Hi, i have an infra red illuminator that runs off a mains adapter providing it with 12V 500mA and i would like to run it off batteries instead. I have a 12V 1300mAh battery to supply it, how do i work out the resistance i need to add in to make this work?

 

Hi mouth_,

You don't need a resistor. Your illuminator should only pull the current it needs. Most mains AC to DC adapters are cheap and provide anywhere from 12V to 18V depending on the load. The illuminator already has whatever current-limiting resistors it needs. Since you are replacing said adapter with a battery of the same voltage, you can hook it up directly.

If the battery is fully charged and the illuminator really needs 500mA, then you'll get a little over 2 hours of operation before the battery needs to be recharged.

 

You don't need a resistor.

+1
If you want to convince yourself, measure the voltage seen by your light under load. I'll bet the adapter is supplying more voltage under load than the unloaded voltage of the battery.

 

Warning: you should research the amount of charge you should take out of your battery before you recharge it. Don't blindly assume the amp*hr rating of the battery means that's what you can take. Discharging a battery too deeply can shorten its life.