I have the following setup: smaller 2015 Lance travel trailer with 2ea 12V lead acid flooded batteries in parallel, BMV-712 and the standard Honda 2kW generator. The trailer's battery charger seems modern (3-stage charging, lots of amps, etc.).
I plan to boon dock (no shore power) regularly and would like to tune the BMV-712 to help keep the batteries charged, healthy, and to know when and how long I should run the generator.
From research, I know I should always keep the batteries above 50% capacity to avoid premature replacement costs. Also, without many hours of charging (e.g., plugged in or on solar), I cannot expect battery capacity to rise above 90% while boon docking.
When charging from, for example, ~60%, the charger will be in the "bulk" mode with high current, and rising voltage. This charge mode is the quickest, but only gets to 80% capacity.
Then the charge mode changes to "absorption" where the charger voltage is held high (14.6v in my case) and then the amps fall off toward ~2amps (in my case) when the charge mode changes to "float" at which the battery capacity is at 90%. To charge from 90% to 100% capacity would take many hours and isn't reasonable while boon docking.
Therefore, while boon docking, I should expect to cycle the battery between 50% and 90% depending on loads and generator run-times.
However, AFAIK, the BMV-712 is designed to treat the 90% charge state as "100% SOC" (?) The BMV-712 decides that when the voltage is the high "absorption" mode, and then the current drops below a "tail current", it proclaims SOC = 100%. But in reality, the battery is at 90% capacity at that moment (i.e., at the end of the "absorption" mode).
If starting from a full-charge (i.e., batteries have been plugged in for a day), the SOC value will be fairly accurate. However, if not starting from a true full-charge (allowing the BMV-712 to auto-sync the 100% SOC value at the end of the absorption mode), the reported SOC value will be 10% lower than reality. So the accuracy of the reported SOC value depends on whether the previous charge completed the float mode? Is this true?
If that's true, then I must tweak other settings to make the SOC value match reality or add-in some fudge-factor or work-around.
I could:
- tell the BMV-712 that the "battery capacity" is 10% lower than the battery specification.
- set my "SOC relay (charge floor)" to 60% instead of 50%, and mentally never let the reported SOC value fall below 60%
Is this a known problem with the BMV-712 SOC value? What do others do to work-around this inaccuracy?
Do other boon dockers have tips on using the BMV-712 to stay confident that the generator is running for the right amount of time?
Thanks!
