Electronic – Is it safe and efficient to charge a Li-Ion cell from mixed alkaline cells

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I’m a live sound engineer by day and very novice electronics hobbyist. Hoping for some advice on a small project I'm attempting.

Although the use of rechargeable cells in my world are becoming more common, alkaline cells are still in heavy use for wireless audio transmitters and receivers. Unwilling to take the risk of a $1 cell running flat during a show, alkaline cells are changed each show. These half-used cells still have usable capacity, yet are thrown out or recycled. It's very common while working large cast theater shows <48 AA used cells are trashed or recycled each night.

In an effort to claim this unused energy from half spent alkaline batteries, I’ve been using a very simple rig using 4 “used” AA batteries which power a Li-ion USB charger which charges a single 18650 cell. I closely monitor the current draw, alkaline cell voltage and temperature. This 18650 cell is used in a larger battery pack to power other uses.

  • If trying to eliminate the risks of mixing cells of unknown capacity and age, could I use a singe AA cell with a DC to DC step-up (.9-1.5v to 5v) into a Li-ion charging type circuit (i.e. TP4056)?

  • If this is a viable path, is it dangerous or harmful for the 18650 to charge at a very low rate (>100mA) and/or long charging period?

Thanks for any insight and help.

Best Answer

The residual energy in the AA cells is not valuable, the fact that the cells are portable, safe and cheap is.

That means using them for battery charging, that could use wall power, which is a few orders of magnitude more powerful and several orders cheaper (than full price batteries at least), is a total inversion of values and waste of time.

I too had access to a stream of partially used discarded AAs, and was able to make a steady donation of them to a charity serving the visually impaired, for use in their handheld illuminated magnifiers. They were perfectly matched to this service, as premature exhaustion of the cell was of no consequence to the users, especially given that they were free. The key here was a reliable stream, which allowed the service to abstain from buying full price cells.

If you insist on trying to recover the energy (I'm trying to imagine a desert island or post-apocalyptic situation where you don't have mains power, but do have the theatre's discarded AAs), then a) mixing AAs of a different charge state will result in premature run-down of the string of cells and loss of efficiency rather than damage (as long as you remove flat cells promptly) b) inverting up from a single cell is the least efficient way to do it, better to use a higher voltage and c) 18650 can be charged by a low current without reduction in life, as long as you continue to observe the normal charging envelope.