Battery chargers that can recharge disposable batteries. How do they work and are they safe?
What happens when you recharge any kind of battery?
- current flows into the + terminal
- a chemical reaction occurs inside the battery, more-or-less the opposite of the chemical reaction that occurred when the battery was discharged
- the chemical reaction recharges the battery and generates heat
What can go wrong?
- the battery can go into thermal runaway and overheat, sometimes explosively
- the battery can develop an internal short and draw excessive current, which can lead to thermal runaway or maybe just wreck your charger
- the battery can develop an internal open and become a paperweight
How to prevent the wrongness?
- monitor the battery temperature (internal if you can, otherwise external) and reduce current if the temp gets too high
- charge really slowly (trickle charge)
- do not overcharge
What else?
In order to "fully" recharge a battery, the charger must actually exceed the rated voltage of the battery (briefly, and at a low current). This overcomes the voltage drop in the internal resistance of the battery.
What do commercial chargers do?
Commercial chargers typically do some combination of constant-voltage and constant-current charging, the slightly fancier ones monitor temperature, and the really fancy ones may also monitor other things.
Why are they not recommended for alkaline batteries?
Each type of battery recharges slightly differently. Old ni-cad chargers may not work well for ni-mh rechargeables, and vice versa. You have to buy a special kind of charger for li-po batteries, lead-acid types, etc.
The real reason that alkaline rechargers are a niche item is that you don't gain much (in terms of recharge capacity or number of recharge cycles) from recharging alkaline batteries compared to other types.
The moral of the story is you can recharge most any kind of battery, but some are better at it than others.
I had a low-stress application for reusing alkaline cells (low current, didn't necessarily need a full recharge), so I tried it myself [original research] although [this ain't Wikipedia]. The first thing I discovered was that the available information is so dead-set against trying this, that it is very difficult to determine what the proper terminating voltage would be. I used a bench supply so I could limit both the current and the voltage.
On the first try, the results were good. On the second try, the cell leaked. After that, I intentionally kept the charge current to C/10 or less, using cells that had never been recharged, and that were not completely exhausted. After some period of time, the cells began expelling gas and electrolyte. The gas leaking was audible (as in, "What's that sound?", and "Oh, it's the cell I'm recharging")
As an aside, cells that are intended to be recharged often have mechanisms targeted at absorbing the gas, or at least safely venting it. Not in this case.
Conclusion: The chemical reaction isn't necessarily "reversible" in an exact sense. When run backwards you get other things like heat and gases. Electrolytic reactions involve things like dissolving metal, and the resulting corrosion products don't automatically know where to go back to. In the end, I quit trying to recharge these. No deaths were reported.
I believe these results will vary with brand (manufacturer), so if it worked for you, you were just more lucky. But this is an example of why it may not work.
Single-use alkalines aren't guaranteed to be rechargable. Why not buy actual rechargable batteries which keep working even over a large number of recharge cycles? It's much cheaper over the long term.
I've recharged lots of 9V alkalines for use in DVMs. Trickle-charge 20mA with a cc power supply set to 9.5V max. It doesn't work well with deeply-discharged batteries below about 7.5V. But you can get some extra life from 9V alkalines if they've only been run down to 8V or 8.5V or so. So, probably a commercial charger will accomplish this without tying up your bench supply.
Yes, sometimes a cell starts venting gas, little popping sounds. I've only seen this on deep discharged ruined batteries where the voltage remains way low despite all "recharge."
There's one good application for all of this. If you score a huge box of "dead" 9v alkalines, you can raise the voltage most of them. Then hook them all in series and use them as woodburners, electrostatic kilovolt supply, carbon arclamp demos, etc. Or just competing on YT/reddit for highest unwise 9V-bank voltage evar. Get yours from indie theaters FM microphone discards, or office buildings replacing smoke detector batteries.
244 batteries in series https://www.youtube.com/watch?v=8hwLHdBTQ7s
490 batteries in series http://www.break.com/video/ugc/490-9v-batteries-351064
If you have some copper sulfate or copper chloride solution, you can demonstrate some issues behind "recharging." Recharging is basically a form of electroplating, of converting the corrosion products back into solid battery plates. Stick a couple of small copper electrodes into your bluegreen solution, then crank up the power supply to high current. Fast-growing clots of black goo! That's dendritic copper growing on the negative electrode; a forest of nanofilaments. Perform the plating at much lower current and you instead get bulk copper metal. Battery design has similar issues: the physical structures produced by the "corrosion" and the "plating" must not degrade over many cycles, and deep discharge (excessive 'corrosion') must not ruin the battery or reduce the number of charge cycles.