this is a eftpos charger from a vehicle. but looking at the input dc to the charger there is two seperate negatives... to the left you can see the red wire to charge sla battery , but the negative doesnt connect up to anything , just goes to the green power connector, can i just join both negatives together ?. i want to use it to power a low voltage led light.

 

Yes go ahead...

 

As Dodgydave says, it seems OK to do as you say, I guess they did this on the PCB as a way of allowing the charger / battery to be switched off and isolated.