Some devices are a pain to open up and change the batteries. If the device is not often used the batteries are usually dead when you want to use it. What a pain.

Easy fix: put the battery on the outside. Also easy to use a plug in adapter so no battery is even necessary. This method has minimal impact on the device, so it is easy to go back to keeping the battery on the inside.

The example here is a digital multimeter with inconvenient access to its battery.

Step 1: Tools and Material

The device to modify, here a digital multimeter.
Battery Clip - hard to find, I found mine at Lowes in the hardware "Hard to Find" section
Dead 9 volt batteries for salvage
Wire

Drill
Soldering Iron.
Misc; small tools

Step 2: Battery Connector

We will salvage the clips from a couple of batteries to make a cable with clips on both ends. One clip goes to the clip in the meter, the other to the battery. If you search instructables for "9 volt battery clip" you will find that the salvage is a frequent topic on instructables. One example is: How to make a 9 volt connector by zelda81596https://www.instructables.com/id/how-to-make-a-9-volt-connector/

For our use we will put 2 clips on the wire. One detail is that the positive wire is connect to diferent sides of the clip at the two ends. This is to make sure the correct polarity is delivered to the meter. Note that the connector needs to be assembled after threading it through the case ( see next step ) Insulate the clip, I used tape, other methods including hot glue also work.

Step 3: Modify the Meter

Open the case to get at the battery. In my case this involved 3 screws. I drilled a hole over the battery compartment, threaded the wire through and added the clips. Connect a new battery to the outside clip, and then check that the polarity of the inside clip is correct for the meter. If everything is fine, disconnect the battery. The battery holder needs to be mounted to the case. I used the holder shown in tools and materials, but you could mount it other ways, perhaps using velcro on the case and battery. I chose to use the battery holder and it was handy to mount this using one of the three screws holding the case together. I had to get a new screw just a bit longer than the original.

Close up the case. Connect up the battery. Welcome to the age of easy battery change.

Step 4: Use a Power Supply

If you have or make a 9 volt power supply you can make another 9 volt clip and connect it to the supply and use it instead of a battery. A couple of notes here:

Make sure the supply is really 9 volts DC do not trust the marking on the supply.
Be careful to observe the correct polarity.
Typically the battery negative is not the common terminal on the meter. In this case you should not use a common ground for the meter supply with the rest of you equipment, Let the supply "float".