To install the batteries in this tester, you will need to remove the battery cover, which is located at the rear of the unit. The cover is the strain relief that connects the wire test lead to the body of the unit. You will need to twist the cover to remove it.
Gardner Bender FAQ
Gardner Bender's frequently asked questions.
Gardner Bender FAQ 
i have a battery charger 240volts in and 12 volts out dunlite brand but there is no power coming out on the dcvolts side ive used a sperry digital multimeter on it but cant get power on the dcvolts side is there anyone that can help me out here
Vince,
First thing to do would be to verify what setting you have the meter on, and what jacks you have your test leads plugged into. What is the model number of the tester that you have?