Extension cord from car battery to power inverter does not work

[carcharger]

I need to have an extension from the car battery to a power inverter in the back seat to charge camera batteries while I am driving.

The built-in car cigar lighter is inop. I am wiring a cigar lighter socket directly to the car battery in the hood. From this socket, I am using a 3 m (10 ft) extension cord from the hood, via the window, that is connected to a power inverter in the back seat of the car.

The problem, whenever I use the extension cord, the charging fluctuates and is totally unreliable/ I tried 3 different extension cords of various brands, all unreliable. If I plug the power inverter directly into the socket at the battery, it works perfectly and powers 120W light bulb with no problem. (As a troubleshooting, I tried to plug the extension cord in another car's working cigar lighter, but the same exact problem presents: with extension cord is fluctuating, power inverter pluged-in directly is reliable)

What is the cause of the problem and how to fix it? Is it a long extension cord or a fuse inside the extension cord or something else?

 

[sagor1]

The wire gauge may be causing a large voltage drop, large enough to cause the inverter to not start up.
for 120W bulb, you need at least 10A at 100% efficiency, so assume you are drawing 12A or so from the battery. A cigar lighter socket is only rated for about 10A, but close. However, those may have poor contacts and cause a bit of voltage drop as well.
Even with 14G wire, you get a voltage drop of 0.75V at 12A in each direction (1.5V total). If using smaller wire, the voltage drop will be even more. For example, 16G will drop almost 1V over 10ft (each way), so for both wires you will get close to a 2V drop.
To have reliable inverter input voltage, use as large a wire as you can, 12 or 10 gauge would be better.
Another point: Some inverters are modified square wave, not true sine wave. Some chargers will work with square wave, but poorly or not at all. If the charger works with the inverter plugged direct to the battery, then the charger is probably ok with your inverter. It is just the inverter then that does not like the long wire length.

 

[Externet]

All the chinese inverters rated for 12VDC do not work on 12V, they want over 13VDC; easy to happen with poor connections and lossy cables.
A possible fix in your case is to locate the inverter in the engine compartment with as short and as firm DC conection terminations as possible to battery and then pray to the chinese gods with the engine running, extending the AC wiring into the cabin.

 

[Diver300]

Some cars vary the charging voltage when the battery is fully charged, to save a bit of fuel. That could be messing things up for you. It's much more difficult to work out if the engine is running from the battery voltage alone than it used to be.

Also, on many cars you should not connect anything to the negative of the battery, but you should connect to the body instead. It is quite common to have a current monitoring device built into the the battery negative lead, and if you connect to the battery negative, you are bypassing that monitoring. You could run the battery down without the battery monitoring circuit seeing any current being taken.

If you use an extension, you could connect the negative of the inverter to the body instead, and use both cores of the extension lead for +ve. That would reduce the voltage drop by 4 times, as there would be half the cable length, and half the current in each core.

 

[carcharger]

More details:

Power inverter I use:

BESTEK 150W Power Inverter DC 12V to 110V AC Converter 4.2A

These are the extension cords I tried, all three causes the 2 x 60W bulbs' light to fluctuate. When I plug the power inverter directly into a working socket, the lights stay steady.

Digit.Tail 12ft/3.6m Car Cigarette Lighter Extension Cable, 15A 250V Fused, Male to Female Socket Plug Lead Cord for 12V 24V Vehicle

SCCKE 14AWG Premium Pure Copper 12V 24V Car Cigarette Lighter Socket Extension Cord, 14.8ft / 4.5m 14 AWG Cable Wire for Car Inverter

Cigarette Lighter Splitter 2 Socket, ZHSMS Car Charger Port Splitter 12V/24V Power Outlet Dual Plug with 10FT Cigarette Lighter Extension Cord

 

[rjenkinsgb]

It says on the inverter that it needs up to 18A input. That is going to be the average power and it may well take high current peaks with gaps between, so that is the absolute minimum you should allow for.

14AWG has a resistance of 8.2 Ohms per 1000 ft or 8.2 milliohms per foot.

That makes the resistance of the 14.8ft cable (29.6ft total conductor length) 243 milliohms, which at 18A will drop roughly 4.4V (and be dissipating ~79 watts of heat in the cable.

That's best case. If it's taking power in pulses the voltage losses and power loss will be higher.


As Sagor said to start with, you need much thicker cable; those 12V extension cables are only really good for much lower current devices, regardless of the claimed ratings.


The wiring in the vehicle to the lighter socket could be 10ft or more and will be adding resistance and voltage drop; at 18A you may well be putting enough load through that to start heating or even melting the vehicle wiring harness and cause damage to that over time.

Lighter sockets are often rated at 10A maximum. Also, cable current ratings reduce drastically when they are in bundles rather than free air, as the heat builds up rather than dissipating.

For a 15ft power cable, I'd say you need something like 8 AWG, 2.1 milliohms per foot. That's still around 63 milliohms total, so about 1.1V drop at 18A and 20W of power lost in the cable.


Ideally that should connect directly to the battery positive via a 20A or 25A fuse and use a single cable, with ground from the vehicle metalwork, as Diver says.

A single wire instantly halves the voltage drop and power losses, and a direct battery connection avoids any risk to the vehicle internal wiring.

(I realise the inverter is not running at maximum load all the time - but the current peaks could easily be 18A or more even when the average load is rather lower).

 

[carcharger]

Great answers, thanks!

More details:

With extension cord, it the inverter can not even power a DJI Battery Charging Hub for Mavic 2 Pro (without extension cord, it powers with no problem). This charging hub has the following specs:
Input Power
17.6 V at 0 to 5.7 A

Output Power
17.6 V at 3.41 A
17.0 V at 3.53 A

 

[carcharger]

More details:

If I plug the power inverter directly to the cigar lighter AND use a 5 m (15 ft) regular household extension cord from the power inverter to the 2x60W light bulbs, the bulbs light up perfectly normal, steady.

 

[rjenkinsgb]

If I plug the power inverter directly to the cigar lighter AND use a 5 m (15 ft) regular household extension cord from the power inverter to the 2x60W light bulbs, the bulbs light up perfectly normal, steady.

You are then using the cable at approximately ten times the voltage, so 1/10th current for the same power!

 

[Diver300]

You are then using the cable at approximately ten times the voltage, so 1/10th current for the same power!

Also at 1/10th current means 1/100 of the power loss.