When Tesla refreshed the Model S in mid-April, they executed a stealth upgrade under the car — the based model 70kWh battery in the Model S 70 and 70D was upgraded to a 75kWh pack, but it's software-limited to the lower capacity. Customers that want to unlock the entire capacity of the battery — a 7% boost for an extra 19 miles — will have to pay a $3,000 upgrade fee for an over-the-air software update to do it.

The refreshed Tesla Model S, with a stealth battery upgrade lurking inside

Said a Tesla spokesperson:

All 70 kWh Model S with updated styling have been built with a 75 kWh battery pack and the additional energy can be unlocked at anytime through an over-the-air software update. We will continue to offer the 70 kWh energy option at but we will no longer produce the packs; a decision that is the most efficient for Tesla and the most beneficial for our customers.

It's an odd decision, we'll admit, though it lets Tesla maintain the starting price of $71,500 for the Model S 70. The decision to move to a 75kWh pack further streamlines the Tesla line-up; both the Model S and the Model X are now offered exclusively with 75kWh and 90kWh batteries.

The part that really has us scratching our heads is the cost of the upgrade: $3,000 to unlock an additional 5kWh equals $600/kWh, which is far above the sub-$190/kWh cost of Tesla's current manufacturing.

This is the first time that Tesla has sold software-limited batteries — from 2012 to early 2013 they sold 60kWh battery packs labeled as 40kWh, with owners offered an optional $10,000 software upgrade to unlock 50% more battery capacity. That situation came about due to similar circumstances — Tesla offered the Model S 40 as a base model but saw demand limited enough that they opted not to produce any 40kWh battery packs, instead starting with 60kWh and software-limiting them for those that had ordered the 40kWh version. Back in 2013 Tesla's per-kWh costs were much higher than they are today, and that upgrade equaled out to $500/kWh — less than today's 75kWh offer.