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Running equipment off the 12V battery

rkmvca

Member
Oct 27, 2017
37
20
Mountain View, CA
Mods, if this thread is in the wrong place or not allowed, feel free to move or remove.


I'm an amateur astronomer, and the power tank that I use to run my equipment is on the fritz, so I want to use my 150W inverter to run the following at a remote site:

- A laptop (~20W)
- A telescope mount with 2 motors (20W max, about 6W typical)
- 2 cameras, 1 with a cooler (maybe 10-15W during the cooldown period, a few W ongoing)

So, 30-40W max, less for the great majority of the time, 2-4 hours. Power inverters usually have about 80% efficiency, so about 25% higher than that out of the battery. I could unplug the laptop and adjust settings so it runs off its internal battery.

My plan is to plug the inverter into the 12V outlet, crack a window and run an extension cord to my equipment maybe 20 feet away. My question is, will doing this damage the 12V battery? I have a 2017 MS 75D, and I have never replaced the 12V battery.
 

swt2c

Member
Jan 31, 2021
79
44
USA
The manual says "The 12V power socket is suitable for accessories requiring up to 15A or a maximum of 180 watts." so I think you'd be okay from that perspective. My suspicion is that it probably wouldn't "damage" the 12V battery per-se, other than by adding more charge/discharge cycles to it as the voltage drops and the system uses the HV battery to recharge the 12V.
 

rkmvca

Member
Oct 27, 2017
37
20
Mountain View, CA
Thanks. Elsewhere, I was pointed to this thread, which seems to answer my question: Max wattage of the rear cigarette plug?

Looks like what I have to do is put it in camp mode so that the car stays on and the 12V battery is constantly charged. I'm testing it now with a 60W light bulb (yes, an old-timey incandescent one).

The last thing that I wanted to have happen was to be in some remote backwoods dark-sky site with a dead 12V battery. In the middle of the night.
 

Rocky_H

Well-Known Member
Feb 19, 2015
7,035
8,646
Boise, ID
This thread will have a lot of the information you are looking for about running things from the 12V system. However, it was started in the Model 3 section, so there will be some overlap with your S75, but also some specific details about where to connect into some things that will be different or not applicable.


Looks like what I have to do is put it in camp mode so that the car stays on and the 12V battery is constantly charged. I'm testing it now with a 60W light bulb (yes, an old-timey incandescent one).
Yes, that sounds like a good way to do that. While the car is on, the DC to DC circuit stays active, so it is continually keeping about 13V passed from the main pack onto the 12V battery system so it can't run down. As long as you are not drawing so much that it's going to blow the fuse through where you are connecting, that should be fine indefinitely.
The last thing that I wanted to have happen was to be in some remote backwoods dark-sky site with a dead 12V battery. In the middle of the night.
Even if the car is off, it won't let the 12V battery die. It constantly monitors the 12V, and it will run down some, and then the car will wake itself up to re-connect the main battery to refill the 12V. And then it will disconnect and go back to sleep. If you are running these other accessories, it will basically just run down more quickly and have to do these recharge cycles more frequently.
 

rkmvca

Member
Oct 27, 2017
37
20
Mountain View, CA
Even if the car is off, it won't let the 12V battery die. It constantly monitors the 12V, and it will run down some, and then the car will wake itself up to re-connect the main battery to refill the 12V. And then it will disconnect and go back to sleep. If you are running these other accessories, it will basically just run down more quickly and have to do these recharge cycles more frequently.

This is good to know. Thanks!
 

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