FIGURE 6A. Plates mounted in cells.
proper ventilation kept in mind. Raw potassium hydroxide
powder should be handled carefully. It is caustic enough
that it will cause burns if left in contact with your skin over
a period of time. So, take reasonable care.
Fill the mason jar with the potassium hydroxide
solution, keeping the level well below the lid; screw the lid
on the jar. An assembled cell with both plates is shown in
Figure 6A, and a shot of the separate plates in Figure 6B.
(Remember, you can use larger plates than shown for
Your cell is now ready for charging!
Note: Commercial cells also use lithium hydroxide
(15g/l) in the electrolyte which further protects the cell at
higher voltage charging.
Each of your Edison cells will produce 1.2 volts.
Edison himself recommended charging these cells initially
at 1.7 volts (for 48 hours) as a forming charge, and that
daily chargings should be made at 1.45 volts to reduce
gassing. Your cell will improve each time you
charge/discharge it. He also recommended only
discharging your cell to . 9 volts while your plates are still
Your cells will take on a charge very slowly, especially
at first. Limiting current to 50 milliamps or less is
recommended for the first charge, though Edison said
larger currents are fine as long as the electrolyte does not
“froth” or exceed 115 degrees. Some gassing at the
terminals is normal and harmless. If liquid levels begin to
get low in the cell, add distilled water only.
After your cell has checked out and is working exactly
as you’d like, you can add 2 mm of mineral oil to the top
of the cell. This will form a barrier against carbon dioxide,
which can harm the cell over long periods of time.
A Unique Project
In trying to think of a fun and useful way to test such
a long lived cell, I struck upon the idea of a “ 20 year
garden light” as a project.
It occurred to me that using these homemade cells in
place of the usual NiCad or NiMH batteries in a solar
powered garden light (which usually only last a couple
years at best), I might wind up with a light whose battery
outlived its solar panel and other components. A quick
test showed that two of the described hobby cells could
FIGURE 6B. Plates outside the cell.