Mike Harding, your help please.

So Derek,

If one was to hook up a well discharged, 80 or 100 amp AGM Battery to a cig socket, would it blow a fuse or not?

Sorry to have to put it into basic terms.

Cheers,

Jim.

PS. Sorry I gave you a serve a few weeks back, sometimes late at night after a bad week at work, and a few glasses too many, we say things we shouldn't. No excuse really. I wish this forum had an edit function at times. Tact is not my strong suit.

Up to 15 amps is OK. Less could blow the fuse.

Get that did ya Jim :)

Am I missing something Ray?

Do you need some more info.

I have seen 10 amp chargers over heat and trip the heat protection on a flat 100 a/h battery but the 15 amp chargers don't.

You need to wire up as required. Lighter sockets were never designed to charge batteries, especially one bigger than the cars starting battery.

Is Ray saying it takes less than that to blow your fuse Jimbo?

I dunno Jim, I think Derek is saying a 15 amp fuse or larger would not blow before the wiring released smoke. The cig sockets are generally 10 amps I think, not wise to increase the fuse value.

The fuse will blow unless there's enough resistance in the wiring to limit current to a safe value.

No, I think Derek is saying you guys must listen.

Lighter sockets are not designed for charging batteries.

They were designed to heat an element that lights ya cig but as long as it's used within spec the purpose doesn't matter.

Quite right, 10 seconds load.

Guys,

I feel some clarification is needed.

A fuse is designed for short circuit protection. ie if you short the power supply to earth the current flow will be infinite(theoretically) because you have no resistance.
Thats why it's called a short circuit. The fuse will blow instantaneuosly.

When a circuit is designed you calculate what the maximum demand will be.
The maximum demand is how much current you can expect to draw.

The current is calculated by using the equation for power(p) in watts = voltage(E) in volts * current (I) in amps.

Once you know how much current you will draw by virtue of what you plug in and its power rating you say my maximum demand is x amps.

Therefore I need a cable from the power supply to the outlet that will carry x amps.
The outlet itself must also have a rating of x amps or more.

Now we need to protect the cable with a fuse that is rated at x amps or lower.
Once the current flow is x amps plus the fuse will blow and not damage the supply cable.

It is possible to carry more than the x amps for a period of time and then what may happen is that heat developes and over time the cable insulation deteriotes and maybe melts and cracks and then a short circuit developes and wulla the fuse blows.

In domestic situations when cables are in conduit or buried or not in free air the current carrying capacity needs to be derated as per the wiring rules.
This is based on a cable of a particular cross sectional area and a particular insulation type having a rated current carrying capacity.

In all cases the fuse is designed to protect the length of cable it is installed in.
It does this by melting or blowing before the wire of the cable does and thus stops the current flow.

Hope I've cleared the air and not fogged it!!!!

Garth