Anyone out there with an EV got a multi-meter with a clamp to measure current draw at boot up? Wondering just how small a battery pack I can get away with as “boot up battery” in the event of a 12V battery issue.
Thoughts precipitated by acquiring two lithium 12V electric blind battery packs of 2600 mAh each and wondering if they could be repurposed as emergency boot packs.
Tim
Tim+ pretended :
I have a Lexus RX450h and measured it, best I can, at 20A but that was literally for a fraction of a second - so quick that I didn't even have time to press the 'Hold' button for the display.
I carry one of these around in the glove box and it's brilliant. Holds charge so well that even after a full year it was still over 80% charged. Never needed to use it on my hybrid yet but used it on 7 or 8 conventional cars and all fired up first time. Highly recommended.
I’m sure it’s good, but I have no need to crank an ICE and I do have two free batteries. Guess I’ll just have to experiment. ;-)
Tim
Thanks for trying!
Tim
During lockdown, next door's Prius battery got too low from non-use, couldn't unlock car, had to get AA to help them "break into" it and take it to dealers.
It really depends on exactly what they do and how much current those things take, which is likely to be car dependent.
The Gen2 Prius hybrid uses 12V power on startup for two main things: opening the contactor inside the high voltage battery (without which the high voltage battery is isolated from the rest of the car), and the inverter coolant pump (the pump runs when you put your foot on the brake, which you have to do to start the car). You can hear if the contactor fails to open and the traction battery is not enabled.
There's a website somewhere with an oscilloscope plot showing the current spikes but I can't find it at the moment. I think the current is of the order of 20-30 amps, but it's a fast spike. It's enough that if your 12v battery is even slightly sad the car won't start (the car doesn't start using the 12v battery so it's not sized for CCA).
I'd imagine an full EV would be roughly the same: there's a chunky contactor to open to enable the traction battery - I think on some Teslas it's 1500A. That's going to take some energy to move it.
OTOH when my 12v battery has been due for replacement a 12v lithium jumpstart pack on top of 10.whatever volts in the battery is enough to shift the contactor. The jumpstart packs are good at current, but don't have much capacity. But they do hold the voltage up when the current spikes.
I'd be concerned your lithium packs are not sized for current spikes if they've not used high current cells in them (power tool packs would be better).
Although you could always give them a go and see.
Theo
Me too. A bit concerned they might literally go out in a blaze of glory if I hook them up. ;-)
Think I’ll have to.
Tim
Is this a thing then? Surely they use static ram to hold settings, ie flash memory?
One other thing to note that a friend of mine said he did not expect when his EV went in for a service under warranty, when it came back the screen had been completely changed as they had put in the latest firmware. He had to learn where everything was again!
Brian
Nothing to do with non-volatile memory. I mean, you can’t boot a PC without power. All the electronics rely on the 12V battery. This is topped up by the EV equivalent of an alternator, a DC to DC converter that converts power from the high voltage traction battery to low voltage DC, but to do this, you need a 12V supply in the first place.
If the 12V battery dies or under-performs, you can’t boot the system, even if the traction battery is fully charged. You can however “jump start” the system with a temporary 12V supply to get things going.
Tim
Brian Gaff snipped-for-privacy@gmail.com wrote:
I got that with my diesel car. It self updated over the air while parked outside the house (it has access to our wifi).
I thought that at least some EVs, even when the main contactor has isolated the traction battery, can feed power from it to the DC to DC converter to charge the 12V battery if its voltage falls.
It seems daft that a car with a huge battery can't be turned on, because the small battery can't be topped up if the car is not used for some weeks.
I guess just like running an inverter off of a battery, the converter draws a significant amount of power and this would lead to unacceptable “parasitic” power drain. You really don’t want to flatten your traction battery.
A healthy 12V battery will hold its charge for a very long time so in practice it’s not a big problem. Certainly a few weeks shouldn’t be any problem.
Tim
It's a safety measure so that there's no HVDC exposed outside the battery pack when the car is parked, in case it's in an accident, or being worked on.
Maybe some have an backup pathway that doesn't need shifting the main contactor. That would be enough to energise the 12V DC-DC and not much else. I can't imagine it would be a good idea to drive like that, because various safety critical things are driven off the 12v - I wouldn't want to lose the dashboard, lights or wipers because I'm driving when the 12v battery is not holding charge.
Theo
Skoda win then. Mine was in for a Charging (firmware) upgrade all my settings were theer - except it thought in km/h.
It could be worse. I was once in a prototype car traveling at the 300km/h top speed of the test track when the dashboard and electric power steering failed. Fortunately the test driver handled things well and the return to base was uneventful.
John
Trouble is that in a modern car, there's quite a bit of qiiescent energy usage. Lockdown showed this up.
I think it just highlighted how many batteries were on a “shoogly peg” so to speak and weren’t in the best of health and needed regular use to top them up.
Modern (and not so modern) cars are pretty clever in their power management. Our old motorhome used to panic me when it hadn’t been used for a while as the remote stopped unlocking the doors.
Nothing wrong with the battery though, it was just that the car had turned off the radio receiver after a few days to save power.
Tim
Cars tend to have a quite high drain these days (alarms, monitoring for keyless fobs, maintaining radio memories, etc.) and it is quite easy to find the 12V battery flat in just 2 or 3 weeks of non-use (I've had that with three different cars). There is no need for a contantly running inverter, just to switch it on for a while every week or two as the 12V battery voltage falls a bit low. The main battery should be able to sustain that for many months. Better to lose some range than not to be able to start at all. Obviously if it is left long enough for th etraction battery to be seriously depleted, then such transfer could be stopped.
The ones that I have heard of that have the feature power nothing but the 12V battery monitoring and charging from the main battery when the main contactor is out, as far as I know.
Even my 2009 Astra has a warning in the owners handbook advising that the battery should be disconnected if the vehicle is going to left unused for for more than a few weeks.
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