Vince said:
Could the charger have some sort of isolating device such as a
capacitor in the charging circuit and thereby prevent discharge
of the li-ion battery when the charger was not energized?
Batteries are DC, capacitors block DC, so that won't work. Really cheap
chargers for less fragile NiCD, NiMH, and lead acid batteries use a
diode to provide both rectification and isolation. In either case, a
transformer provides isolation between the AC line and the battery.
Generally Li-ion chargers use a mosfet to control current to the cell,
and in multiple cell packs there are multiple mosfets to allow
individual control of each cell. Li-ion batteries are notoriously fussy
and require careful monitoring in both charge and discharge to keep the
voltage, current, and temperature within a relatively narrow window. At
any rate, when the charger is unplugged, the mosfets will be off, and
present essentially an open circuit. Some Li-ion cells even have their
own charge/protection circuitry built into each cell. These can be
charged using a simple current limited power source and by nature are
connected to their charger all the time.
For those curious about the chargers, check out the datasheets for some
of the common charge controller IC's out there. Texas Instruments,
Intersil, and Dallas/Maxim among others have a number of products in
that category.