What Fuse to Use?

[Ken_McE]

I have a light I'm working on. It has a 12 volt, 24 amp-hour deep cycle battery. I'm designing it so you can plug it into any car cigarette lighter to recharge it. I'm having trouble deciding what fuse to put on the line for the recharging cord. Any opinions?

 

[MarNav1]

Need more info on the bulb wattage and if anything else will be plugged into it. Then
you can use Ohm's law to figure out the current draw or you could use a digital meter
to figure out the current draw. Beware most stock digital meter's will only handle a
5 to 10 amp load without a shunt so be careful. Ohm's law should get you pretty close.

 

[Ken_McE]

Need more info on the bulb wattage and if anything else will be plugged into it.

Essentially what I'm doing is using an unknown battery - any passing donor vehicle - to charge up my battery. When two 12 volt batteries are connected in parallel the stronger one will naturally try to charge the weaker one. If the donor car is on at the time I figure it'll be putting out 14 volts dc or so. The various bulbs won't really be involved. If the unit is sitting on the floor of someones car taking a charge there won't be much reason for it to be on.

 

[Illum]

if your plugging this into a cig lighter socket you yill need a fuse thats more sensitive or at lower cap than the fuse on the car.

I'd believe you are going to need a charge controller for the battery or you risk overcharging it

 

[MrAl]

Hi there,

Besides a fuse you need to limit current getting to the battery from the car with
say a 1 ohm resistor, or else you have to make sure the battery does not charge
too long. For a 24Ah battery probably 2amps is a good set point so 1 ohm will
work ok as long as the battery is not run down too far. The 1 ohm resistor
should at least have a 10 watt rating or higher and will have to be mounted
where free air flow can get to it.
The only problem with simple resistor charge methods is that if the battery being
charged is run down too far, it draws excessive current. A better way to control
the charge is to build a constant current regulator which will limit current getting
to the battery no matter what the initial state of the battery is before charge.
Normally when a LA battery is working ok the terminal voltage will rise fast so
if it's being charged with a higher current it only lasts for a short time period.
After that the resistor limits the current to a safe level for the battery.
It's when something about the battery does not work properly that it might
draw high current, which could burn up the 1 ohm resistor. Thus, a 1 ohm
10 watt resistor and a 2 amp slow blow fuse might do it. Better yet, use a
circuit breaker rated for 2 amps dc rather than the fuse.