Cheap Chinese stuff often comes at a price, as it's the case with these 10$ calipers. As far as accuracy goes, they're not that bad, and although build quality leaves a lot to desire, that's another issue.
The main problem with these calipers is their battery life, the battery drains too quickly compared to brand name ones. AvE, a youtuber famous for dissecting every tool known to man, did some tests on this kind of knock-off calipers, and found they not only consume 4 times more, but they also consume almost that same amount of current while they're off! This means it's just like you're using them all the time, no wonder why the battery drains up so fast!
There are many fixes for this problem, but one of the most common ones is to simply add a switch, so you can turn them off easily without having to remove the battery every time.
Step 1: Materials:
To proceed with this hack you will need:
- Soldering iron
- Pin header (3 pins will do)
- Jumper for header pins
- Kapton tape
- Some centimeters of copper wire (I used .3mm)
- X-Acto knife
- Steady hands
Step 2: Dissassembly
First we need to disassemble the calipers, here's a video documenting the whole process, it's pretty intuitive as it's only held by some pairs of screws.
The goal is to get the board out, be careful not to drop the LCD screen in the process, do this over a table with the screen facing down.
Try to wash your hands before you proceed with this step. A tiny amount of oil on your fingers could cause a bad connection between the LCD or the buttons and the board. Don't touch the LCD's elastomeric connector, which is the red-pink rubbery thing located at the edge of the LCD, this connector relies on contact to work, if it's dirty it will probably malfunction.
It's recommendable to scrub the PCB with alcohol and some q-tips before reassembling everything again.
Step 3: Cutting the Power Trace
We don't want our calipers to be on the whole time, so we cut the power trace with an X-Acto knife. The trace is located near the battery holder.
Step 4: Soldering the Pin Header
You'll notice there are four pads at the corner of the board, these pads are for power, clock signal, data and ground, respectively. We'll only be using the power one, since we want to put a switch between the battery and the power tab. The ground one will be used to add extra support to the pin header, so we can trim the pin at the base.
The header pins are tinned, and a copper wire is soldered at the center. If you overheat the pins the plastic will melt, you'll probably have to try this a couple of times like I did, trying to solder as quickly as possible.
Once you're done, tin the pads of the board and solder the pin header to them, as shown in the picture.
Solder the other end of the wire to the positive battery pad, loop the wire around so the case can be closed afterwards.
Step 5: All Done
Reassemble the caliper again, be careful not to overtighten the screws, chinese cases are not great, and the screws are prone to destroy the threaded holes.
If everything went fine, then you should have a working caliper, now with a jumper switch to save those expensive batteries without having to take them out every time.
It's also a good idea to carry a spare battery around just in case, as these calipers have no way of indicating low battery level. At low voltages the brains of this thing don't seem to work properly, but the display does, this means you could get wrong readings without even noticing.
As a review, these calipers will do just fine for hobbyists and people on a low budget, they're pretty accurate compared to the real deal, but if you have to use them for something more critical I would suggest getting some good quality ones for the reasons stated above.