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Comment by xxs | Hacker News Reader

>12V battery is tiny and would probably not provide enough power

LiFePO4 is capable of providing massive amounts of current for its size (way higher than a conventional acid one). 100A is not that high amount of current to run even with 4s4p setup. A 10kg battery would be beyond sufficient (should be able to fully power the brake system for 1h use).

Note: jump on the brakes is expected to consume around 1200W

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> "A 10kg battery would be beyond sufficient"

I don't have the exact weight, but Tesla's LFP low-voltage batteries weigh far less than that. Around 2kg at a guess.

Rating is 12.8V nominal, 12Ah, 153.6Wh. Not all that much bigger than a laptop battery!

Because it fundamentally is just a laptop battery. The job of the 12V bootstrap battery is to bring the main control hardware (not even the MCU, I don't think) online so that the DC/DC converter can power the 12V bus from the main battery. There are a few other systems that expect to be powered in the event of a main battery failure, like door locks and charging.

But I really don't think hydraulics like power steering/braking are on that list. The fallback for hydraulic failure is manual pressure, just like it is on any car. It's a naturally redundant system.

2kg would be a monster of a laptop battery of course, esp since it's regular Li-Ion (not the one w/ Fe/P). LiFePO4 should be ~150Wh/kg. So indeed, the battery is 'tiny', like 1kg only. Yet, it'd have no issue supplying the brakes - it's less than 10C discharge rate. LiFePO4 tends to have much higher discharge rate which is the important part not the total capacity.

Either way, the battery is indeed 'tiny'

LiFePO4 can't really be charged and don't like being discharged below freezing.

I don't doubt you could run into problems in extreme cold, but our Model Y SR (LFP pack) has been fine charging in -5C or so. The car is pretty good at managing thermals and making sure the battery doesn't get too cold.

Right, you need a heater to keep things happy! I added a little heater to my LiFePO4 battery packs that live outside. It seems fine like that to take the edge off. Way better than exploding!

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