Charge Efficiency on BMV only settable in 1%-steps is not enough

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brainy asked Oct 22, '19

Charge Efficiency on BMV only settable in 1%-steps is not enough

Hi there, i have a BMV and encounter a problem with charge efficiency. Peukert is set to 1.00 (i have a LiFePO4 Battery with 100AH) and during several weeks the SOC drifts extremly (SOC was shown 78% and i put on the charger but the battery was already full and then it synchronizied) ... the Problem is that if you only have partial charging (over some days/weeks) the SOC will drift away heavily. So i wish that i could set the Charge Efficiency in the BMV to 99.5% or something to have more accurate values.

battery charging BMV Battery Monitor

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Epicycler commented · Nov 08, 2019 at 11:07 PM

I have the same problem.

actual charge / discharge efficiency is also a function of charge / discharge current, the higher the amps the lower the efficiency. At any SOC the coulombic efficiency is the ratio of discharge volts to charge volts. Its difficult to find a single value for efficiency that will allow SOC tracking to work accurately all the time. On cloudy days charging occurs slowly, sunny days the battery is full before breakfast.

Peukert should not effect SOC. Long dry argument but if battery is discharged at high amps and volts is used as proxy for SOC the battery may appear empty. But wait a little and voltage recovers so discharge can continue at lower amps. I hope Victron don't include Peukert in the SOC calculation. Peukert is only really useful for "time to go" assuming discharge is at constant current.

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Answer 1 · 2019-11-07T03:50:07Z

fantail answered · Nov 07, 2019 at 03:50 AM

for a li battery the settings are

register 1 Stack Capacity in A/h ( must be correct for your battery capacity)

otherwise your SOC will otherwise drift high or low.

note if you have a bms on the batteries which is also giving an SOC value these can also drift at low loads leading to inaccurate readings. The BMV will always be more accurate if correctly setup. But the BMV can not account for aging of the batteries as they lose capacity over time.

register 2 charge voltage at which SOC is reset to 100%

register 3 tail current ( time that charge voltage has to be true before SOC is reset)

register 5 peukert for Li is 105

register 6 Charge efficiency 99%

turn all loads off with only the BMV running, then reset the zero calibration value register 9

if this is wrong the BMV will continue to show a current reading when everything is off.

hope this helps

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Answer 2 · 2019-11-10T08:47:48Z

fantail answered · Nov 10, 2019 at 08:47 AM

The charge or bus voltage is only used as a reset point for to 100% SOC to reduce SOC drifting.

The manual explains how the SOC is calculated and gives the formulas.

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