I have the Rayovac ic3 charger. The batteries have a conductive ring
near the negative terminal. The charger will only charge ic3 batteries
at the 15 minute rate. The assumption is these batteries are designed
for the 4c charger, much like RC batteries. I suspect the conductive
ring is temperature sensitive, which could be use as part of the
charging algorithm.

A funny thing about battery charging is that charging at a slow rate,
say 1/2 C, can be more of a problem than 2C. This is because the
changes in terminal voltage are more difficult to detect at the slower
charge rate. This is especially true if the charger uses a switching
power source since you need to sense the battery voltage changers in
the presence of switching noise. If the ic3 uses temperature sensing,
then that problem is solved.