4 ms·

>and the jumper pins It can be started through the converter? That takes something like a hundred amps. It must be pretty beefy.

phire 3y ago
No. For safety, hybrids and electric vehicles physically disconnect the high-voltage battery from everything when it's off with massive relays. So you can end up in the somewhat stupid situation where your high-voltage battery is fully charged, but you can't do anything because the tiny 12v motorcycle battery is flat. Starting simply requires enough 12v current to close a relay, the amps going though the "jumper pins" are tiny. And once the relay is closed, the high-voltage battery can keep the relay closed and charge the 12v battery via the dc-dc converter. The amount of current to "jump" a hybrid or electric car is so small that they really should just install a USB-C cable, so you could just use your phone to jump start it.
- deleted 3y ago
  [deleted]
- sbierwagen 3y ago
  That describes the situation where the 12V battery is flat but the HV battery is charged. But what happens if the HV battery is flat on a non-plug in hybrid? Can it be charged from 12V?
  - ace2358 3y ago
    Wouldn’t you just start the car if the HV was flat and the LV was charged? If they’re both flat I agree, I’m curious what you do.
    - phire 3y ago
      There is no 12v starter motor, it always just uses one of the large motor-generator units to start the engine, and in the Prius drive chain, they require 600v (boosted from 200v from the HV battery). So if the HV battery is flat, you can't start the engine to charge it.
      - ace2358 3y ago
        Wild! Thanks!
  - phire 3y ago
    Yeah, I started wondering the same thing as I was writing that response, as I do actually own a Lexus with the Prius drive chain. For starters, it would be pretty hard to get into that state in the first place. My understanding is they actually report a 10% or 20% charged battery as "completely empty". And the it's completely disconnected, so it shouldn't discharge much. Looking though the service manual, there doesn't even seem to be an error code for "HV battery low", nor any mention of the DC-DC converter being able to charge the HV battery from 12v, it only goes from HV to 12v. So I really suspect if you do happen to end up with a flat HV battery that's too flat to start the engine, it's just going to flash the "Replace HV battery" error code. Theoretically, someone with the right tools could disassemble the battery and charge the individual modules. And the only speculation I can find on google is that "flat HV batteries are very rare" and probably only happen when the HV battery has other issues and needs replacement. Edit: After more googling, apparently Toyota dealers have a special charger called the THS that can charge the HV battery in non-plugin hybrids.
  - tw04 3y ago
    No. Nobody is putting in a 12v to 400v dc to dc converter. There would be no point. I haven’t done the math but I’d be surprised if you got more than a couple miles out of it. If your high voltage battery is completely dead, you’re calling a tow truck.
Moto7451 3y ago
In the case of my Audi, the starter motor is 48v so the 12v system charges the 48v system enough to start. In EVs the 12v system is used to power a contactor that connects the EV battery to the drive system. That’s the clicking sound you hear on power on and power off and another contactor can be heard clicking during charging.

phire 3y ago

No. For safety, hybrids and electric vehicles physically disconnect the high-voltage battery from everything when it's off with massive relays. So you can end up in the somewhat stupid situation where your high-voltage battery is fully charged, but you can't do anything because the tiny 12v motorcycle battery is flat. Starting simply requires enough 12v current to close a relay, the amps going though the "jumper pins" are tiny. And once the relay is closed, the high-voltage battery can keep the relay closed and charge the 12v battery via the dc-dc converter. The amount of current to "jump" a hybrid or electric car is so small that they really should just install a USB-C cable, so you could just use your phone to jump start it.

deleted 3y ago
[deleted]
sbierwagen 3y ago
That describes the situation where the 12V battery is flat but the HV battery is charged. But what happens if the HV battery is flat on a non-plug in hybrid? Can it be charged from 12V?
- ace2358 3y ago
  Wouldn’t you just start the car if the HV was flat and the LV was charged? If they’re both flat I agree, I’m curious what you do.
  - phire 3y ago
    There is no 12v starter motor, it always just uses one of the large motor-generator units to start the engine, and in the Prius drive chain, they require 600v (boosted from 200v from the HV battery). So if the HV battery is flat, you can't start the engine to charge it.
    - ace2358 3y ago
      Wild! Thanks!
- phire 3y ago
  Yeah, I started wondering the same thing as I was writing that response, as I do actually own a Lexus with the Prius drive chain. For starters, it would be pretty hard to get into that state in the first place. My understanding is they actually report a 10% or 20% charged battery as "completely empty". And the it's completely disconnected, so it shouldn't discharge much. Looking though the service manual, there doesn't even seem to be an error code for "HV battery low", nor any mention of the DC-DC converter being able to charge the HV battery from 12v, it only goes from HV to 12v. So I really suspect if you do happen to end up with a flat HV battery that's too flat to start the engine, it's just going to flash the "Replace HV battery" error code. Theoretically, someone with the right tools could disassemble the battery and charge the individual modules. And the only speculation I can find on google is that "flat HV batteries are very rare" and probably only happen when the HV battery has other issues and needs replacement. Edit: After more googling, apparently Toyota dealers have a special charger called the THS that can charge the HV battery in non-plugin hybrids.
- tw04 3y ago
  No. Nobody is putting in a 12v to 400v dc to dc converter. There would be no point. I haven’t done the math but I’d be surprised if you got more than a couple miles out of it. If your high voltage battery is completely dead, you’re calling a tow truck.

Moto7451 3y ago

In the case of my Audi, the starter motor is 48v so the 12v system charges the 48v system enough to start. In EVs the 12v system is used to power a contactor that connects the EV battery to the drive system. That’s the clicking sound you hear on power on and power off and another contactor can be heard clicking during charging.