Trying to measure the voltage on small coin cells is a hassle. Especially those tiny watch and hearing aid batteries. This device does two important things, it holds the tiny cells for measurement, and provides a true voltage measurement taken while the battery is under load. Even a dead cell can sometimes put out what looks like the correct voltage, if it is being read by a modern high impedance digital multi-meter. The slight load makes a much truer measurement.
The device can be made in a few minutes with items found around the workshop. In this case, the meter used is one of those free meters from Harbor Freight. It is well suited for this type of use.
Step 1: Materials Needed
1. The unit is built around a spring clothespin. Find a strong one in good condition.
2. You will need a couple copper weatherstrip nails. These nails are about 1/16" in diameter and 3/4" long. The copper plating makes them easy to solder to. Home Depot sells them.
3. A 1/4 watt resistor to be used as the load resistor. A good size is 1,000 Ohms, but anything from 510 to 2,000 Ohms would work for these small batteries. The 1,000 Ohm resistor provides a load of 1 milli-amp per volt.
4. Two banana plugs, red and black for plus and minus would be good, but any colors would do.
5. Some wire. In this example I used red and black 24ga stranded wire. Cutting off the old test probes that came with the meter would also work and would be a source of banana plugs.
6. Some 3/8 or 1/2" heat shrink tubing. The shrink-seal type is best. It has a lining of hot-melt glue which holds the wires very well.
Step 2: Tools Needed
1. Drill and 1/16" drill bit.
2. Diagonal wire cutters.
3. Needle Nose Pliers with wire cutter for cutting nails.
4. Scissors for cutting heat-shrink tubing.
5. Soldering Iron.
6. Heat Gun or source of heat fore shrinking tubing.
7. Not shown, an Xacto knife or similar for stripping wire.
Step 3: Drill the Clothespin
Drill a 1/16" hole through both sides of the clothespin as square as you can. It should go through the flat area where the two sides of the clothespin come together.
Step 4: Press the Copper Plated Nails Into the Holes
The nails should be a good press fit. Use the pliers to help. Pull the nails until the nail head is firmly seated. Do not leave this assembly on the seat of a chair. ;-)
Step 5: Prepare the Wires
First strip about 1/8" of insulation about 2" back from the end of each wire. Simply cut the insulation with the Xacto knife and pull it toward the end. Then strip of about 3/8" of insulation from the end of each wire.
Tin the exposed wire, on both the small exposed area and the ends.
Step 6: Wrap the Wires Around the Nails
Wrap the end of each with tightly around the nails.
Step 7: Solder the Wires and Clip the Nails
Carefully solder the wires to the nails. Try to keep the wire tight against the clothespin.
Make sure you are wearing eye protection for this next step.
Clip off the nail above the wire. Use sturdy cutters, the nails and fairly tough. Be sure to hold the nail while cutting. The cut nail can come off at high velocity, so be careful.
Step 8: Attach the Load Resistor and Solder
As in the photo, wrap the leads of the resistor around the small bare spot on each wire. The resistor should be close, but not touching the spring. Solder the connections and clip off excess resistor leads. Wear eye protection!
Step 9: Secure the Wires With Heat-shrink Tubing
Cut two short pieces of heat-shrink and slide then on as shown in the photo. Shrink the tubing with a heat source. If you don't have Shrink-Seal type tubing, try tacking the wires in place with some hot-melt glue before sliding the tubing over. The heat source will reflow the hot-melt glue and shrink the tubing.
Step 10: The Finished Battery Clamp
Your project should look something like the one in the photo. Make sure the wires are secure and the resistor is not touching the spring.
Step 11: Install the Banana Plugs
Banana plugs have various ways to connect to wires. The two I found in my junk bow were quite different. In any case, attach your plugs and make sure they are secure.
Step 12: Test Some Batteries!
Plug the banana plugs into the meter. Set the meter to the 20V DC scale, or auto-range DC volts. Open the clothespin and set the battery on one nail-head contact. Don't worry about polarity, it doesn't matter. Close the clothespin and you should see a reading on the meter.
If the battery is Lithium, you should see slightly over three volts as in the photo measuring the good CR2032 battery.
If the battery is Alkaline or Silver, the reading should be about 1.5 volts. In the photo, a questionable LR44 battery is being tested. It is measuring 1.3 volts, which is low. The digital caliper it came out of was still operating, but probably not for long.
The close-up photo shows how the battery mounts in the clamp. With some care, any size coin, or button call can be measured. Small plastic tweezers may be helpful for the tiny watch batteries. Don't use metal tweezers, they can short-out the battery and damage it.