If you want to measure energy capacity you can use the built-in counters to determine the change in energy and coulombs drawn divided by the change in SoC from 100% to as low as you're comfortable going, certainly under 15% at least.

Use the SoC (displayed, not BMS) reading over OBD because that represents what's made available to the driver, plus the additional decimal point over the dash reading.

Take all 5 readings at 100% SoC and later when you end the test. To account for regen I use a battery cycle efficiency factor of 0.98 as that's what I've measure it to be on my Kona. SoC values of course should be entered as e.g. 1.00 and 0.15, or use percent and factor them later as I did.

The calcs are:

Estimated Battery Energy capacity = (ΔCED - 0.98 x ΔCEC) / ΔSoC (kWh)

Estimated Battery Coulomb capacity = (ΔCDC - ΔCCC) / ΔSoC (Ah)

... where the the energy capacity divided by the advertised capacity is the SoH.

For what it's worth I tested this on my Kona twice at around 20,000 km with values close to the advertised value of 180 Ah and 64 kWh, 180.5 and 63.9 kWh in this graph. I log the data during the test but that's not necessary and was only to check linearity of coulombs v.s. energy to the two SoC readings.

 

If you want to measure energy capacity you can use the built-in counters to determine the change in energy and coulombs drawn divided by the change in SoC from 100% to as low as you're comfortable going, certainly under 15% at least.

View attachment 56074

Use the SoC (displayed, not BMS) reading over OBD because that represents what's made available to the driver, plus the additional decimal point over the dash reading.

Take all 5 readings at 100% SoC and later when you end the test. To account for regen I use a battery cycle efficiency factor of 0.98 as that's what I've measure it to be on my Kona. SoC values of course should be entered as e.g. 1.00 and 0.15, or use percent and factor them later as I did.

The calcs are:

Estimated Battery Energy capacity = (ΔCED - 0.98 x ΔCEC) / ΔSoC (kWh)

Estimated Battery Coulomb capacity = (ΔCDC - ΔCCC) / ΔSoC (Ah)

... where the the energy capacity divided by the advertised capacity is the SoH.

For what it's worth I tested this on my Kona twice at around 20,000 km with values close to the advertised value of 180 Ah and 64 kWh, 180.5 and 63.9 kWh in this graph. I log the data during the test but that's not necessary and was only to check linearity of coulombs v.s. energy to the two SoC readings.

View attachment 56075

Thanks. I'll give that a shot once I have the numbers. But why wouldn't number 2 above give you an accurate picture of stored energy?? (OBD2 - [BMS] Remaining Energy at 100% is reading 74.1 kWh (+/- .2 kWh))

 

Well, to answer that you might ask yourself what do we know about the register for "Remaining Energy at 100%"? Does that go down to 0% or does it have some safety margin? Does it take temperature or any other factors into account? We are unaware of the calculations that go into the number displayed and we can't assume that the app publisher made any effort to validate any applicability that the parameter title might suggest.

I use the coulomb and energy counters because I believe they have the highest probability of providing the most accurate data for measuring battery capacity. The inputs to those can only be the instantaneous values of (a) current from the precision shunt and (b) pack voltage, over (c) a short time increment.