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load a 12V battery by the cigarete lighter Answered

Hello guys,

I want to make a publicity trailer with led's that will stand at party's (and on the side of the road in the week).
This trailer will stand alone and will be powered by a 12V car battery.
I would like to charge that battery while moving it with my car by a long cable to the 12V socket in the car.
Should I place a resistor to limit the current or is this not necessary?

The trailer is for my youth movement.

(sorry for my bad english)



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3 years ago

Get a dual battery charging system as used in most 4WD's or caravans.
You want to charge from the alternator or existing battery - for either option you need a dual battery isolator and charger.
Look it up on Ebay, some are quite cheap.
If you wire it like you plan there is a great risk that nothing works or that something goes up in smoke.
For the long distance you also need really thick wires to prevent the voltage drop.
Might be easier and cheaper to put a solar panel on the trailer.
And you should use deep cycle batteries as a starter battery is designed to give a lot of juice for a very short time but otherwise it does nothing while the car is running.
A deep cycle battery can be discharged much further without damaging the battery.


Reply 3 years ago

Thanx for your answer,

Is there no easier solution? I have to use the cigarette lighter because it will be used by different persons (and cars) so I can't build some devices in the car.

And our youth movement has not much money, the trailer will cost us already very much, solar panels are too expensive. And it will be stolen in no time. (theft is a big problem, our previous trailer was stolen)

Why won't it work with a resistor to limit the current? it don't have to charge fast.

The capacity of the battery is no problem because we will use a tractor battery, this is enough for 10m RGB-LED strip.

In case it has to be this complicated, I will charge it at home with a normal charger.


Reply 3 years ago

Problem is the voltage drop and power draw from the battery if depleted.
You need at least 13.2V on the terminals to properly charge the battery.
And without knowing if the battery is really fully charged after the drive you risk damaging the battery in the long run.
The only cheap "solution" that might work is if you use a step up converter right at the end of your charging cable.
Adjust it to about 14V and if no current limitation included in the converter add a suitable resistor so you won't draw more than what cable and converter can supply.
Not pretty but will work for a while.