When low-tech grounds your electric space car

posted by Jeff | Tuesday, August 4, 2020, 8:39 PM | comments: 0

I was super excited to be driving on a short road trip to Naples and back to pick up the kittens. I jumped in the car two days before to fish something out, when I noticed a bunch of warning messages. Specifically, the 12V battery was going to imminently fail. Unfortunately, Tesla didn't have a replacement available, and I didn't feel comfortable going three hours away not knowing if it would make it. We had to get a super-shitty rental to make the trip.

If I'm being honest, the failure probably has nothing to do with the car being an EV or a Tesla. Florida is notorious for killing 12V car batteries in the heat and humidity, and mine has been mostly sitting in the garage for months. The only real issue is that this particular battery is somewhat exotic. It's an AGM battery, common to RV's and boats, but this one has a particularly large capacity. It was covered under warranty and was in stock in less than a day. I'm not that worried about the cost of the rental, I'm just bummed I missed out on the chance to drive the car. I never enjoyed driving until we went all-EV.

So what's the story? Tesla uses 12V batteries to do two important things. They power the computer, which essentially never turns off, and plays an even bigger role now with Sentry Mode, which uses the bazillion cameras to record the surroundings when people get close. (Some calculate the cost of Sentry Mode at 1 mile of range per hour.) The way I understand it, the other thing it does is power the relays that close when the main power train battery is ready to move the car and power the air conditioning. The clicking you hear under the car when you get in is just that. The two places in the car where first responders can cut a wire to prevent high voltage from being in undesirable places actually cuts the wire to the relay. That's kind of neat.

Since EV's don't have an alternator to charge up the 12V battery, they're recharged by the main battery periodically, in some way that algorithmically attempts to preserve the integrity of the 12V. There's some speculation that the thing gets charge cycled a lot, especially since Sentry Mode became a thing, and that's causing these to die at a faster than the normal rate. Indeed, a friend's died outright without warning recently, also a bit over two years. Seems plausible, but the Florida conditions also seem likely. At the very least, there's anecdotal evidence that the software predicting battery death has become more liberal in warning you, maybe because they've had to tow a lot of cars. What's annoying is that all of this amazing technology is beholden to a part that has been around for decades.

It was covered by warranty, but I would've been content to just buy a battery from a local parts store if possible. It would have cost the same as the rental. And if Tesla had one on hand, even better, same day problem solved. It's super easy to replace and very accessible. Tesla's weakest link is service, which has never been able to come up with demand. They've always committed to not making service a profit center. It sounds like they're getting better though, and much of what they do is done by mobile service, where they come to you. Also encouraging, I saw a dude when I picked up the car loading windshields into a Safelite van. I had to get mine replaced after a massive rock induced crack broke mine, and it took five days over a weekend at the service center, which isn't ideal. I need to get it replaced again eventually, as I'm now sporting three significant chips.

All things considered though, it's still the best car I've ever had, and it's fun as ever to drive. I find this especially true since we don't do a ton of traveling these days.


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