Fry's used to sell the Lenmar 5in1 by the dozen.

The Lenmar had always been a really crummy charger with a really nice enclosure.  The general idea is it did a full current charge when it was plugged in, then switched to trickle charge until it was unplugged. There was no power switch or way to manually set the current.  Change batteries & it only trickle  charged until it was power cycled.

The enclosure was of course, the very easy to load, fully spring loaded variant dating back to the legendary Archer which powered the 80's.

This was very useful for batteries which got used once per day & recharged.

Like the original Archer, the Lenmar was measured at 140mA AA current & 10mA trickle current.  If NiMH was selected, it provided full current for 8 hours.  If NiCD was selected, it provided full current for 5 hours.  Lions never encountered a rechargeable 9V or C cell after 1986, but it claims to charge 9V at some ridiculously low current. 

The measured current for AA was much less than the claimed 250mA.  It might be voltage limited.

At some point, the remote controlled Gu-24 light started causing the Lenmar to reset & go into full current mode at random.  It was believed to be the GU-24's phone charger causing a transient on the manes.  There wasn't any other outlet to plug into.  It was an excuse to finally add some manual control to the Lenmar.

The journey begins by fashioning a deep tamper proof screwdriver out of an iFixit.

This leads to a very simple transformer & electronicals.  The transformer is going to be the limiting factor in the charging current.

The 9V leads had to be desoldered to get in.  It has a bunch of voltages from the transformer, a 100uF filter capacitor, & a charred bit.

While looking a bit charred, it actually still worked.  Maybe the heat dried out the 100uF capacitor, making it more sensitive to transients.  The lion kingdom whacked in a 220uF replacement.

It has a very simple center tapped transformer.  The high & low leads give it 120Hz through diodes.  Never documented all the voltages.

The brain is a HCF4060 14 bit counter.  It's driven by a resonator.  Bit 14 starts low, causing it to make the full current.  When bit 14 turns high, it makes trickle current.  The battery type changes the speed of the resonator.  It has different positive terminals for AAA, AA, C/D to vary the current. 

The long desired interface was to manually put it in full current mode with the timer or to manually put it in trickle charge mode.  The problem with using the timer is it would always be vulnerable to glitching back into full current mode.  The decision was made to just disconnect the timer, manually select full current or trickle current with a switch.

Most of the time, the 10mA trickle current was enough to do what lions normally do with the batteries.

In the long term, it would ideally have a real time clock with a supercap as the timer & the trickle current would be stepped up to 20mA.  The HCF4060 was driven by 14V through the crusty resistor. The RTC would have to be isolated & its output boosted to 14V.  The RTC would take switch input to put it into trickle charge mode or to reset it.  It would save electricity to remove the HCF4060 & its resonator.