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Boondocking with BMV-712: SOC error? Best practices?

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!

BMV Battery Monitorcharging batteryabsorptioncharge controller errorfloat
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1 Answer
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ben answered ·

Hi,

I also boondock a lot, just like you. You have made a good analysis.

It is true that lead acid doesn’t like deep discharge, and it doesn’t like not getting back to fully absorbed either.

And it is also true that absorption takes a long, slow time, which makes it quite difficult with a pure solar setup unless you have a large ratio of solar to bank capacity. (It’s also hard with a generator, unless you like listening to it run all night.)

The above are facts independent of whatever state estimation you choose to do with your BMV. So you need to size your system appropriately, or plan for more frequent replacement of your batteries, or at least plan for the possibility. (Or you could upgrade one day to LiFePO4, which loves not reaching a 100% charge.)

For the BMV, you are on the right track: you can tune the parameters to alter where it “syncs” to 100% and/or adjust the capacity so that it remaps your range to something you understand, living in your rig.

You can configure it so that synchronization happens at the true 100% for your battery, either by adjusting the absorption voltage and/or the tail current threshold.

Keep in mind that the BMV will always just be an approximation, and it will drift further off the longer you go between 100% resyncs. That is just the nature of these devices.

Both the BMV and the battery will be happier if you get them to 100% more often. Whether that matters much just depends on your preferences (and maybe your budget). Good luck!

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