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Using an inverter to charge a computer: recommendations? cautionary tales? effect on 12 v battery?

MaryAnning3

Supporting Member
Nov 3, 2018
349
142
California
I am thinking of getting an inverter and using that to charge a computer and maybe some other things. My macbook air battery has a capacity of about 50 Watt-hours, which is roughly about 1/3 of the useable capacity of the 12 volt battery of the model 3, so it seems reasonable, but I wonder if 12 volt battery might be adversely effected or if there are other issues I may not have considered?

Also, does anyone know what the charging protocol for the 12 volt battery is in a model 3? I have seen it mentioned that Tesla used to let the charge drop to 50% and then charge the 12 volt from the traction battery. It seems unlikely that it still works that way, since 50% is too low to let a lead acid battery drop to routinely. So I would guess that there is a higher number or different protocol. Does anyone know about that?

Have people had any negative or positive experience with using the 12 volt power port to charge computers or run small refrigerators? Thanks.
 

MaryAnning3

Supporting Member
Nov 3, 2018
349
142
California
PS. The inverter I am thinking of getting is the Bestek 300 Watt pure sine wave. I am thinking that the power port only puts out 12 amps and about 150 Watts, but I could not find a smaller inverter that was pure sine wave. Maybe it is okay to have that buffer anyway? I am not really sure. Also, my Macbook air is kind of old and uses magsafe 2; I don't think it can be charged via USB C or anything like that as far as I know.
 

GHammer

What a long strange trip its been.
Feb 1, 2016
920
2,162
Wren, Oregon
I run a 400w inverter, a Coolatron solid state cooler and two phone chargers all the time, no problems. The capacity of the 12v battery is well over 150Whr, the limit of the accessory plug is determined by the wire size to it and the fusing. Deep cycle 12V lead acids can go well below 50% repeatedly safely.

The cooler takes 36 watts and my laptop charger is 70w the phones are 12w each for a total load of 130W

Also, in my experience, true sine wave is not needed for computer chargers, the power supply doesn't care.
 

Uncle Paul

Well-Known Member
Nov 1, 2013
6,296
6,858
Canyon Lake,CA
Issue is that Tesla 12v lead acid battery is not deep cycle. They are designed to offer lots of juice, for limited periods of time Deep cycles deliver slower drain, but for longer periods of time. This is the same type of battery ICE vehicles use for their starter motors.

Lots of people want to use their cars large Li batteries to power their homes, power tools, refrigerators, CPAP machines etc. Nobody has yet come up with a Tesla approved way to do that. Tells you something.

Small draw can be easily handled by Tesla USB plugs or the lightly fused 12v accessory plug some have in the hatch.

Nobody knows for sure, how draining the battery, through the 12v accessory battery will effect the life of that battery.

Uninformed logic would suggest that it is pretty easy to tap, but it again may reduce the life of that battery and reduce the voltage that other internal systems need to tap reliably.
 
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Uncle Paul

Well-Known Member
Nov 1, 2013
6,296
6,858
Canyon Lake,CA
Perhaps my biggest concern with adding additional signifi.cant draw, is that Tesla have lots of finely tuned sensors managing the power systems. Adding an extra draw might trigger a warning or cause another system to not perform

Imagine that the upcoming Pickup Truck from Tesla would be the first to offer power take off points for construction and field workers. Would give that truck a huge differentiation point against all the other truck manufacturers. Believe I saw a Ford truck commercial energizing a bunch of flourscent or LED lights. This is something that truck owners would want.
 
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MaryAnning3

Supporting Member
Nov 3, 2018
349
142
California
If the car is running though I believe the HV to 12volt inverter will be handling load not the 12volt battery.
Interesting. I wonder how we can confirm if that is the case. Also, my impression is the the 12 volt port is active only when the contactors are closed. (Is that true?)
It is unclear to me when or if the 12 volt port is powered by the battery.
 

MaryAnning3

Supporting Member
Nov 3, 2018
349
142
California
Issue is that Tesla 12v lead acid battery is not deep cycle.
It would be interested to know more about the battery and the charging protocol. Does the battery have the thick, solid electrodes associated true deep cycle, more porous electrodes used for high current, or somewhere in between? I guess you are saying porous although Hammer was thinking otherwise. All I really know about the 12 V is its technical capacity is 45 amp-hours (540 Watt-hours), but how much of that we would want to actually tap into routinely depends on the nature of the electrodes as I understand it.
Also, a related question: at what percentage discharge does the 12 volt charging kick in?
 

GHammer

What a long strange trip its been.
Feb 1, 2016
920
2,162
Wren, Oregon
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MaryAnning3

Supporting Member
Nov 3, 2018
349
142
California
The real question is what kind of inverter do you need to keep your fridge running during a power outage?
I know you are joking, but, well, the power is there since a fridge just uses a few kWh per day so the traction battery has the capacity to run a fridge for about 20 days in principle. That would be an interesting hack. Peak draw for a fridge is i believe about 6 amps (700 Watts).
 

MaryAnning3

Supporting Member
Nov 3, 2018
349
142
California
Well, I bought the inverter. Funny thing is we are converting 12 Vdc at the power port up to 110 VAC and the plugging in an apple charger that drops that down to about 8 VDC to charge the Li-ion Mac battery. I wonder how much is lost in all that (and if a serious hacker could create something that takes a direct DC to DC path and is more efficient)?

How much power would you guess we take from the model 3 to get 50 Wh into the MacBook battery?
 

MaryAnning3

Supporting Member
Nov 3, 2018
349
142
California
If the car is running though I believe the HV to 12volt inverter will be handling load not the 12volt battery.
Do you mean when the screen is on? If that's true then maybe there is no effect on the 12 volt battery?

I get the impression the 12v power port is only active when the screen is on, though i haven't used it at all yet myself. Does anyone have recent experience with that?
 

aviphysics

Member
Oct 6, 2019
41
11
Bay Area
I know you are joking, but, well, the power is there since a fridge just uses a few kWh per day so the traction battery has the capacity to run a fridge for about 20 days in principle. That would be an interesting hack. Peak draw for a fridge is i believe about 6 amps (700 Watts).

I am definitely joking, but there is potentially a lot of sense in there. The battery in my model 3 could run my home for 2 days. If I could recharge it with my solar, the number of days depends only on the weather.

The issue is that I have a difficult time imagining a prolonged power outage where I wouldn't prefer using my car to just go some place else (unless it is some kind of apocalypse situation.)
 
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Big Dog

Active Member
Mar 7, 2016
1,612
1,602
Irvine, CA
I am definitely joking, but there is potentially a lot of sense in there. The battery in my model 3 could run my home for 2 days. If I could recharge it with my solar, the number of days depends only on the weather.

The issue is that I have a difficult time imagining a prolonged power outage where I wouldn't prefer using my car to just go some place else (unless it is some kind of apocalypse situation.)

Unless you have a battery backup such as a PowerWall, doesn't solar shut down during a blackout (to protect the linemen).
 

aviphysics

Member
Oct 6, 2019
41
11
Bay Area
Unless you have a battery backup such as a PowerWall, doesn't solar shut down during a blackout (to protect the linemen).

In this hypothetical, you would install a grid disconnect and the inverter/battery charger connected to the house from the car would trick the solar into thinking the grid is online.
 

outdoors

Always roaming
Aug 10, 2014
1,665
2,853
in the moment
Have had very good luck using the inverter in both 3 and S. Have used the Bestek brand.

Didn't work on my Expresso machine. Was able to get my sousvide to work for a short run, but water in an EV sounds like a bad idea. Especially if driving.

Little off topic. Heating blankets that plug in 12v are pretty nice.

Would love to see a 12v plug in frunk for a mini fridge.
 
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