I have plenty of used alcaline batteries and I bought a charger for them...the only problem, charging current is too high and it causes the batteries to leak, if not at the first charge, it happens after a couple of times.
Problem is not the battery wasted, but that the acid that comes out eventually ruin the charger and if the battery seal get broken undetected and you put it in a device, you may ruin the device too...
So, i decided to make one, using a few modules available online.
This is customizable, and it can be configured with one power supply module, a voltage and current regulator and battery holders. You can set the modules dimensions and move them around to arrabge them as you like, then print the case and put everything together.
The one I made is for two AA battery holders, but you can add as many as you like and change the size of the holder's dimensions for smaller or bigger batteries.
The charging circuit will be ok for AA,AAA,NiCd,NiMh and even Lithium ones, but YOU have to set the charging voltage and current, this is why I called it STUPID.
The smaller the charging current the more time it will take to charge, but less heat will be produced with a benefit for the life of the batteries, rechargeable ones included.
Just consider that the charging is in common, so if you put together batteries with different charge state, the fullest one will discharge into the emptier one, and if the difference is big, the current will be high and may ruin the batteries.
To have external adjustment for current and voltage, the trimmers of the regulator have to be removed and the external ones wired instead of them.
My first try was replacing the 10 Kohm trimmers with the same value potentiometers.
That gave me voltage up to 12v but as I would probably need maximum 4.2 volts in case I want to make a Li-ion charger, I replaced them with 1 Kohm ones that give me a maximum of 6 volts but an increased accuracy when turning the knob; the same for the current one, so you can choose 1K with nice accuracy or 10K for the range of the module.
I did charge an alkaline battery as first test and it works great, set the volts to 1.5, current to 50 milliamperes and after a few hour the voltage reached 1.5V and the current started to decrease.
So, it is working as I wanted and it reduces the current as it gets closer to the target voltage.
My purpose was to have a slow charger and this is what it works well for, but you can use higher currents to charge faster.
The battery holders I got have some parts on the sides made to hold the battery tight in there. If you get the same, cut them away otherwise the batteries will be hard to remove.
They are for AA and AAA fits too.
Parameters settings: https://youtu.be/6gFzzNn-Wa0
Charger test: https://youtu.be/wXOcIXNaP28
These are the modules you can use:
There are 1K and 10K potentiometer links and also a more precise gauge
Gauge 4 digits
AA battery holder
AAA battery holder
C battery holder
D battery holder
18650 battery holder
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Pellets for extruding machine