Here is a low current charger I designed in an attempt
to extend the life / recharge regular non rechargeable alkaline
batteries. The trick to doing this is three things.
- Use a low current over a longer period
- Charge before they become too drained
- Charge to no more than 110% of the cells capacity (eg 1.5v charge to 1.65v and stop)
The nice thing about using alkaline batteries is that they have no
internal discharge unlike Ni-Cd and Ni-Mh rechargeable’s, and are
therefore suited to low current drain applications such as remote
controls, clocks or things you don’t use often such as torches.In my
tests I have found the lower charge rate, the better the charge and the
less chance of a cell leaking electrolyte. Also, if a cell becomes too
flat or completely flat, it will not take a good charge and will also
probably leak electrolyte and possibly even pop open. The idea here is
to keep them topped up. Lets say you have fresh batteries in a torch and
you used it for a while. The cells have drained to around 1.3v for
example. Put them on gentle charge with this circuit, monitor the
voltage and stop when it reaches 110%. That would be 1.65v for one cell
or 3.3v for two cells in series. Do not charge beyond 110% or there is a
risk of cell leaking or even popping open / exploding. Its also
advisable not to try to charge an alkaline battery that is completely
flat. They don’t absorb a charge and just leak in my experience. Some of
my tests I done outside in the winter (around 2°c) and I found the
cells hit 1.65v quite fast but didn’t absorb much of it due to high
internal resistance at cold temperatures. Charging should be carried out
at room temperature, around 20°c.
Here is a constant current supply circuit schematic diagram using the
LM317 variable voltage regulator. It is a very simple circuit for
charging alkaline batteries. It will provide a stable constant current
which is adjustable by switching different values of resistors. The
input voltage must be at least 6v higher than the battery(s) you wish to
charge. The LED, BC548 and 470Ω resistor provide an indication of
current flow to show that your battery connections are good. They can be
omitted if you wish to make the circuit simpler. I used a 12 way rotary
switch set to 5 way to select different resistors to give output
currents of around 5, 10, 20, 30 and 40mA. The idea being for 9v PP3
types I would use 5mA. For AAA’s 10mA. AA’s 20mA, C’s 30mA and D’s 40mA.
This is just my guideline, you can try what you like! Just remember
more current is not good for charging alkaline non-rechargeable
batteries.
Electronics Circuit Application
