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.