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General question about 12 volt inverter hookup in vehicle

wjbell

Jul 27, 2023
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I have a 3000 watt inverter in my truck. I haven't mounted inside the truck to prevent it from getting the heat from the engine compartment. So I have to run the positive wire through the firewall to the battery. I'd say it's about 8 to 10 ft Max. I'm using a set of jumper cables for the wire. They're not that great of a gauge so for the positive feed I use both the positive and negative of the jumper cable and twisted them together to make the positive lead. Same with the negative but the negative is only about a 1 ft stretch mounted to a grounded section where my console mounts inside. Now this is a two-part question. First, the positive cable heats up where it connects to the battery. To the point where you can't even touch it. It's gotten to the point where the wire started deteriorating and eventually broke off. One of the questions is is my negative connection too weak. In other words, if my positive connection was big enough gauge wire to allow proper flow but the negative wasn't as heavy gauge allowing the throughput that it needs to match the positives amperage flowing through. I guess my question is would that cause the positive connection at the battery to fry? And I know I need a breaker or a fuse in between there but I've been using it without one which is bad. So I'm going to rewire it this weekend I ordered some 10 gauge battery cable / welding lead wire. I'm going to use that for the positive and the negative and find a better place to ground it. So I guess my question is if you have a strong positive and a weak negative will the positive tend to heat up and melt off or pop a fuse? And is 10 gauge wire enough to allow the flow for a 3000 watt inverter drawing maybe 750 watts of power?

Thanks for any help and insight on this.
 

crutschow

May 7, 2021
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One of the questions is is my negative connection too weak. In other words, if my positive connection was big enough gauge wire to allow proper flow but the negative wasn't as heavy gauge allowing the throughput that it needs to match the positives amperage flowing through.
The is no difference in which connection has the higher resistance, since they in series and just add together to make the total resistance of the circuit.

750W will draw a minimum of 750/12 = 62.5A (likely closer to 75A actual).

Having that amount of current flowing through the car chassis is a concern.
Best would be to run both positive and negative wires directly back to the battery.

And you should add a 100A fuse or DC breaker near the battery connection, so you don't burn up the truck if there's an accidental short.

20' of 10ga wire (round trip) will drop about 1.5V @ 75A and dissipate about 110W of power.
That's problematic, since 10ga wire is rated for a maximum of 52A (see here).

8ga wire is rated for 75A and would reduce the drop to about 1V, dissipating 75W of power.

Much better would be to use 2 or 4ga jumper cables, or use at least three 10Ga wires in parallel for both positive and negative.
 

wjbell

Jul 27, 2023
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Okay thank you for the reply. I guess I'm not sure about the 10 gauge maybe I was looking at the 10 ft length. Here's a screenshot of the wire that I was going to order. If I use the same size wire for the ground but just go to it the chassis with a good connection do you think that would be okay. I know it's not optimal but will give me any problems? Like I said, the biggest thing I'm running off of this is a low powered microwave. I think it's only pulling a out 5 to 700 Watts.

Oh and also, I have a 100 amp breaker ready to put in place. So putting it directly to the battery for the breaker is the best way to install it? Would it make much difference if I put the breaker at the inverter side of the power wire?
 

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crutschow

May 7, 2021
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I'm not sure about the 10 gauge
Looks like 1ga, so it's plenty large for that current.
If I use the same size wire for the ground but just go to it the chassis with a good connection do you think that would be okay.
No, it's not a good idea to run that much current through the chassis.
The connection would be through the battery ground connection to the motor and then the motor connection to the chassis, but I don't think that motor to chassis connection is designed to carry that much current (75A).
 

wjbell

Jul 27, 2023
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Looks like 1ga, so it's plenty large for that current.

No, it's not a good idea to run that much current through the chassis.
The connection would be through the battery ground connection to the motor and then the motor connection to the chassis, but I don't think that motor to chassis connection is designed to carry that much current (75A).
Okay thank you very much. Will run both directly to the battery with the 100 amp breaker.
 

wjbell

Jul 27, 2023
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Okay thank you very much. Will run both directly to the battery with the 100 amp breaker.
One other thing, I might be dealing with space constraints going through the firewall with two 1 gauge wires. What do you think the smallest gauge I can use safely for this application?
 

Bluejets

Oct 5, 2014
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Voltage drop will be your biggest enemy especially @ 12v.
Cable may well be rated for gazillion amps but voltage drop will get you.
Use as big as you can, open up the firewall hole if necessary.
Plenty of online v/drop calculators .....
 

kellys_eye

Jun 25, 2010
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I know you have a 12V inverter but this is the reason why 24V inverters were created. Half the current for the cables!

It would be far easier to located the inverter IN the engine compartment and arrange proper cooling for it and take the AC to where it's needed. For heavy current applications it is ALWAYS best to use the shortest physical cable run length when talking high current. I suspect the inverter is only used when you're stationary anyway so constant heat shouldn't be an issue and a simple deflector (possibly supplemented by a fan) is all that would be necessary. What about against the firewall INSIDE the cab? Under the passenger seat perhaps?

Don't disregard the actual connection points at the battery. Even the tiniest amount of resistance caused by a poor or inadequate (size) joint will create enough heat to cause long term problems. The battery connections should have adequate surface area at the clamping points.

Lastly, your microwave will have a cooking (heating) power rating which is usually around HALF the actual power it uses hence a 750W microwave can easily use 1000-1500W watts in use (albeit for a short period).

YMMV
 

wjbell

Jul 27, 2023
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Voltage drop will be your biggest enemy especially @ 12v.
Cable may well be rated for gazillion amps but voltage drop will get you.
Use as big as you can, open up the firewall hole if necessary.
Plenty of online v/drop calculators .....
Thank you.
 

wjbell

Jul 27, 2023
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I know you have a 12V inverter but this is the reason why 24V inverters were created. Half the current for the cables!

It would be far easier to located the inverter IN the engine compartment and arrange proper cooling for it and take the AC to where it's needed. For heavy current applications it is ALWAYS best to use the shortest physical cable run length when talking high current. I suspect the inverter is only used when you're stationary anyway so constant heat shouldn't be an issue and a simple deflector (possibly supplemented by a fan) is all that would be necessary. What about against the firewall INSIDE the cab? Under the passenger seat perhaps?

Don't disregard the actual connection points at the battery. Even the tiniest amount of resistance caused by a poor or inadequate (size) joint will create enough heat to cause long term problems. The battery connections should have adequate surface area at the clamping points.

Lastly, your microwave will have a cooking (heating) power rating which is usually around HALF the actual power it uses hence a 750W microwave can easily use 1000-1500W watts in use (albeit for a short period).

YMMV
Thank you sir. It's like you read my mind. I had my previous inverter (RIP) in the engine compartment. It melted down big time. Ribbon cables inside looked like an old guitar where somebody cut the strings. It's just too hot under there, even for a fan to cool it. It is kind of Midway in a vehicle now so I was actually thinking of moving it up on the floor in front of the passenger seat. That will take away two and a half feet of wire.

My old setup was kind of hokey anyway. For the power wire I cut an old set of jumper cables that were cheap in the first place and twisted the two wires together for my positive and on the battery side used one of the jumper cable clamps to clamp it on the battery terminal. And then the ground was just grounded to the nearest ground I could find inside. Now that I hear you guys advice I'm not surprised and how it was failing. I'm just lucky I didn't get a fire.

Thanks for the advice. I think I'm going to do it clean this time with proper wire and running both positive and negative directly to the battery with 100 amp breaker right at the battery. When I get it hooked up I'll post results to this thread.
 

ivak245

Jun 11, 2021
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If you mount the inverter inside the cab, it will make a dandy heater.....unless you live somewhere where you need airconditiong!
 

wjbell

Jul 27, 2023
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If you mount the inverter inside the cab, it will make a dandy heater.....unless you live somewhere where you need airconditiong!
It doesn't put out that much heat actually. It has a thermostat controlled fan and it hardly ever comes on. And I do live where you need air conditioning, Sacramento California. We have seen 110 115° days.
 

Bluejets

Oct 5, 2014
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It would be far easier to located the inverter IN the engine compartment
Have you stuck your head inside ANY engine compartment ????
Excessive heat, water ,oil, vibration, not to mention bugger-all space to mount an auxiliary relay let alone a 3000w inverter.

but this is the reason why 24V inverters were created
Dohhhhhh !!! I'd say it's a 12v vehicle...no???
 

kellys_eye

Jun 25, 2010
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Dohhhhhh !!! I'd say it's a 12v vehicle...no???
Not questioning that fact but simply illustrating the reason why such devices were invented.

Have you stuck your head inside ANY engine compartment ????
Yeah, many! True, space can be at a premium but with enough determination it can be found. Even modern 3000W inverters aren't THAT large! Without knowing the specific details of the vehicle (or a handy manual detailing it) I'm just guessing though.

Another method would be to mount another battery BESIDE the inverter and charge that battery through a blocking diode from the original charging circuit - this need not be a heavy current circuit, just enough to recharge/maintain the battery. I used this method for many marine applications (usually capstan winches) where a battery cable run direct to the winch motor would mean the length of the vessel (from the engine room) to the winch motor) so a separate battery was fitted locally to the winch motor and a charging wire - via aforesaid blocking diode - was run back to the engine/alternator. Indeed, the use of blocking diode/charging is commonplace in many RV-type situations where aux batteries are fitted.

This 'auxiliary' battery need not be overly large either. A small (say 65Ahr) standard car battery would easily suffice - probably something a LOT smaller if your inverter isn't demanding a constant LARGE load. The microwave example might use 100A for 3 minutes which equates to only 5 amp-hours draw from a battery - the battery can supply that current and the charger (alternator) need only deliver a 'minor' current (10A) over the next 30 minutes to recharge it.

It works in a marine application - it'll certainly work in a car/truck.
 
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wjbell

Jul 27, 2023
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Not questioning that fact but simply illustrating the reason why such devices were invented.


Yeah, many! True, space can be at a premium but with enough determination it can be found. Even modern 3000W inverters aren't THAT large! Without knowing the specific details of the vehicle (or a handy manual detailing it) I'm just guessing though.

Another method would be to mount another battery BESIDE the inverter and charge that battery through a blocking diode from the original charging circuit - this need not be a heavy current circuit, just enough to recharge/maintain the battery. I used this method for many marine applications (usually capstan winches) where a battery cable run direct to the winch motor would mean the length of the vessel (from the engine room) to the winch motor) so a separate battery was fitted locally to the winch motor and a charging wire - via aforesaid blocking diode - was run back to the engine/alternator. Indeed, the use of blocking diode/charging is commonplace in many RV-type situations where aux batteries are fitted.

This 'auxiliary' battery need not be overly large either. A small (say 65Ahr) standard car battery would easily suffice - probably something a LOT smaller if your inverter isn't demanding a constant LARGE load. The microwave example might use 100A for 3 minutes which equates to only 5 amp-hours draw from a battery - the battery can supply that current and the charger (alternator) need only deliver a 'minor' current (10A) over the next 30 minutes to recharge it.

It works in a marine application - it'll certainly work in a car/truck.
Not questioning that fact but simply illustrating the reason why such devices were invented.


Yeah, many! True, space can be at a premium but with enough determination it can be found. Even modern 3000W inverters aren't THAT large! Without knowing the specific details of the vehicle (or a handy manual detailing it) I'm just guessing though.

True, I have a 2004 Ford expedition 4WD and have plenty of a space.

Another method would be to mount another battery BESIDE the inverter and charge that battery through a blocking diode from the original charging circuit - this need not be a heavy current circuit, just enough to recharge/maintain the battery. I used this method for many marine applications (usually capstan winches) where a battery cable run direct to the winch motor would mean the length of the vessel (from the engine room) to the winch motor) so a separate battery was fitted locally to the winch motor and a charging wire - via aforesaid blocking diode - was run back to the engine/alternator. Indeed, the use of blocking diode/charging is commonplace in many RV-type situations where aux batteries are fitted.

Yes, a battery isolator. You have that in between the normal battery for the truck and the extra battery for the inverter. That allows the inverter to only use the power from the inverter battery without touching the main battery but allows the inverter battery to be charged by the system. Like you said, if you could have that set up with your inverter battery right next to the inverter under the hood while being able to keep the inverter cool enough that would be the perfect setup.

With the heat under the hood and living in a hot climate I think my best option is to get as close to the battery as possible without being under the hood. That's going to be the front floorboard. Maybe later I can add an extra battery for the inverter on the front floor board also with the isolator.
This 'auxiliary' battery need not be overly large either. A small (say 65Ahr) standard car battery would easily suffice - probably something a LOT smaller if your inverter isn't demanding a constant LARGE load. The microwave example might use 100A for 3 minutes which equates to only 5 amp-hours draw from a battery - the battery can supply that current and the charger (alternator) need only deliver a 'minor' current (10A) over the next 30 minutes to recharge it.

It works in a marine application - it'll certainly work in a car/truck.
 

sarika78

Jan 7, 2024
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"Installing a 12-volt inverter in a car might be a practical method to power small electrical gadgets on the road. To minimize excessive drain on the vehicle's battery, ensure appropriate installation. Always utilize proper gauge wiring and secure connections to ensure safety and dependability while enjoying the benefits of portable power."
 

wjbell

Jul 27, 2023
38
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Jul 27, 2023
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"Installing a 12-volt inverter in a car might be a practical method to power small electrical gadgets on the road. To minimize excessive drain on the vehicle's battery, ensure appropriate installation. Always utilize proper gauge wiring and secure connections to ensure safety and dependability while enjoying the benefits of portable power."
I'll be damned, the instructions. I knew I forgot sumpin...
 
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