wiring a 350w inverter directly to battery? help.

Hi guys,

I was planning to run a few of my gadgets in mr car (1991 civic 1.5L manual
std-hatchback, no frills) and bought a inverter because I neither have nor
want to spend the ridiculous prices they ask for buying ciggarette adapters
for each device, AND didn't think I would have been able to fit 5 of them
together anyway. The fact that my vanilla car never even came with a
ciggarette lighter socket is another story. I also upgraded my battery to a
nice optima yellow top of 51R size.
I concluded that it would be a much better investment to just buy an
inverter, into which I could plug any device with its own ac adapter that it
already comes with. (though not as clean as the other solution, with
inverter bulk, adapter bulk and DC-AC-DC conversion. ok whatever.)
Well now that I have it, I am not sure how to wire it in. I found that the
wire that feeds the lighter plug is live, even though there is no lighter.
So I was thinking of wiring the invereter to taht, but didn't think it
would handle the current requirements that I plan. Lets see: laptop 60w,
battery charger 20w, camera 20w, GPS 1w etc etc..so I expect a total of
around 120W power. Which is 120w/12v = 10Amps approx. ---> So I would design
for 15-20 amps.

So my question is about the practical part: what wire(gauge etc) to use,
and how to physically wire it to the battery? Inverter stays in the cab, so
the wire would need to come throught the firewall. Is it possible to not run
two full wires to the battery and maybe ground one side somewhere closer
inside the car? Or is this totally unadvisable for large currents?
Also do I need a fuse or any other electronics? Where should I put it?
Anyone done this in their car?

Please help!
Thanks,
Ak

 

350 watts at 12 volts is around 30 amps. Too much for the convenience outlet
in the car. Most cars don't have a cigarette lighter any more, only the
convenience outlet which is fused at 15 amps. Check the install instructions
for the inverter. It should give you wire sizes and directions. Make sure it
is fused close to the battery.

 

12V to 120V inverters are switch-mode inverters. This means that they
rapidly change between drawing virtually no current and drawing quite a
bit of current. If you are drawing an average of 15 Amps, the peak
current will be much higher. Decoupling caps in the inverter smooth this
out somewhat, but inverter manufacturers skimp on the capacitors.

What this means is that for purposes of wire heating, you can follow the
charts for AWG, wire length and current draw. However for efficient
operation of the inverter you need to bump up the wire gauge a bit. with
respect to wire heating you need a minimum of 12 AWG regardless of
length. 12 AWG wire can handle 15 A without overheating.

You can run a ground lead to the battery, or ground the the body
somewhere in the cabin. You can use an existing ground stud, shared with
other components, but do not tap in to an existing ground wire.
Personally I think it is cleaner to run both power and ground leads all
the way to the battery. No extra work and minimal extra expense. On my
Civic there is an under-hood fuse box, and a fat wire from the battery
goes into this fusebox and is connected with a screw terminal. Very
convenient spot to tap into for your own high-power accessories.
Remember to put in a fuse close to where you tap into battery power. If
you plan on dimensioning for 15A, I would use a 20A fuse. For ground I
use the same chassis-mounted stud that the battery ground lead goes to.

If you go with two wires to the battery you will end up with a fairly
significant wire length round trip. That, together with the peaky nature
of the current draw would lead me to suggest 8 AWG wire. It is
conventional to use yellow for positive, unswitched supply and black for
ground.

Often times there are unused holes in the firewall, plugged with rubber
plugs. Great place to pull wires through. Remember to use a grommet
wherever you pull wires through sheet metal. If you go to
http://www.hondaautomotiveparts.com or
http://www.hparts.com/instructions.php3 you can find installation
instructions for accessories to your car. Read through them as they
often give locations of unused holes in the firewall. The instructions
for security systems are a particularly good source for that kind of
info.

If you can't find an unused hole, you need to drill your own. Be careful
not to put holes in important stuff on the back side of where you are
drilling! A somewhat uglier approach is to look for the rubber grommet
where one of the car's main wire harnesses go through the firewall. If
the grommet is large you can sometimes punch a small hole in it next to
the factory wires and pull your own wires through that way.