While pulling together a post for the specific alternative energy blog, it did dawn on me that one of the zener circuits could be used as means of charging alkaline cells in a manner that should yield better results than the previous pulsers discussed, which do work, but also have a percentage of heat failures. While one cannot expect perfection from the circuit because real issue is diffusion of ions within the battery cell structure itself. This particular use is outside of the realm of the original application which was primarily lead acid batteries. Nickel Cadmium batteries and Nickel Metal Hydride batteries can also be effectively charged with this circuit.
This is a lot more sophisticated than the previous circuits discussed, however this one can be adjusted more finely for current flow through the cells. By using a higher value zener and adjusting the value of R1 to protect Zener, more cells in series (aka higher voltage batteries) can be charged. As shown it is suitable for a 6 volt lead acid or Nickel Cadmium or Nickel Metal Hydride battery array of 6 volts (5 cells) or 4 alkaline cells experimentally.
Increasing the voltage of the zener to something above 12 volts will allow it to be used for 12 volt lead acid, NiCd, NiMH or experimentally- alkaline types. If you use the cells to make up for an approximate 13.8 to 14.4 fully charged condition, the charger will be automatic- using cells of lower combined voltage of the same type of cell- it will continue to charge and will not shut off as it will never achieve the reference voltage needed to shut down except in a few rare cases.
It all depends on what you have available. R1 serves as a current limiting resistor to the Zener, so adjust that accordingly as needed.
Higher voltage SCR’s can be used, and you can probably use the mystery unmarked SCR’s you have in a box somewhere as well.
This one has not been built yet– but considering the source is an ON Semiconductor Zener Diode handbook, I see no reason it could not work, and when I free up some time, I will likely construct one for testing purposes. Until that time I cannot tell you what to expect for measurments or other adjustments. At the bottom are 2 useful links.
If you attempt to charge Alkalines with this you do so at your own risk. There are too many variable involved so no concrete guidelines are available. use common sense, try limit current through AAA cells at 100 to 250 milliAmps maximum, AA cells to less than 300 millAmps, C cells 700millamps and D cells at 1.4 Amps Maximum.
Nockel Cadmium and Nickel Metal Hydrides are a different matter. The C and D cells can handle 1 to 1.5 Amps maximum, and the AA and AAA cells can handle bout 1/2 ampere maximum.
The ON Semiconductor Zener Diode handbook can be found here
This is the On Semiconductor homepage The web makes it easy to locate reference designs and application notes as well as the always useful datasheets.
In a worst case scenario, with the correct values, you can replace a power tool dharger that has failed because they are largely using the same principles- only different methods to achieve it.