Running a 12 volt inverter off a dual battery set up

[Hammer24]

Just got a few questions about running a 12 volt inverter off my second (deep cycle) battery. I don't see many of you in the US using inverters or dual battery set ups but they're very common here in aus for overlanding or long distance remote trips. When I bought my TJ, a previous owner had already set up the dual battery system, with a double ciga plug (cigarette lighter style accessory outlet) mounted on the back of the centre console that he said is running straight off the second battery. I use one of these plugs to run my small electrothermic fridge with no problems.

So my first question is; I've seen very small inverters (150 to 200 watt) that are designed to plug into a cigarette lighter, but the bigger ones (I'm looking at running a roughly 800 watt inverter) come with leads that are meant to be attached directly to the battery terminals. If I was to assume that the double ciga plug outlet goes straight to the second battery, could I just solder a ciga plug onto the end of the leads of a bigger inverter and plug into that accessory socket?

Second question is, how can I determine how large of an inverter I can safely run?

Also, the smaller 150 watt inverter doesn't have a point for chassis earthing but the bigger ones do, why's that? And how would I earth a larger one to the chassis anyway?

Cheers

 

[someguysjeep]

i'm no electrician but voltage transfer ability is related to wire diameter. to small a wire cooks when you flood it with to much current. so my guess would be the wires supporting the cig outlet would not be sufficient to run the higher powered unit. i unless the current transfer rate is below or can be adjusted below the wire capacity.

bottom line is wire size vs power draw. we don't power our winches with speaker wire.

 

[dbbd1]

You will need to hook up the larger inverter directly to the battery. Divide the watts output by the volts and you will get the amps (current draw) that the inverter requires, at peak output. Most power ports/ciga outlets are usually rated for 10 amps max and are limited by the size/gauge of the wiring to them. See that thick cable on your new inverter? That's the size of wire that your ciga would need going to it also. It doesn't have it (at which point, your next weak link would be the cheap materials used in the ciga outlet). If you want to shortcut your installation and make it temporary, you could use a set of high quality, high current jumper cables to quickly connect to the battery.

 

[dbbd1]

Just to further clarify-

It may not make a lot of sense to some people as to why you need a thick cable going to the inverter when you are just plugging in this skinny little power cord for you device.

It goes back to watts. Again, divide watts by the voltage. For the sake of discussion, we will use a 1200w inverter. At 12 volts, peak, it will draw a nominal 100 amps. This requires a fatty. On the output side at 120 volts, we only have 10 amps of current. Here, we only need a skinny. Watts will stay the same, in and out.

How many watts is your new inverter?

 

[Steel City 06]

It's going to depend on the amp-hours of your batteries. If you're using a single isolated battery rated to say, 60 amp hours, you'll have to determine the required runtime. As a general rule of thumb, 10 watt-hours equals one amp-hour at 12 volts. So to run a 600 watt inverter at full power, you will get approximately one hour of runtime. For battery life, it's recommended that you do not deplete more than 50% depth of discharge. So you're realistically looking at about 30 minutes at 600 watts.

Note that's based on actual power consumption, and not the maximum rating of the inverter. So you could use a 1500 watt inverter and only use 300 watts from it and get an hour of normal runtime before hitting 50% depth of discharge.

If you're willing to idle the engine during periods of high use, that makes a big difference. Not only does the alternator supply a large amount of current, it also raises the supply voltage a volt or two, making for more efficient power conversion.

I have thought about something similar for another car. My F150 has an aftermarket alternator rated at 270 amps (used to have a 370 amp alternator) that puts out 220 amps at idle. I thought about installing a 3000 watt sine wave inverter so I could use the truck as a mobile generator. I could probably use up to 2500 watts continuously at idle, and the batteries would act as a bit of reserve if I ever have to take it to 3000 watts.

 

[Steel City 06]

There are several companies that make up to 370 amp alternators for our Jeeps.

These are the three I recommend (in order):

1. Singer alternators
2. DC Power Inc
3. Mechman alternators

3000 watt sine wave inverters are far cheaper than they used to be thanks to the solar industry.

Make sure you understand the difference between a true sine wave inverter and a modified sine wave inverter and a square wave inverter. Square wave inverters and even many modified sine wave inverters will fry some electronics.

 

[Hammer24]

Thanks for all the feedback guys. I'm not very good with electrical stuff but what you've all said seems to make enough sense. Maybe I will just run a small 150 watt inverter out of the ciga outlet which will be enough to charge my laptop and other small things, just whist driving. I'm not confident properly wiring in a bigger inverter and everything to suit and am also not too fond of my Jeep burning to the ground. Again thanks for all the feedback guys.

 

[Goatman]

We don't really need extra batteries in the US. Your problem is that your country is upside down on the planet and all the volts start to fall out of the terminals. Either that, or those metric volts aren't compatible with our standard volts.

Even if an inverter has that little cigarette plug I would wire it direct to your battery using larger gauge wire and an inline fuse. Those cig plug wiring is marginal to use as intended, let alone for an inverter that will draw power much longer than the brief time needed to heat the lighter part and that will continue to load the wires and generate a lot more heat and resistance.
If you can find a pure sine wave inverter any device (especially electronics) will be much happier and live a lot longer. Hardwire the inverter with fuse and a switch inline to decrease any vampire load it creates to reduce strain on the whole system.