There were two times in the same week during October 2012 that I needed to purchase some wafer batteries (watch batteries). The first was for a bore sight for an AK47 (three batteries @ $4 each) and the second for a clamp meter (three batteries @ $6 each). I normally use cheap Chinese NiMH rechargeable batteries for most things around the house but since there are no rechargeable wafer batteries I was stuck buying replacement alkaline cells. During that same time my neighbors replaced the expensive batteries in their dog's invisible fence shock collar. I remembered a project that I had started years ago but never finished. An alkaline battery recharging station.
I looked around in a junk project box and found some resistors, diodes, and transistors. I had everything I needed without having to run to Radio Shack. It was a good thing because I built it at night and all the stores had already closed.
I'll have to add a build list and schematic here in the future. For now I'll post a picture that has a faint schematic on the clipboard as well as a few stats. The output of the charger is 1.9mA and it draws 2 watts. I used a 10K and 100 ohm resistor, two switching diodes and a transistor:
I built the circuit on a breadboard and used hot glue to hold the power brick wires to it:
A shot of the back:
Power is being supplied by an old Fellows paper shredder brick:
The first test was a dead AA battery (1.17 volts). I put it in a battery holder out of one of those free Harbor Freight LED flashlights:
After 6.25 hrs the battery had a charge of 1.42 volts:
Once I saw that the trickle charger did indeed work I wanted to try it on a wafer battery. I first needed to mount it to something a little more permanent so I grabbed an old 2x4 and some brackets from a mini blind:
I used machine screws and nuts to hold the battery in place:
The battery I was charging was for this $12 Harbor Freight clamp meter. Funny how you can buy it for $12 but replacing the batteries will cost you $18. If Harbor Freight wasn't an hour drive for me I would have just picked up a new meter instead of buying batteries:
Initial voltage was 0.57V:
After 3 hours of charging the voltage was up to 2.45 (this is a 3V battery). It was time to close up the shop so I disconnected the power and quit for the day. The next day I saw that the voltage had dropped to 1.62 volts. I'm guessing the battery was just old Chinese junk (I bought the meter 8 years ago) and will never hold a charge but I'll have to do some further testing. In any event, I did not like using a screwdriver and wrench to place the battery in the charger. I grabbed some pens out of my desk drawer and removed the springs from them:
Using spring tension to hold the battery in is much better:
Since I wanted to charge both wafer and AA/AAA batteries I needed to construct a second set of contacts. The first set was far too small for larger batteries:
Now that is more like it:
Note - You are probably seeing missing links to some pictures above. I still use Notepad to create the html code and have a template with plenty of extra references to pictures to save on typing. As I continue the page I will add pictures and descriptions and these missing links will disappear.