Tesla Model S owner and battery tinkerer Jason Hughes published this week some of his findings following his tear down of several Tesla battery packs. Hughes is best known among the Tesla community for salvaging two Tesla Model S battery packs in order to create a massive energy storage system for his off-grid residential solar installation even before Tesla introduced its own line of stationary energy storage products.

Now Hughes’ new findings raises a few questions about the actual capacity of Tesla’s battery packs.

Hughes has been running tests on cells from a “85” kWh battery pack, which had less than 1000 miles or 5 charge cycles when he started tinkering with it.

In a new post on TMC this week, Hughes claims Tesla’s advertised 85 KWh battery pack actually only has a capacity of up to 81 kWh and ~77 kWh usable range after accounting for 4 kWh of “anti-brick” buffer.

He pointed out that the battery cell counts in the now discontinued “60” kWh battery pack and the “85” kWh battery pack don’t add up.

Hughes broke down the cell counts in each pack:

Tesla’s “85” kWh pack consists of 16 modules of 444 cells for 7,104 total cells.

Tesla’s “60” kWh pack consists of 14 modules of 384 cells for 5,376 total cells.

Based on this break down, the cells in the “60” kWh packs should contain 11.161 Wh of energy, while the “85” kWh packs should contain 11.965 Wh of energy, but when tested, cells from both packs averaged 11.36 Wh.

If you extrapolate Hughes’ findings by multiplying the number of cells by the average capacity, you end up with 80.7 kWh for the “85” kWh pack and 61.1 kWh for the “60” kWh battery.

Hughes then verified these results by comparing them to the usable capacity of his Model S and Tesla’s own Battery Management System (BMS) reported 76.5 kWh of capacity after a 4 kWh buffer.

As Hughes points out, rounding down 61 to an even 60 makes sense, but how and why Tesla would have rounded up 80.7 to 85 is a mystery. We have reached out to Tesla for a comment on Hughes’ findings and we will update this post if the company decides to comment.

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Update: 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.”

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Here you can watch a time-lapse of a tear down of one of Hughes’ “85” kWh battery pack:

Now it would be important to note that even if all these findings turn out to be correct, it wouldn’t change much of anything for the average Tesla owner. The company advertises both the energy capacity and the estimated range of its battery packs. Most buyers are only looking for or only care about the range, which gets tested by the EPA and is not being challenged here.

What is interesting here is that it gives us some perspective on Tesla’s battery pack ratings, which are notoriously the highest capacity battery packs of any production electric car on the market. At the moment, Tesla officially offers 70 kWh, 85 kWh and 90 kWh battery packs, though it recently started discontinuing the 85 kWh battery in certain markets.

The second best is the electric Mercedes B-Class’ battery pack, which is rated 36 kWh, but guess who supplies Daimler with this battery pack? You guessed it: Tesla.

Most electric car manufacturers advertise the full capacity of the battery pack and not the usable capacity. For example, the Nissan LEAF’s 24 kWh battery has a usable capacity of about 21.3 kWh. The difference with Tesla based on Hughes’ findings is that not only the usable range is not 85 kWh, but also the actual full capacity could be significantly less than advertised.

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