markcx avatar image
markcx asked

125Ah AGM Super Cycle tail current


I am using a BMV-712 to monitor my 125Ah AGM Super Cycle battery and charging with a BlueSmart IP67 25A charger.

The BMV is telling me that the battery is at 100% when the charger is still in the absorption stage.

My current BMV settings are:

Charged voltage: 13.2V

Tail current: 3%

Charge detection time: 10m

Peukert exponent: 1.14

Charge efficiency factor: 95%

Please can someone tell me if these are correct or what I should change them to. Datasheets and manuals don’t seem to have all the answers.


battery chargingBMV Battery MonitorAGM Battery
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2 Answers
JohnC avatar image
JohnC answered ·

Hi Mark.

Tuning a BMV correctly can be seen as an onerous task. But the only way..

If it's overcalculating SOC, then it's to do with Charge Efficiency being set too high, or Peukert too low. With AGM's, try Peukert at 1.25 as a start point. If that's too much, and the SOC drifts downward, then find a median point. Or even go further..

Drift of some minor magnitude may always remain, so you 'sync' it to 100% when you know it's fully charged (a return-to-base thing). For me, that would be an acceptance of charge (the *tail*) of about 1% of Float V, but only when above the 'Charged V' setting. You may find 13.8V and 2% tail ok, but only really for you to determine. This is just for syncing..

And if these figures suggest to you that your Pb's aren't as efficient as you'd like them to be, take a deep breath (ie. suck it up). VRM reports mine as 76% overall. Breath taken long ago.

Understanding your BMV is the key. That's the bit to work on, and ask again if you need more help to get on top of it. Good luck, don't give up..

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doug-morford avatar image
doug-morford answered ·

I don't agree with all of what JohnC said, but the general message to tune and play with the settings is absolutely correct.

My strategy was to discharge my batteries to as close to 50% discharged as possible. The consumed Ah reading is helpful when doing this, but the resting battery voltage being what it should be at 50% is what's important, as the Ah rating of a battery varies throughout its life. For me, I was shooting for around 12.2 - 12.25 volts. Resting means no load for a half hour. When you get them at 50%, play with your Peukert till the capacity on the BMV reads 50%. The temp compensation matters here too, but it would be hard to dial it in like this. My battery's manual showed % of full capacity at different temperatures. But the BMV setting wants change in % capacity per degree of temperature change. You'll have to do some math to make the conversion.

To make the increase in capacity correct as the battery is being charged, you need to play with the charge efficiency. Hint: the default 95% is bs. It's not that high. This one takes some tinkering through multiple discharge and charges.

To make the bmv reset at a reasonable point, set the charged V to just below the voltage where the charger goes from absorption to float. I set mine to 12.5v. Also set your float % (% of battery capacity) to just above what your charger is set to. This might be difficult because you probably have two chargers: solar and AC. One will have a lower float. Find out in the settings or by watching it as it switches from absorption to float. Set your bmv float to just above this.

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Related Resources

Victron BMV battery monitors product page

VictronConnect VE.Bus charging manual  

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