Zil said:
if you have small wires and poor connectors from the battery to the appliance, you will pull more amps from the battery. The appliance power is measured in watts, lower volts higher amps, higher volts lower amps. More resistance getting the power to the appliance, equals more amps hours from the battery.
I expected this too, but with resistance heating it does not happen. The less volts that make it to the appliance, and it will take less amps too.
I've noticed the same thing with some computer fans. My meters are not lying.
The Ciggy plug and receptacle are simply poor designs. Convenient and standard, yet poor designs. YOu can feed the receptacle with 10awg, and have 10 awg from plug to appliance, but there will still be significant resistance, heating, and voltage drop across the plug and receptacle.
The higher the load the more of all three. 13 amps is asking too much from even a well wired receptacle especially for the duration required by some of these appliances to complete their job.
Higher quality receptacles and plugs do exist , but it remains a fact that it is a spring loaded connection. If/When the 'nipple' spring backs the connector out of the socket, then that nipple contact now has an extremely poor connection and heats up significantly. Then the spring loses its spring, and then the device no longer functions.
Here is a Blue Seas plug and receptacle, which have a 'locking' design. I've still never bothered to put this one into use, in favor of the Anderson connectors.
No soldering required.
It is only rated for 15 amps, and I think, that is generous.
But no doubt it is a better design than most ciggy receptacles and plugs.
Check out this lesser ciggy plug which failed powering my laptop, max of 7.5 amps.
Check out the discolored steel spring which is fully compressed.
The fuse is showing overheating damage too. Yet it never blew.
The plug which came with my mattress heating pad , and appears to be the same as on the RoadPro, was made a bit better, with stronger ground and nipple contact springs, and NO internal fuse, which has been the weak point in every design with the fuse, that I have come across.
I am not going to recommend not fusing something.
The house battery needs to be grounded to the chassis, if it is getting recharging current from the alternator.
13amps is also in Peukert territory.
http://en.wikipedia.org/wiki/Peukert's_law
Peukert says the higher the load the less overall capacity the battery has to give. Simple math is not going to account for a 13 amp load for x amount of minutes subtracted from overall battery amp hour capacity.
These 12v heating appliances might work well enough, and be convenient, but replacing the energy taken from the battery is a factor I think many are not considering. Also if relying only on the alternator for recharging, you are very likely not starting with a fully charged battery when operating such a device.
Just do not be too surprised when the battery fails.
I kind of think it is irresponsible of the manufacturers of these devices to ask for such capability of a ciggy plug and receptacle.
But it is a hard spot for them. The electrical demands can be met but the standard convenient ubiquitous 12v receptacle, in a best case scenario, is at or well past its limits. Once it heats up it degrades, further reducing its capability to meet the devices demands, on and on, until failure.
