Battery Drain Fix for Chinese Digital Calipers

The Problem

I’d be lost without accurate digital calipers. Inexpensive imported ones work great, but they all seem to have one annoying flaw: They use an always-on microprocessor and lack a hardware power switch, so the tiny button cell battery discharges even when the calipers are “off”. Every time I reached for my calipers, there was a 50/50 chance of a dead battery!

The Solution

Hack it! Add a tiny slide switch so the power is completely cut when you turn it off.

Patient #1: Harbor Freight 4″ (100mm) Calipers

I got this little guy years ago, but it still seems available in 2024. I disassembled the caliper body (peel off the chart on the back to reveal the screws) to remove the PCB. I de-soldered the lower battery contact from the board, turned it 180 degrees, and used a drop of CA glue to keep it in place. I added a small slide switch between the rotated contact and the original PCB pad. There is a programming port (?) on top with a handy slide-off cover where I mounted a slide switch. I routed the cables carefully so the cover could close, and the job was done.

Patient #2: Kynup 6″ (150mm) calipers from Amazon

Here’s the link in 2024 but I assume the names will change but the tool will stay the same. These bigger calipers are great because they can display fractional inches. I hate Freedom Units, but sometimes I just really want to know what 0.109375″ is in Fractional Freedom Units (7/64″).

I got lazy with this one. There was no handy programming port to mount the switch, and I didn’t feel like peeling off the back of the body and go poking around. So re-used the sliding battery cover as a switch. I added a piece of copper foil tape behind the side battery contact and brought it up past the edge of the battery door. I used Kapton tape to insulate the same battery contact so it couldn’t touch the battery. Then I added copper foil tape to the inside of the battery cover so it wrapped around the edge. When the cover is open, the circuit is broken, but when you close the cover it roars to life!