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Tesla’s hacked Battery Management System exposes the real usable capacity of its battery packs


Like most automakers, Tesla measures the battery capacity of their electric vehicles by the total energy potential of the pack rather than the total usable capacity. For example, the original Nissan LEAF’s 24 kWh battery had a usable capacity of about 21.3 kWh.

What is particular to Tesla is that the company uses the battery capacity in its branding – a Model S 75 has a 75 kWh battery pack, but as we previously reported, those are rarely perfectly representative of the pack’s usable capacity. We now have more details on the actual usable battery capacity of each of Tesla’s battery packs, which highlights some better bargains and some options to avoid for Tesla shoppers.

Jason Hughes, a Tesla owner and hacker tinkerer, was the first to reveal the discrepancy in Tesla’s advertised battery capacity versus the actual capacity in the pack and the available capacity.

He did it through a tear down of the since discontinued 85 kWh battery pack of a Model S. He found that the 85 KWh battery pack actually only had a capacity of up to 81 kWh and ~77 kWh usable capacity. At the time, a Tesla spokesperson sent us the following statement:

“The battery pack in Model S is designed to meet everyday driving needs as well as provide long range for road trips. The total available energy from a battery depends greatly on conditions and can vary based on factors such discharge rate and temperature. It is very difficult to replicate the exact discharge profile at home to extract the maximum available energy in a battery pack.”

That’s fair enough, but Hughes, who is well-known for his hacking achievements with Tesla vehicles, has since gained access to Tesla’s battery management system, which he tested on several vehicles, including a brand new Model X 60D:

Now the data is directly from Tesla’s software and not a calculation based on the capacity of cells from a tear down of a pack. He gathered similar data from other Tesla models. Here’s a list he sent to Electrek:

  • Original 60 – ~61 kWh total capacity, ~58.5 kWh usable.
  • 85/P85/85D/P85D – ~81.5 kWh total capacity, ~77.5 kWh usable
  • 90D/P90D – ~85.8 kWh total capacity, 81.8 kWh usable
  • Original 70 – ~71.2 kWh total capacity, 68.8 kWh usable
  • 75/75D – 75 kWh total capacity, 72.6 kWh usable
  • Software limited 60/60D – 62.4 kWh usable
  • Software limited 70/70D – 65.9 kWh usable

As you can see, sometimes the rounding is actually in Tesla’s disadvantage on total capacity, but it’s never the case for usable capacity.

The buffers are there to optimize the range calculations and again, this ’rounding’ doesn’t affect the advertised range, which most car buyers prefer to work with when taking a decision anyway, but for those who like to know the battery capacity of a vehicle, it can be considered misleading.

The particularly striking and perhaps worrying point that Hughes is highlighting is the impact on the upgrade from a Model S or X 60 to 75. All Model S 60 or 60D and Model X 60D are equipped with the same battery pack advertised at 75 kWh, but with a software restricted capacity of 60 kWh. Tesla is offering an over-the-air upgrade for the 15 kWh difference for ~$10,000.


But as Hughes points out, Tesla’s BMS shows a software-limited capacity of 62.4 kWh and if you decide to get the upgrade, it goes up to 72.6 kWh since that’s the usable capacity of the pack. That’s only a 10.2 kWh difference.

To be fair, Tesla advertises the upgrade as a 25% range improvement and not a 15 kWh increase – even though it’s very much implied.

It’s something that we already reported when Tesla first announced the software-limited Model S 60, but it’s now even more clear with Hughes findings: the “75 range upgrade is just not a good deal, but the Model S 60 is a bargain.”

It’s not good for your battery pack to charge it daily at 100%, but charging a Model S 60 at 100% is not a problem since it’s like charging a Model S 75 at 86%. It also increases your charging speed since you will always have at least a 10 kWh buffer even at a higher state of charge.

In other words, you get almost the exact same car as a Model S 75 for daily driving, but your top speed is 10 mph slower and you are short 30 miles for the few occasions when you charge your car to 100%. That’s for a $6,500 difference at the purchase – ~$10,000 after tax if updated after delivery.

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Avatar for Fred Lambert Fred Lambert

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