DIY 1.5v Alkaline Battery Charger

Description

I've been researching the possibility of recharging 1.5vdc Alkaline batteries for many many years, I hate throwing things away, just ask my wife!   The trick I've found to recharging them is not to overload while charging, causing them to overheat and start leaking.

I originally started recharging them by using a solar powered pathway light (small round puck style) knowing they operate at around 20-30mA.  This worked perfectly but would only recharge them to about 1.4vdc which is the recharge voltage for NiCad batteries, and of course it could only be done while the sun was shining.  So, I detached the solar panel from the little circuit board and replaced it with a low voltage variable power supply.  My research found that the charge on a fresh AA Alkaline battery was around 1.6-1.7vdc with a current rating of about 2000mAh (+/-).

A little more research, along with trial and error, I discovered the best charge current to be about 10% of the battery's rated current.  For instance, a typical AA Alkaline battery is rated at 1.5vd@ 2400mAh current.  I've found that recharging @1.7vdc @10% rated current worked well and did not cause overheating; in this case the recharge current would be 240mA.

Originally I had placed a current control in the circuit to adjust the current but found it wasn't needed.  I have yet to see a battery pull anything above 200mA and generally drops pretty quickly after starting to recharge, usually below 100mah.  I believe this has something to do with the internal resistance of the battery.  After charging for awhile the current slowly creeps down and the voltage goes up.

You can daisy chain multiple battery holders in the circuit but you cannot recharge more than one battery at a time. This is due to the fact that multiple batteries will pull more milliamps and that total will go into both batteries and cause overheating in the batteries, a big no-no when recharging alkalines.  I have AA, 18650 Li-ion, and 9v battery holders in my charger.  The AA holder can be used to charge AAA batteries by putting a spacer at the negative (springy) end, I use a small nut.

If you plan on recharging 9v batteries then you'll need a wall adapter that provides 12vdc or more, the buck converter will accept up to 30vdc.

NOTE:  You can recharge NiCad, NiMh, and Li-ion rechargeable batteries with this charger as well.

NOTE:   There are some batteries that will not take a charge because they are open or shorted.  Open batteries will not show a rise in mA when inserted, shorted batteries will max out the mA meter, these batteries are simply no good and should be disposed of.

You should visually check each battery for leakage before and after recharging, I've recharged a couple hundred batteries but have only had two leakers.

A habit I've gotten into is to leave them sit over night and recheck the voltage, I've had a few that have lost a bit of charge overnight so I put them back in the charger again, usually they're ok after another recharge.

How to use the charger

Once you have it all wired up simply plug in the wall adapter and you should see the dc meter light up.  Flip the switch to the charge position then using your little itty bitty screwdriver adjust the voltage by turning the little itty bitty screw on the variable resister to get the voltage you desire (see chart below).

Once you have your voltage adjusted flip the switch back to the battery voltage check position, then simply place the battery in the holder and flip the switch back to the charge position, you should see the ammeter rise then slowly come back down.  Once it reaches zero the battery can be considered charged, though I have seen some batteries only go down to 20-30 milliamps after a 24 hour charge.

Parts List:  (can be gotten from Amazon or eBay)

LM2596S Adjustable DC-DC Buck Converter

M4430 Digital DC Volt/Amp meter

Various Battery Holders depending on what sizes you want to recharge.

A 3-30vdc wall adapter, old 5vdc cell phone chargers work great if you’re not planning to recharge 9v batteries.

Something to mount it all on.