12V battery disconnect switch + boring story
Posted: Wed Jun 10, 2026 9:52 pm
For some it may be useless, but it could have saved me from undercharging and damaging the car's traction battery in the winter. There is a module in ZD D1 that monitors the voltage of the 12V battery and if its voltage drops, it switches on the contactor in the battery to start the DC/DC converter and recharge the battery and then turns off. And since we have permanent energy guzzlers in the car (Remote control receiver, radio memory, God forbid the original sharengo tablet), the 12V battery is always draining something and even the fact that it was -20°C outside didn't help it. When I knew that I wouldn't be using the car for a long time (a month or more), I unplugged the connector on the 12V battery under the front hood. But once I parked the car with about 10% battery and I didn't know that I was leaving it for a long time and I didn't want to open the hood and unplug the connector (purely my laziness 
), snow fell and the car remained "forgotten" for about 2 months parked outside in the cold. Result? 12V battery discharged to 2.5V and the traction battery did not light up Ready, on the contrary it lit up the Battery with an exclamation point 
Reason? all cells at the lower permissible voltage limit except for one which had 2.2V for who knows how long. I charged the cell via the external balance 26pin connector on the battery but the cell was already seriously damaged. The range was about 30km and that too with limited vehicle speed because the damaged cell was "soft" and under the pressure its voltage dropped extremely sharply. But more on that later 
I knew it was because of my laziness because I didn't want to open and close the hood, especially in such cold weather. Then it occurred to me that I should install a 12V battery disconnect switch that would be easily accessible from the cabin.
the most suitable place was on the sheet metal under the vacuum tank (the flattest piece of sheet metal and nothing obstructing it on the other side). I drilled a hole. I painted the edge of the hole with a thick layer of metal paint so that the car wouldn't start to rust in that place and installed the disconnector.
I opened the wiring harness where it enters the cabin. I traced the cable that goes directly from the battery connector and cut it off, connected a cable of the SAME CROSS-SECTION to it and sealed the whole thing in shrink tubing with glue. On the other side, I pressed copper eyelets with the necessary hole for the screw and put a protective rubber band on it.
Then it was necessary to move the vacuum tank away from the sheet metal and the disconnector. From the inside, I screwed 3 M6x50 screws to the stop and from the outside I tightened them with a counter nut. Then, another 3 nuts with an expanded base screwed in the opposite direction to form a new wall for the vacuum tank and finally only the last 3 nuts that hold the tank.
Done
Now I can turn the 12V battery off/on at any time in 3 seconds from the comfort of the cabin so that it doesn't discharge and at the same time so that recharging it during a long stop doesn't drain the large traction battery.
), snow fell and the car remained "forgotten" for about 2 months parked outside in the cold. Result? 12V battery discharged to 2.5V and the traction battery did not light up Ready, on the contrary it lit up the Battery with an exclamation point Reason? all cells at the lower permissible voltage limit except for one which had 2.2V for who knows how long. I charged the cell via the external balance 26pin connector on the battery but the cell was already seriously damaged. The range was about 30km and that too with limited vehicle speed because the damaged cell was "soft" and under the pressure its voltage dropped extremely sharply. But more on that later
the most suitable place was on the sheet metal under the vacuum tank (the flattest piece of sheet metal and nothing obstructing it on the other side). I drilled a hole. I painted the edge of the hole with a thick layer of metal paint so that the car wouldn't start to rust in that place and installed the disconnector.
I opened the wiring harness where it enters the cabin. I traced the cable that goes directly from the battery connector and cut it off, connected a cable of the SAME CROSS-SECTION to it and sealed the whole thing in shrink tubing with glue. On the other side, I pressed copper eyelets with the necessary hole for the screw and put a protective rubber band on it.
Then it was necessary to move the vacuum tank away from the sheet metal and the disconnector. From the inside, I screwed 3 M6x50 screws to the stop and from the outside I tightened them with a counter nut. Then, another 3 nuts with an expanded base screwed in the opposite direction to form a new wall for the vacuum tank and finally only the last 3 nuts that hold the tank.
Done
Now I can turn the 12V battery off/on at any time in 3 seconds from the comfort of the cabin so that it doesn't discharge and at the same time so that recharging it during a long stop doesn't drain the large traction battery.