12V capacity theoretical vs reality

[Shadow Catcher]

A miscalculation?
I bought a Waco refrigerator which draws a max of 4 Amps. This was to go into the back of the Subaru Outback Wagon which has a 12 power outlet in the rear, except when you have the Macintosh sound system (which mine does). So much for that idea
The idea was that when we reach camp the refrigerator comes out and is plugged into 110AC or the trailer battery system.
Alternatives that I see possible.
Tap into the 12V line that comes from the TV battery to the trailer plug it is 10 ga (I measured) and bring it up through the trunk.
Run a line from the trailer battery forward to the back of the car.
Run a separate line for the refrigerator from the battery.
Increase the size of the 10ga line to 8ga or 6ga and tap in.

[Dale M.]

Run separate line for fridge....

Dale

[Cliffmeister2000]

4 amps times 12 volts = 48 watts. Don't plug it into a cigarette lighter.

[Eric K]

Most cig. lighters (accessory outlets) have a 10 amp fuse. If the cooler only pulls 4 amps at 12 volts DC, then it should be fine.

This will be fine while you are running, but car batteries don't like big discharge cycles. Three to four hours without the engine running will pull 12-ah to 16 ah out of the battery. 20 amp hours is as far as I would pull an automotive battery. (Deep cycle batteries are a different story, 30% to 50% can be quite acceptable.)


Eric K

[Shadow Catcher]

The cigarette type plug is what comes with the refrigerator, I was thinking in terms of replacing it with an Anderson Power Pole (I will be using them for the solar panels).
The TV battery is already starting to look vaguely like an octopus with separate lines for the trailer brakes and power to the trailer battery, oh well what are two more lines

[Engineer Guy]

In Industrial applications, I've used this trick often. When you look at the ohms/1,000' wire size Chart, you'll see that 2 - 16 Gauge wires ALMOST equal 1 - 12 Gauge wire in ohms. Smaller gauge wire is more easily run sometimes. So, if you can use one wire run of what you already have in place, this trick is one way to double available current safely. I know from direct experience that Trailer Brakes need their own runs, but perhaps there's some 'wiggle room' for ya to customize your application.

[bdosborn]

The cigarette type plug is what comes with the refrigerator, I was thinking in terms of replacing it with an Anderson Power Pole (I will be using them for the solar panels).
The TV battery is already starting to look vaguely like an octopus with separate lines for the trailer brakes and power to the trailer battery, oh well what are two more lines

Cutting the plug off voids the warranty, but I think you got a used Waeco? The Waeco has an automatic voltage disconnect and 3 levels of disconnect - 10.1 V 11.4 V 11.8 V. When I plugged mine in at the back of the truck, I found it wouldn't turn on with the highest disconnect level selected. I had to turn it down to the medium level to get it to start. I looked at the charge line running back to the rear of the truck and found it was a #14 awg protected with a 40A fuse. This is the factory tow package too! There was so much voltage drop on that little wire that the waeco disconnected itself. I wouldn't mess around kluging in little wires, do yourself a favor and run an automotive quality, copper #12 AWG to the back. Then there should only be about a 2% voltage drop with a 5 amp load.

http://www.nooutage.com/vdrop.htm

Bruce

P.S. I don't know why you couldn't tap off the #10 trailer charge line as long as you add a 20A fuse to protect a #12 tap or run a #10 tap.

[Engineer Guy]

To deal with the real vs. the theoretical in this question, use a Voltmeter to measure voltage drop at the Trailer Plug while the Subaru is running. In the Trailer, turn on ~4 Amps 'worth' of load to simulate Fridge load. Your Voltmeter will tell you if the Fridge is likely to hit the low voltage cutout setpoint. Fridge current draw is periodic, and highest when full of warm Pop, etc.. Plug it in at the Trailer or another Receptacle to measure 'reality'. A periodic 4 Amps draw may be just fine.

The Trailer Battery will draw the most charging current, obviously, after it's been used. When first driving away after use, keep the Audio System volume moderate to keep current draw moderate IF that's even an issue. The pounding Rap music may have to wait until the Trailer Battery charges some! ;-0

Some folks like Fuses; I use RV-style Circuit Breakers because they reset after transient issues. I can disconnect them and find any shorts/problems with an Ohmmeter, if req'd. Convenience [in not replacing Fuses] is king in the boonies, as it is in Home Breaker Panels vs. Fuses of old... Fuses are cheaper, however.

My hunch: adding a rear Cig Lighter Receptacle to the existing #10 wire likely will work. Add a 30 Amp CB/Fuse at the Battery if you like. Another limiting factor is the max available output of the Subaru Alternator before wiring losses, since we're not talking the HD Alternator on a Dodge Diesel here! ;-) Subaru is likely acknowledging that their system can handle upgraded Audio OR auxiliary Cig Lighter current draw - but not both - given Factory tendency to undersize wiring. Charging an 'additional' Trailer Battery is possibly taxing the system already. No point in running more wiring of any size if other factors limit available current. The Subaru Op Manual will tell you the Fuse rating - and max acceptable current draw - when an Aux +12 VDC Cig receptacle is installed vs. upgraded Audio.

See 'Handy Bob's' opinions, including wire sizing, here. RVers with Solar Systems swear by his sometimes-over-the-top advice:

http://handybobsolar.wordpress.com/2009/07/22/solar-power-that-works/

My point above was that IF you wanted to upsize wire, parallel runs is one old trick to do so. My opinion, kept to myself, was that your existing #10 wire run would be sufficient since there's likely other limitations already. Here's a Wire Chart w/fine print qualifiers:

http://www.powerstream.com/Wire_Size.htm