Comment by flutas
3 years ago
I've honestly wondered for a while how many devices (from phones to cars) have features like this that haven't been documented yet.
Also how many engineers have worked on features like this without whistle-blowing over behavior like this.
I can’t change the 12V lead acid battery in my EV without using a reverse engineered OBD-II dongle. If you don’t use the dongle to reset the charge circuit, it fries the new battery in about a month.
Here are incorrect directions explaining how to do it:
https://www.mybmwi3.com/forum/viewtopic.php?t=17838
Step 14 requires the magic dongle.
Note that they are not disconnecting the main battery, so they are risking electrocution from the >> 100V DC batteries.
There are some comments about not letting the old battery get into a low voltage state.
That’s tricking the charger into not overcharging the new battery to death.
This is actually not specific to the EVs but something all German car brands started doing. They made their alternators/chargers of the 12V battery overtly complicated and you have to use a dongle to tell the car you replaced the battery and with what kind of battery.
My friend once replaced her battery, exact same one in a BMW X3. The car immediately went into a limp mode and would refuse to go faster than 5mph until we connected a dongle and told it that the battery was replaced with the exact model that was already in there.
There's an argument they did it for "battery lifespan optimization" which there is a semblance of truth, because there are different kinds of lead acids. The reality is they found a new way to force the majority of people into dealerships.
Registering batteries has been a thing for BMWs for at least a decade. The dance around keeping windows open etc is a little more annoying, but nothing out of the ordinary.
another reason not to buy BMW added to the list
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What a jackass thing to do to vehicle owners. Changing the battery is a normal maintenance action you can do on any competently designed vehicle in less than 20 minutes.
Does anyone know of a sort of "buyer beware" website where these sorts of gotchas are catalogued by users? I don't fully trust any vehicle manufacturers to be beyond pulling this sort of thing.
>any competently designed vehicle
I guess you skimmed over the name of the manufacturer. ;)
> I can’t change the 12V lead acid battery in my EV
Aside from that not having anything to do with it being an EV, it's worth mentioning that many newer EVs (most of the ones sold, perhaps) use a lithium 12V battery now, not lead acid. So in general they ought to last longer anyway. Plus Tesla, at least, doesn't 'register' batteries the way BMW does.
What is the story here exactly? Is there an official way to replace the battery that doesn't require a dongle? What does the dongle do exactly? Why does a new battery get drained if you don't follow this process carefully?
The charger learns how worn the old battery is, and overvolts old ones to get a bit more useful life out of them. When you disconnect and reconnect the battery it doesn’t reset the training algorithm, so it overvolts the new battery, reducing its lifespan to roughly 30 days.
There’s no official way to reset the charge algorithm without a dealer-only dongle, so you take it to the dealership to replace the battery (~$400 labor, $100 parts).
They could solve the problem by adding a “register 12V battery” option to the service menu, or by having it prompt the next time you start the car after 12V power is interrupted.
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.... just imagine how many instructions you can hide in a 64-bit address space (I'm thinking of you intel hacker magic)