Circuit Design for Managed Draining of NiMH Cells

Question

One of my pet peeves is the prevalence of devices powered by odd numbers of standard batteries (AA, AAA). Since I try to avoid using disposable cells and all of the chargers that I've so far owned charge cells in pairs, this means that it is almost impossible to get the odd cell to the same state of discharge as any other. Therefore it is impossible to charge said odd cells without damaging one of the pair.

I'd like to create a simple device that could safely bring any two rechargeable cells to the same level of discharge. I presume that other than mechanical and wiring bits, all that would be needed would be resistors and diodes. But, being an electrical retard, I'm not quite sure... not at all sure, really, where to begin.

My immediate focus is on LSD NiMH cells; but discussions that are applicable to other chemistries are more than welcome.

Answer 1

Ideally, you would use a charger that independently charges each cell (the Lacrosse BC series chargers comes to mind).

If you want to balance the voltages of multiple charged cells, you could easily place them in parallel with each other. Ideally, they will all have roughly the same V & charge before you put them in parallel with each other.

Ideally, you'd have a recharging device that has a built-in balancing optional capability. In that case you would just leave the cells in the charger & start the balancing option & it would indicate when the cell balancing is finished.

If you do not have a charger with a balancing option, you could build your own parallel setup by using individual battery holders that each have red & black wires. You would connect all of red wires together & all of the black wires together (do NOT make a connection between any of the red wires & any of the black wires or you will create an electrical short-circuit when your cells are placed into their holders). Insert your cells into the holders & your cells will self-balance after a time.

Another connectivity alternative (if you don't have individual cell holders) is to wrap aluminum foil around 2 large craft sticks (i.e. foil wrapped completely around each stick separately). Then hot glue the craft sticks to a board with the distance between the craft sticks a tight fit for your cells. After the glue dries sufficiently, insert your cells with all of the + terminals touching 1 craft stick & all of the - terminals touching the other foil wrapped craft stick. Your cells will self-balance after a time.