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Can you charge an EV with a dead 12v?

  • 29-09-2020 3:11pm
    #1
    Registered Users Posts: 5,789 ✭✭✭ the_amazing_raisin


    I realise this is a general question with EV specific answers but it's an interesting topic I though of earlier


    I was watching one of Bjorn Nylands old tests of his old Model X where his 12v battery dies and he's unable to charge:





    I'd known that when the 12v dies the car won't start up but it was a bit of an eye opener that he couldn't get the charge port to unlock. That seems like it may be Tesla only though as I know that some EVs have a string or switch you can pull to manually unlock the charge port



    I'm curious for other cars, will you be able to charge if the 12v is dead?



    From my (limited) understanding of the charging protocol, the car uses resistors to signal the charger that it's ready to start charging. That implies that there's something running on the EV end to set the resistance.



    As far as I can tell, there's no 12v power from the EV charger to the car, so that would imply that if the 12v is dead then you can't charge.


    I guess the main thing to remember would be to try and keep the 12v battery in good condition, and if you're going on a trip, consider bringing a 12v booster pack.


Comments

  • Moderators, Category Moderators, Motoring & Transport Moderators, Regional East Moderators Posts: 58,818 CMod ✭✭✭✭ unkel


    Pre-facelift Tesla Model S has the 12V battery very deep down near the front firewall, so not accessible, particularly if your automatic frunk opener doesn't work because of a dead 12V battery :p

    Remove the nose cone and there are positive and negative terminals though, connected to the 12V via a 50A fuse. You can jump start the car through these, just like any other normal car. Very unlikely you need to do this though as a Tesla is never asleep and it constantly monitors the 12V battery and if too low, tops it up from the high voltage battery (with an impressive 2.5kW DC-DC converter). But of course if the 12V dies on you (and it does after about 2-3 years, it sees heavier use than a battery in a normal car), then you'd need to jump it

    Never understood why Tesla (or any other manufacturer for that matter) did not replace the 12V lead acid with a drop in LiFePo4 replacement. Adds no more than a couple hundred quid to the cost of producing it, has far higher capacity and can go to a much lower voltage while still working and you don't need high cranking amps (the strong point of a lead acid battery) if you are not going to crank any fuel explosion engine into life :p

    "Wind is Ireland's oil" - An Taoiseach, 25/05/2022



  • Moderators, Society & Culture Moderators Posts: 31,241 Mod ✭✭✭✭ Gumbo


    I realise this is a general question with EV specific answers but it's an interesting topic I though of earlier


    I was watching one of Bjorn Nylands old tests of his old Model X where his 12v battery dies and he's unable to charge:





    I'd known that when the 12v dies the car won't start up but it was a bit of an eye opener that he couldn't get the charge port to unlock. That seems like it may be Tesla only though as I know that some EVs have a string or switch you can pull to manually unlock the charge port



    I'm curious for other cars, will you be able to charge if the 12v is dead?



    From my (limited) understanding of the charging protocol, the car uses resistors to signal the charger that it's ready to start charging. That implies that there's something running on the EV end to set the resistance.



    As far as I can tell, there's no 12v power from the EV charger to the car, so that would imply that if the 12v is dead then you can't charge.


    I guess the main thing to remember would be to try and keep the 12v battery in good condition, and if you're going on a trip, consider bringing a 12v booster pack.

    The nosecone pops out by hand once you know where to pull. The battery terminals are there to charge the 12v.

    So I suppose the answer is yes, a dead 12v battery leaves an EV is a very similar position as an ICE. No worse, no better.


  • Registered Users Posts: 5,789 ✭✭✭ the_amazing_raisin


    unkel wrote: »
    Pre-facelift Tesla Model S has the 12V battery very deep down near the front firewall, so not accessible, particularly if your automatic frunk opener doesn't work because of a dead 12V battery :p

    Remove the nose cone and there are positive and negative terminals though, connected to the 12V via a 50A fuse. You can jump start the car through these, just like any other normal car. Very unlikely you need to do this though as a Tesla is never asleep and it constantly monitors the 12V battery and if too low, tops it up from the high voltage battery (with an impressive 2.5kW DC-DC converter). But of course if the 12V dies on you (and it does after about 2-3 years, it sees heavier use than a battery in a normal car), then you'd need to jump it

    Never understood why Tesla (or any other manufacturer for that matter) did not replace the 12V lead acid with a drop in LiFePo4 replacement. Adds no more than a couple hundred quid to the cost of producing it, has far higher capacity and can go to a much lower voltage while still working and you don't need high cranking amps (the strong point of a lead acid battery) if you are not going to crank any fuel explosion engine into life :p


    Yeah I decided to look up the Tesla manual for the Model S/X and there's definitely a bit involved in just opening up the car with a dead 12V


    I suppose that's a point in favour of my Leaf, manual switches to open everything. Battery sitting on top right under the hood



    Still has the problem with the 12v discharging that every EV has.


    I'm guessing the reason every legacy manufacturer uses lead-acid is because they're already producing heaps of them for their ICE vehicles. I guess Tesla just uses them to save cost. It'd be nice to replace with a set of LiFePo cells as you can probably get a higher capacity battery, which would suit the 230v inverter I'm planning on fitting to my next car :D


  • Moderators, Category Moderators, Motoring & Transport Moderators, Regional East Moderators Posts: 58,818 CMod ✭✭✭✭ unkel


    I have an inverter fitted to the frunk. Was handy when we had a power cut recently, I could still work from home

    Or I could charge up the car for free and them charge up my power wall for free with my car so it can in turn power my house for free. Free. There I said it :p

    "Wind is Ireland's oil" - An Taoiseach, 25/05/2022



  • Registered Users Posts: 4,872 ✭✭✭ zg3409


    Generally 99% of the electrics on an EV are driven off 12v battery. Such as door unlocking (most cars have hidden physical bypass key) . If you cannot open bonnet or trunk to access battery some cars have a special charging point to hook booster battery to.

    Most cars have 12v battery checking and charging systems, some better or worse than others that fill small 12v battery from big battery. Downside it drains big battery over time known as phantom drain for example while parked for 2 weeks in airport parking.

    The 12v is intended to work emergency systems if main battery goes flat or fails so hazard lights work etc. Some cars disconnect the 12v if it gets very low, with a button to reset it and go again. This is done to both keep car going, and also prevent 12v battery damage. A similar system is also installed into many emergency vehicles like ambulances to prevent dead battery at accident scene. .


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