I got bored one day, so I cracked open a battery charger I got.  I was curious if it was just a dumb charger when it came to 9v batteries like many chargers out there, or it actually monitored the state of charge.  The model of the charger: EBL-C802. 

With it opened up:

It quickly became obvious why this charger requires charging AA/AAA cells in pairs.  It charges two cells in series.  It's likely done this way as a cost reduction measure.  Sadly the main chip that controls the charging is missing a label, almost certainly sanded off.

Below is a schematic for the AA/AAA charging circuit. There's two circuits for charging two pairs in total.  It's a pretty simple circuit. A mosfet is turned on/off to control charging and voltage of the two cells in series is monitored by the controller chip though a 10K ohm resistor.

Below is a schematic for the 9V charging circuit. There's two circuits as two 9V batteries can be charged at a time. This circuit is a bit more interesting. It doesn't operate as a dumb charger as the controller chip is apparently monitoring the battery voltage via a resistor divider.  The circuit trickle charges the battery even when the B772 transistor is turned off via a 3K ohm resistor.

I have some other observations to note about the charger, but it's close to midnight for me. I'll be sure to update the page later.