I have noticed that my CZero seems to use more amps while discharging than it stores in the battery while charging. This must be wrong because it would eventually lead to negative values for the Ah remaining in the battery. At the same time, a possible error in the amps measurement is mentioned on the forum. I found what I believe is the amps error using a continuous data set for a trip involving driving, slow charging and fast charging. This trip is shown in the figure below.
Figure: A trip of about 200 km with both slow and fast charging.
The data shown in the figure was collected with the OBDZero app. The app computed the battery amps at 4 to 5 second intervals using the formula provided on the forum by garygid:
PID 373 D3,D4 as (((D3 * 256) + D4) - 32768) * 0.01 = pack amps In
Note that I chose amps in rather than amps out with a corresponding change from -0.01 to 0.01
In Excel the data was sorted by date and time then the precise time step in hours between each measure¬ment set was computed. The amps were multiplied by the time step to obtain the Ah for each time step using this formula:
Ah = (Amps0 + Amps1)*h/2
Where Amp0 is the amps in one measurement and Amp1 is the amps in the next measurement, the time step h later. Then the accumulated Ah were computed as a running sum starting with 100% = 37.3 Ah. 37.3 Ah was the battery capacity shown by the EvBatMon on that day. The result is shown as the blue line in the figure. The downward trend in Ah compared to the SoC is easy to see.
By simply adding 0.66 amps to each measurement I was able to move the blue line so the two lines were on top of each other.
Figure: The accumulated Ah after adding 0.66 amps to each amps measurement
As the amps are negative while discharging and positive while charging adding 0.66 effectively reduces the amps used while discharging and increases the amps stored. This balances the budget. Therefore adding 0.66 amps calibrates the amps measurement at least on my CZero.