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Is it safe to connect a large inverter to the 12V battery?

BrettS

Active Member
Mar 28, 2017
2,108
2,511
Orlando, FL
Indeed, as far as I know there is no fuse on the 12V poles.

I want to charge a big 12V battery from my Model S (long story!) on a very remote location.

I want to connector jumper cables and see how that works out.

There absolutely is a 50 amp fusable link on the 12V poles. A number of people have accidentally blown it. There is some additional detail and a picture in this post from this thread:

Is the battery post under the nose cone fused?

Yes, I believe there is a fuse, but I'm pretty sure the fuse is actually sitting on top of the battery. Let me see if I can find the photo I saw a while back that showed this...

- - - Updated - - -

Found it here:

12 V battery pictures

View attachment 50877

Would be interested in hearing what kinds of fuses those are and how hard it is to get to them to replace them.
 
Jan 22, 2016
176
110
Tonto Basin, Az
Just hooked up my 400 W inverter to the charging lugs with 220W load. I noticed the voltage at the lugs jumped to 14 V....presumably because the DC-DC converter kicked in.

Do you guys think I might overcharge the 12V battery in the car? or will it just top off the battery to 14.5 and then shut off.
 

FlatSix911

Porsche 918 Hybrid
Jun 15, 2015
6,512
5,917
Silicon Valley
thx flatsix, where is that fuse ???
There absolutely is a 50 amp fusable link on the 12V poles. A number of people have accidentally blown it. There is some additional detail and a picture in this post from this thread:
Is the battery post under the nose cone fused?
Here is the Battery-Mounted Fusebox Layout ... the 12V posts are confirmed to use a 50A fuse :cool:
View attachment 274834
upload_2018-1-21_11-28-40-png.274834
 

David99

Active Member
Jan 31, 2014
4,850
7,021
Brea, Orange County
Just hooked up my 400 W inverter to the charging lugs with 220W load. I noticed the voltage at the lugs jumped to 14 V....presumably because the DC-DC converter kicked in.

Do you guys think I might overcharge the 12V battery in the car? or will it just top off the battery to 14.5 and then shut off.

I ran an inverter for a while. As long as there is a high load on the 12 Volt system the DCDC converter will keep the voltage at around 14.1-14.2 Volt. I'm not sure if that is bad for the lead acid battery in the long run. I don't know what these batteries are able to take for a longer period. I was under the impression that 13.8 Volt is the max for a sealed lead acid battery as a continuous voltage. But I might be wrong.
 

FlatSix911

Porsche 918 Hybrid
Jun 15, 2015
6,512
5,917
Silicon Valley
I ran an inverter for a while. As long as there is a high load on the 12 Volt system the DCDC converter will keep the voltage at around 14.1-14.2 Volt. I'm not sure if that is bad for the lead acid battery in the long run. I don't know what these batteries are able to take for a longer period. I was under the impression that 13.8 Volt is the max for a sealed lead acid battery as a continuous voltage. But I might be wrong.
I did a few tests with my 750W inverter and found no issues with the 12V battery.
The main high voltage 350-400V traction battery recharges the 12V when required. :cool:
IMG_7750.jpg

IMG_7745.jpg
 
Last edited:

David99

Active Member
Jan 31, 2014
4,850
7,021
Brea, Orange County
I did a few tests with my 750W inverter and found no issues with the 12V battery.
The main high voltage 350-400V traction battery recharges the 12V when required. :cool:

Yeah, that's exactly my experience, too. Even if the car it parked/locked and 'off', the DCDC converter will always kick in to keep the 12 Volt system/battery at a good level. The only concern was that I measured the voltage and it was at 14.2 which might be a little high for the lead acid battery in the long run.
 

FlatSix911

Porsche 918 Hybrid
Jun 15, 2015
6,512
5,917
Silicon Valley
Yeah, that's exactly my experience, too. Even if the car it parked/locked and 'off', the DCDC converter will always kick in to keep the 12 Volt system/battery at a good level. The only concern was that I measured the voltage and it was at 14.2 which might be a little high for the lead acid battery in the long run.
I am seeing 13.4 - 13.6V input from my 12V battery when testing. :cool:
 

widodh

Model S 85 and 100D
Jan 23, 2011
6,853
2,771
Venlo, NL
I did a few tests with my 750W inverter and found no issues with the 12V battery.
The main high voltage 350-400V traction battery recharges the 12V when required. :cool:
750W? That's 62,5A at 12V. Posts above suggest that there is a 50A fuse.

For how long did you run a 750W load on it? Because with 50A the posts behind de nosecone should only be able to provide 600W.
 
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FlatSix911

Porsche 918 Hybrid
Jun 15, 2015
6,512
5,917
Silicon Valley
Indeed, as far as I know there is no fuse on the 12V poles.
I want to charge a big 12V battery from my Model S (long story!) on a very remote location.
I want to connector jumper cables and see how that works out.
See my post above... 50A protection is shown on the fuse diagram.
750W? That's 62,5A at 12V. Posts above suggest that there is a 50A fuse.
For how long did you run a 750W load on it? Because with 50A the posts behind de nosecone should only be able to provide 600W.
Remember to use 80% of the rating for sustained loads...the design voltage is 12-15V.
 

widodh

Model S 85 and 100D
Jan 23, 2011
6,853
2,771
Venlo, NL
See my post above... 50A protection is shown on the fuse diagram.

Remember to use 80% of the rating for sustained loads...the design voltage is 12-15V.
Yes, but still. You can't draw 750W from the nosecone that way I assume, right?

50A fuse at let's say 14V would be 700W, not 750W.

So is the fuse really 50A? Or is it 60A and designed for 50A in all environment?
 

David99

Active Member
Jan 31, 2014
4,850
7,021
Brea, Orange County
OK, so we can't get enough current from the nosecone terminals. Can we attach directly to the 12V battery terminals - are they accessible enough?

Yes that's what I did. You need to remove the cover above the frunk. Depending on the year built, the battery is either to the left under the air filter or in the center. There is a main 240 Amp fuse so you are still protected from disaster. The DCDC converter is rated for 2.4 kW. I would limit to 1 kW to make sure the car has the rest available when it needs it.
 

zap fizzle

Member
Jan 2, 2018
62
45
on the internet
Yes that's what I did. You need to remove the cover above the frunk. Depending on the year built, the battery is either to the left under the air filter or in the center. There is a main 240 Amp fuse so you are still protected from disaster. The DCDC converter is rated for 2.4 kW. I would limit to 1 kW to make sure the car has the rest available when it needs it.

Thanks. There are a lot of wobbly 1950s utility poles between our house and "civilization". It would be very nice to be able to run the fridge and [gas-fired] central heating off the car for a while next time the power is out for a day. Glad to hear it's possible; it was easy enough with the LEAF.
 
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Dr. Smoke

My former name was Edmond. Before that, Quantum`.
Oct 26, 2016
101
105
Edmonds, Washington
Really, connecting to the 12v system is not a good solution. Best would be having a qualified shop install a HV inverter near the rear HVJB and run 120vAC outlets where convenient. Camping mode.
 
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David99

Active Member
Jan 31, 2014
4,850
7,021
Brea, Orange County
Really, connecting to the 12v system is not a good solution. Best would be having a qualified shop install a HV inverter near the rear HVJB and run 120vAC outlets where convenient. Camping mode.

Ideal would be a two way connection to the grid so every EV would act as an energy buffer for the grid. Tesla knows this but wants to sell this concept as a separate product (Powerwall), so there is no hope Tesla will implement it in their cars. They also don't want you to get free energy from Superchargers and then power your home with it.

For DIY home use I think going into the 12 Volt system is the cleanest and safest. It also requires no artificial camper mode. If there was a simple way to connect to the high voltage system, I would prefer that. It's just too much trouble and much more expensive.
 

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