charging 12V, do you need to disconnect leads?

My understanding of this is that it is 'similar' to a flat battery on a conventional engine car. You are not trying to charge the dead battery, but supply enough current to 'start' the car, after that the car's logic should take over and begin charging the 12v battery (which begs the question of why the 12v battery lost it's charge in the first place).

You should connect the external battery to the 12v terminals without disconnecting anything else (making sure you get the polarity correct!). Then turn on the car, once the car has 'started' disconnect the external battery, but keep the car 'on' whilst it charges the 12v.

fairnuff said:

My understanding of this is that it is 'similar' to a flat battery on a conventional engine car. You are not trying to charge the dead battery, but supply enough current to 'start' the car, after that the car's logic should take over and begin charging the 12v battery (which begs the question of why the 12v battery lost it's charge in the first place).

You should connect the external battery to the 12v terminals without disconnecting anything else (making sure you get the polarity correct!). Then turn on the car, once the car has 'started' disconnect the external battery, but keep the car 'on' whilst it charges the 12v.

Click to expand...

your instructions refer to jump starting, I was referring to charging the 12V - reason I would want to use my own charger instead of using the car is only because it would appear many people question the cars ability to charge effectively hence the complaints of dead batteries

even if I did do the above - can you turn the car on (so the car can charge the 12V) then lock the car and leave? would the car not switch off after a short while like this?

My apologies.

If you change the 12v battery you need to disconnect the HV. Personally, I would disconnect it to charge the 12v, just like taking it out of the car to charge

FWIW, I just returned from a month away. I left the 12V negative lead disconnected during my absence to minimize 12V battery drain. When I returned, I connected my Schumacher SC1304 charger to the 12V battery, leaving the 12V + lead attached and - lead disconnected. I plugged in the charger, which indicated the battery was a bit low and let it auto-charge at 15A to 13.8V. I disconnected the charger then reconnected the - lead. No problems, worked fine.

I definitely wouldn't try to charge with both + and - leads attached. You want the charge circuit dedicated to the battery charging, not powering the car components. Ideally (as indicated in the manual) you would disconnect both leads, but it isn't necessary and removing the + lead is a bit more of a bother.

I have done this (charged the 12V without disconnecting anything) with no ill effects. As long as the car is off I don’t see the problem. 🤷‍♀️

Taking the battery out of the vehicle is probably more than necessary but disconnecting the positive terminal would be preferential. Negative is "common ground" but the positive terminal is the entry path to all of the circuits. You are technically isolating the system with one side disconnected but the recommended procedure - to avoid problems - is to isolate the positive terminal.

If the positive cable is not connected to the battery, there is no possible connection the HV circuitry.

Maybe it wasn't a good idea but I left my 12v on a tender for a few weeks without disconnecting either terminal and there were no ill effects. I don't think the car tries to charge the battery itself unless it falls below a certain threshold which should never happen if it's on a tender.

FWIW, I also left my car for two weeks without disconnecting the 12v or hooking up a tender and the 12v was still fine when I got back.

I have left my car twice for 31 days and 34 days and just left it in the garage locked, didn't use the app for anything, just left it sit and let it do it's own thing and came back to a live car. It goes into a deep sleep and you have to use the fob button to wake it as it doesn't respond to the proximity of the fob until you hit a fob button, then everything is back to regular. Only lost 2% of the the HVB.

fairnuff said:

My apologies.

Click to expand...

no need to apologise! thnx for the input

Dylan said:

I have done this with no ill effects. As long as the car is off I don’t see the problem. 🤷‍♀️

Click to expand...

do you mean you charge battery with leads connected and just disable the HV battery?

Caliden said:

do you mean you charge battery with leads connected and just disable the HV battery?

Click to expand...

Sorry that was unclear - I edited the post.

When I first got the car home, I'd already read about 12V issues so I checked the voltage. It was in fact quite low, probably drained during transport and/or PDI.

I attached a 2A charger overnight without disconnecting anything. Is it a good idea? Probably not, I was being lazy, but it was fine.

mdmattingly said:

Maybe it wasn't a good idea but I left my 12v on a tender for a few weeks without disconnecting either terminal and there were no ill effects. I don't think the car tries to charge the battery itself unless it falls below a certain threshold which should never happen if it's on a tender.

Click to expand...

I did the same thing for 15 weeks with no problem.