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I parked my HEV for 5 days with 30% charge only to find it completely dead today. I mean completely dead, even the frunk wouldn't open electronically. Plugging it into the charger did nothing, it would not charge.

I followed the 'jump start' instructions using an AGM battery charger on the 12V accessory battery and after about an hour of charging, it came to life and I was able to charge the drive battery. The problem now, however, is that it will not drive. It will not start or go into gear, despite all the electronics seeming to be working.

There is a service light on and the app diagnostics says the Electronic Drive Unit and Anti Lock Braking System need service.

Very disappointing for such an advanced vehicle to essentially turn into a 9000lb paper weight after simply sitting in my garage. Never, ever had such an extreme problem with my Tesla
 

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I parked my HEV for 5 days with 30% charge only to find it completely dead today. I mean completely dead, even the frunk wouldn't open electronically. Plugging it into the charger did nothing, it would not charge.

I followed the 'jump start' instructions using an AGM battery charger on the 12V accessory battery and after about an hour of charging, it came to life and I was able to charge the drive battery. The problem now, however, is that it will not drive. It will not start or go into gear, despite all the electronics seeming to be working.

There is a service light on and the app diagnostics says the Electronic Drive Unit and Anti Lock Braking System need service.

Very disappointing for such an advanced vehicle to essentially turn into a 9000lb paper weight after simply sitting in my garage. Never, ever had such an extreme problem with my Tesla
The electronics will come on long before there is enough energy in the battery to flip the high voltage contactors on. Unless you are using a high amperage charger, I'd give it a few more hours.
 

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2019 Chevy Volt, Voltech with 7.2 kW optional Charger
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You haven't given the 12V enough time to charge.... It may take a bit and it could be a weak 12V....

I've seen all of the above issues with random lights or odd things happening in the Chevy Volt community all due to 12V issues.
 

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You haven't given the 12V enough time to charge.... It may take a bit and it could be a weak 12V....

I've seen all of the above issues with random lights or odd things happening in the Chevy Volt community all due to 12V issues.
Why are the12v batteries going dead? Defective? I thought the drive batteries keep the 12v charged.
 

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GMC Gas / Diesel / EV Technician
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Why are the12v batteries going dead? Defective? I thought the drive batteries keep the 12v charged.
It will until the main battery gets so low. 30% charge and letting it sit 5 days is the problem. The vehicle still runs the cooling system to maintain battery temps. Plus if the software did a OTA update during that time. Real question is why was it allowed to sit 5 days at such a low state of charge anyway? Plug it in when you go on trips
 

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It will until the main battery gets so low. 30% charge and letting it sit 5 days is the problem. The vehicle still runs the cooling system to maintain battery temps. Plus if the software did a OTA update during that time. Real question is why was it allowed to sit 5 days at such a low state of charge anyway? Plug it in when you go on trips
I think that’s the key… I travel a good bit and have left my hummer sitting unplugged for well over 5 days, but usually at ~65-~80 % charge, havnt had a problem yet…
 

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Seems like a pretty material design flaw. Hopefully an OTA can remedy. I can see lots of scenarios where the car sits unplugged for more than 5 days
Tesla kinda has/had the same issue with cars when not plugged in and HV pack is <20%. They seem to have fixed it a bit right now or at least improved the longevity of the 12v by using a smaller custom 12v lithium battery they designed or get somewhere. They seem to address though by putting the their cars into a deep sleep when HV is at lower charge and it takes a manual request to wake up the car that can take a lot longer than usual; i.e. modem checks for that request a lot less frequently and everything else is purely off versus standby.
 

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The vehicle still runs the cooling system to maintain battery temps.
The HV battery should be used for cooling, the HVAC compressor is normally 400V, not 12v on EVs. The blower motor for cabin cooling will be 12V.

Here is a photo of a Bolt AC compressor, notice the orange wires, meaning high voltage.

Wood Gas Cylinder Automotive lighting Machine
 

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2019 Chevy Volt, Voltech with 7.2 kW optional Charger
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The HV battery should be used for cooling, the HVAC compressor is normally 400V, not 12v on EVs. The blower motor for cabin cooling will be 12V.

Here is a photo of a Bolt AC compressor, notice the orange wires, meaning high voltage.

View attachment 3189
The 12V is still used to wake the vehicle up and power the computers to do that job.
 

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Bare minimum I’d hope they’d have the truck send a notification when the 12v battery gets low instead of it being a complete surprise. Ideally, there’d be a routine that checks the 12v at a certain interval, and has it kick on the HV system to recharge itself so that it never strands you or ends up dead.
 

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Bare minimum I’d hope they’d have the truck send a notification when the 12v battery gets low instead of it being a complete surprise. Ideally, there’d be a routine that checks the 12v at a certain interval, and has it kick on the HV system to recharge itself so that it never strands you or ends up dead.
As far as I can tell the Hummer only checks the 12V voltage once a day.
 

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As far as I can tell the Hummer only checks the 12V voltage once a day.
It makes me curious why they don't implement a 12v battery maintenance routine that checks the voltage and intervenes to recharge it before it gets slow low it can't turn on the HV system. Heck, maybe there is one that just doesn't work well. Thankfully we should only be a software update away from a solution to this, apparently, continuing issue. The irony of an EV truck with a 212kWh battery pack having a ~1.2kWh 12v Achilles' heel.
 

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Forgive me, if this does not apply to the Hummer. The Jaguar I pace has a similar issue with the 12 volt battery. If you leave the Jaguar for a few weeks, the 12 Volt battery will be run down since the car never really shuts down.

The work around is to once a week schedule the car to precondition. This starts the car and conditions the interior but also charges the 12 volt battery. It runs for 30 minutes and then shuts down. This uses less than a half of one percent of the high voltage battery per week. I have no idea if this would work with the Hummer.
 

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Almost every EV forum has threads about this very issue, this is not unique at all to the Hummer EV. With features like OTA updates, phone as a key, connected services, these cars end up with some amount of computing power running most of the time. The vehicles should be smart enough to have the HV battery charge the 12V occasionally, but this seems to be a common poor design area.

That said, I have left my Bolt at the airport for up to 8 days, with no issue. I try to park it with 50-60% battery charge at the airport. At home I always plug it in. I have not had a 12V failure on my Mach E either, but have not left it unplugged for a really long time yet. But many on the Mach E forums have had this issue.
 

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I think that’s the key… I travel a good bit and have left my hummer sitting unplugged for well over 5 days, but usually at ~65-~80 % charge, havnt had a problem yet…
Yes ~ Agree. I have only used my EV1 sparingly with only 800 miles on it. Keep it unplugged @ 75 -80% sitting for 20+ days in between uses w/ no issues. But have to admit every time I drive it ~ WOW!!!
 
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