Three Model 3s, three dead 12V batteries, one common thread

Three separate owners, three Model 3s, three flat 12V batteries in one week. All three cars had spent the previous fortnight in an airport car park.

What they had in common

None of the owners knew each other and none of the cars were the same year. What they shared was a fortnight parked outside with Sentry mode left on, and a 12V battery somewhere past its fourth birthday.

Sentry keeps cameras and processing awake, which the high-voltage pack supports happily. The 12V is what carries the load between the pack's top-ups, and a battery that has lost capacity cannot do that for two weeks. The pack was still at a healthy charge on all three cars. That is the part owners find surprising.

Why it looks like a bigger problem than it is

A Model 3 with a flat 12V is completely inert: no handles, no screen, no charge port. It reads as a catastrophic failure of an expensive car, and the first thought is the traction battery. In every one of these three it was a part that costs a fraction of one.

What we did

All three got the lithium 12V and the configuration that goes with it, plus a check of the charging path. Two of the three also had a parasitic draw worth investigating, and in one case it was a hard-wired dash camera on a circuit that never sleeps.

If you park long-term

• Turn Sentry off when the car will sit for more than a few days.
• Leave it plugged in where you can, even at a low amperage.
• Get the 12V tested before a long trip if the car is past three years old. It is a ten-minute job.

None of that is exotic advice. It is just the thing nobody mentions when you buy the car.

Written from the week's repair orders in Shop-Ware, anonymised. The counts here are one week; the guides carry the long-run figures.