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

Watts_Up

Active Member
Mar 4, 2019
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In a galaxy far, far away
And they also monitor the current going to the battery? I'm guessing there's a solid state switch in there between the 12V DC/DC bus and the battery to allow them to measure charge current.
I noticed also that the inboard DC/DC converter can also recharge the battery when driving, by setting the voltage to 14.5 instead of 13.5 V and then going back to 13.5 V.
 

Watts_Up

Active Member
Mar 4, 2019
3,708
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In a galaxy far, far away
How do you change the voltage?
Sorry, may be I was not giving a clear explanation.

I installed a 12 V Battery Monitor, which generates a countinous voltage graph of the 12 v battery.
You can access this information using a smartpone App connected with Bluetooth.

I noticed the different voltages, such as about 12.5 V when the car is sleeping, 13.5 V when the car weak up and running,
and 14.5 V when the battery is charging which can occur when the car is parked or when the car is running,
 
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mspohr

Well-Known Member
Jul 27, 2014
10,044
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California
Sorry, may be I was not giving a clear explanation.

I installed a 12 V Battery Monitor, which generates a countinous voltage graph of the 12 v battery.
You can access this information using a smartpone App connected with Bluetooth.

I noticed the different voltages, such as about 12.5 V when the car is sleeping, 13.5 V when the car weak up and running,
and 14.5 V when the battery is charging which can occur when the car is parked or when the car is running,
Thanks. I also have one of those and have noted that behavior.
 

miimura

Well-Known Member
Aug 21, 2013
6,540
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Los Altos, CA
Those voltages are typical for an AGM type battery. 14.5V is typical for active bulk charging, 13.5V is typical for float charging, and 12.5-12.7 is typical for when the battery is supplying current to a small load. The car is just implementing those charging strategies and the whole 12V system follows those voltages. Since the Model 3 can actively monitor the battery charging current, the PCS is regulating voltage to achieve the desired charging current while simultaneously supplying excess charging current to the rest of the 12V systems in the car. An AGM battery is very elastic, for lack of a better term, to charging voltage. In other words, the charging current changes relatively slowly as you vary the supply voltage. A LiFePO4 battery like the new one in the refreshed Model S is very inelastic because the charge/discharge curve is very flat. I wonder how Tesla has managed that issue with such a small battery.
 
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n2mb_racing

Active Member
Jun 14, 2014
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durham, NC
Those voltages are typical for an AGM type battery. 14.5V is typical for active bulk charging, 13.5V is typical for float charging, and 12.5-12.7 is typical for when the battery is supplying current to a small load. The car is just implementing those charging strategies and the whole 12V system follows those voltages. Since the Model 3 can actively monitor the battery charging current, the PCS is regulating voltage to achieve the desired charging current while simultaneously supplying excess charging current to the rest of the 12V systems in the car. An AGM battery is very elastic, for lack of a better term, to charging voltage. In other words, the charging current changes relatively slowly as you vary the supply voltage. A LiFePO4 battery like the new one in the refreshed Model S is very inelastic because the charge/discharge curve is very flat. I wonder how Tesla has managed that issue with such a small battery.
Probably a dedicated charging controller for the 12V battery. I think it's actually lithium ion, based on these photos:

3.6v per cell, 4s battery, 6.9 Ah. Strange cells, though. maybe pouch cells?
 
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Snowstorm

Active Member
Dec 8, 2016
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Ontario Canada
I was able to connect a 1000w inverter to my model S and tested it for about an hour at 500w output. Used 4awg wires to 12V battery terminal with an inline 100A fuse, connected to 1000w pure sine inverter. The inverter is charging another battery/inverter intended for house backup in an emergency. Absolutely not for regular use, but successful proof of concept to power my fridges, lights, furnace and internet.
 

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Snowstorm

Active Member
Dec 8, 2016
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Ontario Canada
A potential limitation beyond the DC-DC which is rated for 2000W or so is the fuse on the positive battery terminal. I can imagine if we are drawing enough current, the DC-DC may push the current in but this fuse would blow. Does anyone have an idea what is the rating of this fuse?

The alternative is to attach the inverter cable to the terminal of the DC-DC converter instead, which as a 250A fuse. However, the same problem may exist if it pulls hard from the battery before the DC-DC kicks in.

1630165480269.png
 

n2mb_racing

Active Member
Jun 14, 2014
1,181
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durham, NC
A potential limitation beyond the DC-DC which is rated for 2000W or so is the fuse on the positive battery terminal. I can imagine if we are drawing enough current, the DC-DC may push the current in but this fuse would blow. Does anyone have an idea what is the rating of this fuse?

The alternative is to attach the inverter cable to the terminal of the DC-DC converter instead, which as a 250A fuse. However, the same problem may exist if it pulls hard from the battery before the DC-DC kicks in.

View attachment 702499
You could bypass that fuse and tap your inverter to the unfused +12V terminal and add your own 100A fuse. Then the car can charge the 12V as needed and you can pull whatever you need from the battery.

Reminder, this only works on the Model S/X. Not the Model 3/Y. Those have to go to the DC/DC.
 

Snowstorm

Active Member
Dec 8, 2016
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Ontario Canada
You could bypass that fuse and tap your inverter to the unfused +12V terminal and add your own 100A fuse. Then the car can charge the 12V as needed and you can pull whatever you need from the battery.

Reminder, this only works on the Model S/X. Not the Model 3/Y. Those have to go to the DC/DC.
I am attached directly to the battery terminal now, but the fuse comes into play anyway as the DCDC would push current into the battery through that fuse. So if I pull more than the fuse rated current out, the DCDC would attempt to replenish it and blow that fuse. If I tap into the side of the connected after the fuse (connected to the DCDC), I risk drawing more current from the battery and blow the fuse before the DCDC kicks in.

So I am trying to fins the rating for that battery terminal fuse and not draw more current than that fuse is rated for. If that is day 100A, then no worries. But if that is say 50A then I don’t know what to do.
 

FlatSix911

Porsche 918 Hybrid
Jun 15, 2015
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Silicon Valley
A potential limitation beyond the DC-DC which is rated for 2000W or so is the fuse on the positive battery terminal. I can imagine if we are drawing enough current, the DC-DC may push the current in but this fuse would blow. Does anyone have an idea what is the rating of this fuse?

The alternative is to attach the inverter cable to the terminal of the DC-DC converter instead, which as a 250A fuse. However, the same problem may exist if it pulls hard from the battery before the DC-DC kicks in.

View attachment 702499
I believe that the fuse was listed at 50 Amps in another battery thread... I'll search and post the link. ;)


 
Last edited:

Snowstorm

Active Member
Dec 8, 2016
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Ontario Canada
I think the nose cone terminals of the pre refresh has a 50A fuse, but those terminals are not there on the refreshed model. The unknown is a fuse on the positive terminal of the 12V battery on the refresher model s. I can’t see any obvious marking on the fuse, maybe I will take it apart and look harder.
 
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cuti

Member
Aug 4, 2021
12
44
Fremont
I have been using a 2000-5000 surge for years on model S with the inverter connected to the 12 volt and it is always kept topped up with the dc/dc automatically whether the car is on or off. I also run the largest Massimo cooler they make (think CX50) from a wired in 12 volt power socket tied into the liftgate circuit. No camp modes etc is necessary if tied into an always hot circuit. I just got back from a trip where I was using both... First time using the electric cooler/freezer was really a plus. The cooler plus a c-pap machine used about 4 miles for every five hours of use. Fully powering up a house uses much more of course and depending what you power up. Usually we are powering lights and TV and movie player at the cabin but all of that stuff uses much more than the above described pieces of equipment.
with your wiring, can I pull 1000W per hour for 4 hours straight on everyday ?
 

Snowstorm

Active Member
Dec 8, 2016
1,556
1,495
Ontario Canada
Get a jackery battery pack and connect divided through that as a pass through I use that for the iceco portable frig.
I have tried to use a jackery 1500 pack to backfeed into my house panel while being charged by the car. It works, as long as I keep the power draw under 1800w. However, the fastest I can charge it is with 2xAC charger and it max out at 560W, pulling about 700w from the car, probably 750-800W from the HV battery/ (HV DC to LV DC to AC to DC losses). So if I try to draw more than 560W, the battery will drain. It is good for the most critical loads like communication, lights, fridge, gas heating appliances and sump pump, but not all at once or continuously so. Better than nothing backup, not full home solution.

The Jackery 2000 can be charged at 800-1000w through the AC, so that might be better. However you will likely need a 1500W inverter and be pulling 100A from the car, not sure if that would cause problems. I have only pulled maybe 60A max so far.
 

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