with how your alternator's regulator is programmed to charge the batteries. It also could be that when you are sitting at idle the charge amps are low enough for long enough it will reset the charge state on the Victron.
Below is some text copied from the manual. The tail current is the charging amp threshold that resets the Ah percentage and Ah counter, and the charged detection time is how long that voltage must be detected before doing those resets:
03. Tail current
Once the charge current has dropped to less than the set tail current (expressed as
percentage of the battery capacity), the battery is considered as fully charged.
Remark:
Some battery chargers stop charging when the current drops below a set threshold. The tail current must
be set higher than this threshold.
Default Range Step size
4% 0,5 – 10% 0,1%
04. Charged detection time
This is the time the charged-parameters (Charged Voltage and Tail Current) must be met
in order to consider the battery fully charged.
Default Range Step size
3 minutes 1 – 50 minutes 1 minute
Hope this helps!