automotive EFTPOS battery charger

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 instead of a eftpos unit..

I suspect it is done like that so that when the DC power connector the the charger board is not plugged in there is no leakage back in the the charger board to discharge the battery. I have never heard of eftpos chargers so I know nothing about them. By "it" do you mean the power supply that feeds the charger or the SLA battery ?

Les.

the green socket has 3 terminals. a power and negative , and a negative just for the battery.