Introduction: MT99 Multimeter Battery Mod

This is a back cover replacement for Mustool MT99 multimeter

(the MT77 and MT99PRO models are similar).

If you're thinking about buying this kind of multimeter but the lack of rechargeable battery keeps you on the fences, here is a 3d printed case that solves that :)

I wasn't too happy about the battery life given by the two coin batteries so I've made this case that houses a battery, power charger and voltage booster if needed. Those original two coin cell batteries were giving out about 6V, but it works just fine with less than that (3.7V battery).

The frame has space for any battery up to 47mm X 47 mm x 8mm thickness, when using the covered version.

For the second model (edge) you can use the original back cover, screwed in place with the original screws. It has the advantage of about 13 mm thickness clearance for the battery, and less printing time.


  • access to a 3d printer
  • PLA filament
  • the 3d model files from the link
  • 3.7v rechargeable battery
  • Battery Charger Module with Usb port
  • wires and soldering iron
  • 4 screws, 15mm long - same as the original ones

Step 1: Download the 3d Model

You will need to download the models from the link below and choose one that suits you better.

A slicer of your choice would do the dirty work before the 3d printer does it's magic.

You will need some supports for the USB cable hole, depending on the print position.

Some settings for the 3d print can be found on the linked page too.

Download the 3d models

Step 2: Fitting the Usb Charger Controller Board

You can use any board that is compatible with your battery. The board can be fixed with small screws or double sticky tape.

The one I'm using it's a "Micro USB 5V 1A 18650 TP4056 Lithium Battery Charger Module Charging Board".

I've got it from uncle Ali, the one with the express thingies :)

This particular model has voltage protections and a usb C charging port.

Step 3: The Original Back Cover Option

If you opted for the model without the cover then you'll have to screw the original cover on the

3d printed part. The original screws can be used to attach the cover to the printed part.

Step 4: Final Assembly of the Case

Using wires and a soldering iron the battery is connected to the charger board. The multimeter power leads are soldered in place of the original battery holders. Those holders can be easily removed with some pliers.

It is very important to shrink-wrap, tape or use some isolation method for the top of the charger board. You don't want it to touch the main board, it might release the magic smoke :)

The sides of the front case (black) have some little tabs, two for each side. You'll have to cut some lines on the 3d printed case so the tabs can connect. In the second picture you can see circled in green where the cut is made.

Finally the case is assembled with the front bezel without using too much force. It should snap in place with ease if you start along the connector's edge. It's easier to close-it like a book, pivoting on the side.

You can use 4 longer screws to secure everything in place but you'll need a drill to make the holes on the corner.

Step 5: Results ...

This is how it looks after the final assembly. The added thickness was about 6mm, which doesn't look too bad :)

When the battery needs recharging the multimeter will flash it's Lcd screen.

This multimeter is really nice and it works a lot longer now.