This is fascinating info! I've bookmarked the link the link to this thread.
1) Average Battery Cycle Efficiency = (20.3/20.7) * 100% = 98%. Looks good.
But, does this figure fall as battery degradation happens? Or maybe it stays pretty constant but the capacity simply drops a bit?
2) Cumulative Charge Cycles done = 20700/120 = 172.5.
3) Lifetime Check.
... What's gone wrong? ...Would make sense, as Regen charging's no different to mains charging, and is cycling hence degrading the battery (v slowly!) in exactly the same way.
I'm always cautious of posting things like this because from experience it's usually ignored and my time is wasted. But I like analysing this stuff and that's partly why I bought an EV. I don't mind explaining it to anyone interested. Many things can be determined from those four
cumulative Ah and kWh values, but you have to work them out yourself, as the BMS does. The BMS of course uses this info to figure out some of the things it needs.
As an analogy to the battery pack, if you imagine the battery to be a 100 litre tank of water attached to a pump/turbine via a restrictive pipe, you'll see more intuitively how this works. The tank has a "full" mark (100% SOC), an empty mark (0% SOC) and a starting level somewhere in-between, let's just say at the 52.5 L mark (SOCo = 52.5%).
We're going to assume that the pump is perfectly efficient in both directions, in other words 100% of the energy going in or out results in an exact equivalent pressure head change to the flow of water. We also have a flow sensor that can measure water volume as it passes in or out of the tank.
If you measure the water volume flowing in and out separately (Ah) you know that every time you pass the original water level at 52.5 L those numbers
will be the same because water is
not lost while in the tank. We'll assume of course that the water doesn't doesn't evaporate. "Coulombs" (Ah) doesn't either in a battery (at least as best as I'm aware!)
If you use the pump to raise the level from empty to full (adding up the energy needed to pump it in) and then exploit that water pressure to generate power (adding up the energy gained) as the water level drops back to empty, you can be sure sure that you will get
less energy
out than you put
in because of losses in the intermediate restrictive pipe. The equivalent for battery pack losses is its internal resistance, which gets worse as it degrades.
Since the four cumulative values were zeroed at the same time (at the factory, or later after a BMS software update) you know that whenever the water level passes 52.5 L (SOCo) the ratio of the cumulative energy
gained at the pump as water drains out to the cumulative energy
lost required to pump the water into the tank represents the
energy storage efficiency of the water tank. If our restrictive pipe was not present the efficiency would be 100%. See the first image below.
Technically you have to measure storage cycle efficiency of the tank (or battery) when it is at 52.5 L (SOCo), otherwise your result is in error by the difference in the current water level (charge level) from that when the values were all zero.
My Kona returned 98% efficiency some time ago, now 97.8%, see image. Note that my SOCo is 52.5%, a coincidence, lol! The 96% above is the first time I've seen anyone post values for an "older" EV and that seems to indicate that it does drop over time, and that's entirely expected based on Li-po datasheets.
Regarding your driving efficiency, since it's impractical to start and stop at the same exact SOC it's better to only look at the
change in values over a trip of known distance. Read those four values at the start of a trip and again at the end. The net energy used is
ΔCED - 0.98 x ΔCEC where 0.98 is your determined battery cycle efficiency applied to the regen. The net coulombs used is simply
ΔCDC - ΔCCC.
If you carry out such a test over the majority of the batteries capacity you'll be able to extrapolate out an accurate estimate of that capacity, see the second image.
In this graph the
slope of the black
trendlines of Ah and kWh represent an
extrapolation to 1, or in other words to 100% of the battery capacity, advertised at 180 Ah and 64 kWh. The two thin black trendlines are sitting over the blue and red data points so closely you can barely make them out. This also demonstrated that SOC is more closely aligned to change in Ah than kWh. So, range diminishes faster at lower SOC.
Regarding cell voltage balance I'll note that the car measures this without loading the traction battery, not the case when you're using an OBD2 app. So, it's not entirely accurate but close. Another note is that the cell voltage values have a granularity of 0.02 V and that is almost certainly a truncated value meaning 4.140 could actually be 4.159. The average cell voltage on that app is simply the
pack voltage divided by the number of series groups (88) and is
much more accurate on a per-cell basis. The Kona cells top out at 4.156 V I think and it seems higher (and risker) than your Ioniq at 4.147.
