Converting watts to amps?

Hi Boo Boo,

Power (in watts) is equal to the voltage times current (in amps).

Thus the maximum current draw that the outlet is rated for is 10 amps.

Amp-hours are different again - generally used for battery capacity or power consumption. One amp-hour is the amount of current usage if one amp is drawn for one hour. (ie: current multiplied by time in hours).

It can get more complicated when dealing with battery capacity, which I'm sure one of the battery experts will elaborate on, however loosely speaking, if you have a 90 amp-hour battery, it would be flattened if 10 amps was drawn for 9 hours, or five amps for 18 hours, etc etc.

The math gets somewhat more complicated if you add a power inverter to the mix, and start dealing with 240 volts as well.

Hope this helps!

Charles

Theoretically you are almost correct.

Watts divided by Volts gives Amps (not amp hours), but this is only in theory.
Voltage drop due to either undersize or overlong cabling will affect the actual results.

Hope this helps, others may explain it all more clearly.

Disco.

Charles and Disco above have given good explanation but just for interest in engineering terms it means just a little more.

It means that the socket and wiring can supply 10amps continously without overheating damage when the voltage from the battery (typical reference 13.8v) is such that there is 12v on the socket.

Behind this is the rating of the wire and contacts - different wire covering has different temperature ratings and in supplying the 10 amps above the wire thickness could be different - depending on length of run etc.

Designer can use either thinner high temp wire or thicker low temperature wire to achieve a result and get cost savings at expense of performance.

Hello Boo Boo,

in the case of a ciga lighter socket, there'll be localised heating in the spring loaded contacts.
This pushes up the temperature speak resistance of the metallic conductors on both sides (plug/socket).
Higher resistance means more voltage drop and more heat again, a self feeding loop which is only stopped by a significant temparature gradient which forms around the hot spots and the cooler wire conductors attached.

What this boils down to: a cold set of contacts have a higher current carrying capability than a hot set.
Traditionally ciga lighter sockets were meant to deliver 10A for 15~20 seconds. Then, there'd be some time before another cig gets lit. So this is a typical intermediate duty application, allowing for the contacts to cool before the next burst of current.

But then, along came other uses for 12V power which are more of a continuous duty nature.
Because there's hardly any time for cooling in a continuous duty situation, some derating is in order.

Depending on the quality and type of material (ceramics or plastics) surrounding the contact areas, these sockets have different continuous duty ratings.

Expect no more than 7~8A (~90 Watts) continuous from them @ 25 degrees ambient.

If you need a higher continous current from these, you could wire two of them in parallel.

Hope this explains it.

cheers, Peter

Thanks fellas greatly appreciated.

Your calculations are correct - manufacturer's ability to deliver what's promised is faulty.

There isn't a single "120 watt" socket in the rear of a car/4WD that can deliver that.