Confusion regarding battery capacity

[ishareit]

Part of the battery is reserved to prevent complete discharge which would likely destroy the battery. You have no access to that part of the battery. You will never get the nominal battery rating for your use. If the battery is at 100% regen is not possible and you better be ready with the brakes to stop the car.

Isn't that the reason why there is only 72.6 Kwh usable? Or am I understanding it wrong?

That's how I understand it as well...

 

[ishareit]

Maybe @wk057 or @BigD0g can help explain how these trip stats are calculated...

 

[cezdoc]

Sorry, but rounding errors with what? Even if I assume that the remaining battery in my case was 6% (rather than 5%), the usable capacity is still 67 kWh, not 72.6 kWh. Also, assuming that the trip stats factor in any regen, the usable capacity gets even worse. 63 kW battery used with regen would be less than 63 kW without regen, no?

An explanation of the trip rounding error from @wk057 's usable battery capacity thread is here:

"Additionally, the trip meters are handled entirely by the MCU (CID). They rely on just watching the BMS data and constantly computing the used power. I've found it to almost always underestimate power usage due to missing small amounts of data. You can prove this by rebooting the CID while driving and checking the trip meters. You'll notice they don't update for some time during the reboot."​

As far as regen goes I got my head around it by doing an example calculation: imagine you have a battery with exactly 100 kWh available at 100% and use 10 kWh on a journey but also generate 5 kWh of regen. The car trip meter can only measure energy as it leaves/enters the battery so calculates net 5 kWh used. But the battery recharging isn't 100% efficient - let's assume of the 5 kWh sent back to the battery, 20% is lost during re-charging so it only adds 4 kWh taking the state of charge back to 94 kWh / 94% at the end of the trip. So in this example trip the energy used is 5 kWh for the battery going from 100% -> 94% (-6%), and the "usable" capacity from the trip would then only be 5 kWh / 6% = 83.3 kWh. In reality the charging efficiency is better but in general you can see that any inefficiency in recharging from regen will result in a lower apparent battery capacity calculated from the trip. I hope that makes sense!

 

[murphyS90D]

The motor is a 3 phase AC motor. During regen that is what is generated. It must be rectified to DC to charge the battery. That is never close to 100% efficient.

 

[SuisseDriver]

So one explicattion given is that the measurement of the DC current from regen is higher than what is really store in the battery. Why not but even if I consider 10kWh of regen (which is humongus as my measurement was on mostly road and highway trip), and that you would lose 3% in the effective storage into the battery (the maximum number usually given for that), it would be 0,3kWh not measured. We are very far from the 7kWh of difference.

Also, we know that 0% is not 0% but that is why we are supposed to have 72,6kWh instead of 75kWh.

It seems that nobody has a clue why we have an effective capacity usable roughly 12-13% lower than the rated capacity. That's super weird.

 

[animorph]

Too many engineers here!

I think the accuracy of the actual battery ratings (not the badge ratings) should be very close.

Trying to integrate instantaneous power into an estimated energy usage is probably fairly inaccurate. Especially if it is just sampling and prone to skipping samples when the MCU get busy. We have no reason to believe this number is more accurate than the battery rating. And yet nearly all complaints on this subject are along the lines of "why doesn't my battery have the XXX kWh Tesla said it had?" instead of "why is the trip kWh so inaccurate?".

I haven't seen any of the 1000 existing previous threads discussing this discrepancy between the trip calcs and the battery rating come to a clear conclusion. The two numbers don't match. The trip calc is always significantly lower. For whatever reason, the trip calc misses some of the energy being used.

For those interested in this problem, please research past threads. There's no lack of them.

If you're really interested, do some more testing. Is AP energy usage being captured? MCU energy? Do a trip without regen to see what that does. Does frequent high acceleration impact the trip calc accuracy? Temperature can affect battery capacity. Maybe the trip calc doesn't see power loss due to the internal resistance of the battery (more accurate at lower power usage than high in that case). Do some experiments and let us know the results.

 

[SuisseDriver]

Nowhere are we speaking of instantaneous power.
The trip meter is reporting a capacity usage which is 13% lower than the rated capacity. That is not just a small error but something massive.

I understand that it is as it is. My issue is more than I am using a lot ABetterRoutePlanner for my trips. And I cannot really enter a 110km/h consumption based on the car info because ABRP is probably considering 72.7kWh of capacity and not 65kWh so you have to use different numbers which are not the one you can see on the MCU...