How OEMs calculate Battery Capacity, and why it's not totally their fault they are wrong.

For years now, savvy members of the EV community, especially the El Moto community have noticed that their usable battery capacity falls quite short of the manufacturers' labeled marketing and nominal battery capacity. I myself was quite irritated by the pure BS I saw from what I knew were the real numbers, and what was being marketed as nominal or "maximum" charge. These numbers were just plain old wrong, and confuse the whole market.



What if I told you this 14.4 is actually 11.6?


Well, it's time to lift up the rug and show why it is total BS, but not really the manufacturers' fault, per se. I say per se, because some of the math and variables are, shall we say, less than 100% accurate. Let's start with how to do the calculation according to the government for EV incentives. This part is actually very simple and also, very wrong. (Battery Max Voltage)*(Battery Amp Hours)=Pack capacity in watt hours (Wh)


Looks simple enough right? In fact it is dead wrong, and you can measure it yourself; in fact I encourage you to. Let's start with Zero batteries because these are the batteries I know the best. By no means is this tomfoolery limited to Zero though, being that the equation above is wrong to start with. I can understand a lot of the confusion around this matter. So I'll lay out the base constants here, 116.4vdc is the maximum voltage a Zero will charge to naturally. 128 Amp Hours is the theoretical max Ah this is based off of battery construction of 28S 4P, battery nerd speak. Each "battery brick" is 32 Amp Hours, multiply that by 4 (28S 4P) for 128Ah, now, Let's do the math:


(116.4 vdc)*(128 Ah)=14.9kWh ... wait what? That's not right.... until you look under the hood. So let me explain, Zero keeps their batteries in great shape by only charging the battery to 4.157 vdc per cell with 28 in series (4.157 vdc)* (28 cells in series) = 116.4 vdc maximum. That's the reality of it, however the cells don't really start to have immediate degradation until 4.25 vdc. This is the maximum possible to charge them to, which is not actually possible to do and ride the bike after because you will be over the controller's operating ability, but it is the number that is in the SevCon and theoretically possible to do, just not in a Zero. Lets add this back into the first equation : (119 vdc)*(128 Ah)=15.2kWh ... wait what? That's not right.... until you look under the hood deeper. Get way down in there.


Let's explain again, the claimed max Amp Hours on the batteries is 32Ah per "brick" (28s4p) however, the real max capacity of the bricks is 30.25 Ah which brings the max pack Ah down to 121 Ah. Lets add this back into the first equation : (119 vdc)*(121 Ah)=14.4kWh ... Thar she be. Now, lets move onto the described nominal which one would calculate based on the 50% SoC voltage of a battery pack times the Amp Hours again, just like the below equation :


(Battery 50% Voltage)*(Battery Amp Hours)=Nominal Pack capacity


So once again, the real 50% pack voltage and the settings in the controller for 50% pack voltage don't exactly line up. The real voltage at half capacity is 102vdc; the value in the controller based on charging the cells much higher than Zero does is 104vdc. Seems like a small difference, right? Lets check the math :


(104vdc not real)*(121Ah not real)=12.6kWh ... the advertised nominal value. (102vdc real)*(121Ah not real)=12.3kWh ... but wait, there's more... That is using the maximum Ah the cell is possible to hold when charged to a blustery 4.25vdc per cell instead of Zero's 4.157! Now, I happen to have made a lot of aftermarket parts for Zero in the past, and their BMS reports real Amp Hour capacity as 114 Ah for the new batteries, much more reasonable. Lets add this back into the equation : (102vdc real)*(114Ah real)=11.6kWh ... OMG, we finally have the usable capacity.


If you drain a Zero to 95vdc, which is the calculated 0%, and charge it up to the full 116.4vdc this should be the amount that you get. Try it and let me know if you can stuff that much in there! Make sure you measure your starting and stopping voltage so you know it is drained to 95 volts and topped up to 116.4 volts.



Math, how does that work?

So I know Zero batteries really well, and all 3 of the production electric motorcycle manufacturers publish their Max and Nominal capacities. The interesting thing you will notice about the capacities below is that the percentage skew between the advertised max and advertised nominal is just about the same for each manufacturer. Check out this table :


So in the above table we added in an estimated Actual capacity for Energica and Harley based on the match above and we see actual capacities of the motorcycles. As much as I want to call BS on this, what really matters is that all the OEMs are calculating battery capacity wrong in the same way so they are comparable. Given the similarity of the % difference from Max to Nominal, I would say basic logic states that this is true, so it's not that bad. On top of this, the calculations, while possibly using disingenuous values, are based upon a set of government rules, which are wrong. Physics changes from time to time but this isn't one of those times. I can't fault any manufacturer for doing what the government forces them to do, and since the percentages are so similar it seems they are all doing the same funky calculations. At least we have consistency on our side as consumers. And now you know how to calculate actual numbers.


If you want the easiest method to see how many real kWh a battery has from a production vehicle, it seems to be true that 80% of the advertised max kWh will give you a reasonable number. (Advertised Max kWh) * 0.8 = ~Actual Capacity


In closing I'll leave you with a little teaser : If the Actual battery capacity is 80% of the advertised max capacity, does that mean that your battery is available for a free replacement under warranty the moment you drive it off the lot? Think about it...

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