What is the difference between gross and net capacity?

In general, an HV battery consists of many individual cells. These cells have precise specifications for how they are intended to be used, so they are as cycle-resistant as the cell manufacturer promises. Aside from temperature windows and maximum currents, this is also about the voltage window.

According to the supplier, the cells can be used from 2.8 V to 4.35 V. The low voltage is the so-called lower cut off voltage and the high voltage is the so-called upper cut off voltage. These practically define "empty" and "full". Discharging the cell even lower will result in deep discharge and over charging it will damage the cell to the same extent.

Of course, neither is good for the longevity of the cells. Between the voltage range is the gross capacity of the cells. In the case of smart vehicles there are 107 cells with a total capacity of 66 kWh.

Car manufacturers then usually further limit the voltage window of the entire battery pack. This also depends on how the respective car manufacturer wants to use the battery and what their priorities are. In the case of smart vehicles, the upper final voltage of the cells is maintained at 4.35 V, but the battery is switched off earlier, when the first cell reaches 3.46 V.

This area then results in the usable capacity, the net capacity of 62 kWh.

Internally in the vehicle, the battery knows that it is not yet completely empty at 3.46 V. The real state of charge (rSoC) is then still more than 0%. But the state of charge (SoC), which is displayed to the customer, is then at 0%.

Older smart vehicles still have some air above and below, not just below. So it is no problem for them to constantly charge the battery to 100%. Of course, this comes at the expense of the daily usable capacity.

In newer smart vehicles, like many other electric cars, you can set a charging limit manually. Set to 90% for non-critical everyday ranges and 100% for long-distance journeys.

It is also important to note that these voltage limits vary from manufacturer to manufacturer. But it is common for the limits in the car to remain the same in winter and summer. Although an electric car has less usable range in winter, there are other reasons for this. Technically speaking, the capacity is always the same.

This post was originally written for my personal Twitter and Linkedin accounts. I then published it here so it won't get lost in the depths of social media. I would also like to be able to see how my statements have aged in 10 years time.

Replies 5

  • I note you said "Older smart vehicles still have some air above and below, not just below. So it is no problem for them to constantly charge the battery to 100%." Which Smarts do you include in "older"? I have a Smart EQ 2019 whose handbook recommends no higher than 80% for the sake of battery lifr.

  • Every smart fortwo and forfour ever build, as they are technically at least 7 years old.

    Yes, i know the hand book notes that. But that's just German Angst and not actually necessary. Who ever wrote that thing didn't knew it better and most likely copied the text from a had book of a laptop, phone or something like that.

  • I always charge the EQ fortwo to 100% as a consequence using a dumb charge point, (because smart charge points do not work on my car,) but I have always worried about increasing the battery degradation as a result of doing so.

    Are you suggesting I no longer need to be concerned about battery life?

  • Yes, I am. :) Your EQ can handle that without any problems.

  • That's a relief. Not that I can change the charging regime, but at least I no longer need to worry about it.

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