TDLR: My 2015 Model S got a reconditioned battery, under the 8 year battery warranty. While the cells in the original battery were fine, with ~6% degradation, the battery case had suffered water ingress. Tesla looked after us with courtesy car(s) as it took a five weeks.

New battery? No, not the keyfobs. Though I’ve replaced them plenty. Or the 12V lead-acid, Tesla replaced that last year. No, the big one, 6 thousand cells of the 70kWh battery under the floor of my Model S.

The first symptom was one I’d seen early on in our ownership – so I knew it was serious, a grounding fault on the HV system. This can be any of the cabin heater, coolant heater, motors or battery itself. The car went into a limp mode with no regen braking and a 100kW power limit. Driveable, but not an excessive amount of power for a 2 tonne car.

11 November: Tesla took the car in, and we got a 5-year-old Model X 75D as a courtesy car. Sounds great but in reality it probably has the lowest range of any Tesla. In rain it’s easy to get under 2 miles per kWh, and the range then is only slightly better than our benchmark car, the awesome Hyundai Ioniq 28kWh. (BTW in 5 years it’s had no HV issues!) The doors are fun, but it’s just too big on many of Oxfordshire’s roads. In the UK this size car is really only practical on large roads and motorways. First world problems.

Tesla spent 3 days testing the HV systems, I guess the computer isn’t clever enough to isolate the fault very well, finally they opened the battery pack- to find water ingress. Some early cars have seen this due to the aircon draining it’s condensate water onto the fuse cover at the top of the pack; not the best design feature. Others have noted corrosion to the steel screws holding the pack lid on, I don’t know if this happened to ours.

So we’re waiting on a replacement battery from the Gigafactory. In the meantime we’ve swapped the MX for another MS with a half decent range. It at least has free supercharging!

17 December: Heard this week that the pack has been replaced. The app also shows the various repairs as “in progress”. It also shows software v2022.44.2 which indicates the MCU2 has been fitted. No news yet on when we’ll get it back, however!

20 December: Work completed, which consisted of a new charger module, part of the high voltage harness and the high voltage battery itself. The battery is a remanufactured one, with the same useable capacity as the old one, 65kWh, so no change there. I had read that Tesla are making a 90kWh 350V battery for the older cars, but we didn’t get one of those.

The new MCU has genuinely refreshed the car, it starts up much faster and navigation and entertainment features are quick, with no lag. The 7 year old MCU1 was feeling very old! In addition one of the service comments was that supercharging errors I experienced were probably due to lack of memory on the MCU1, so I don’t see the upgrade as just a nice-to-have item. The fact that it’s upgradeable at all shows that Tesla are serious about keeping the older cars on the road and up-to-date. The car is now running the latest software and while it doesn’t have every feature that a brand-new car would have, it has most of them.

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