The amplified ones... More hit-or-miss than...well... a Motorola Minitor V amplified charger.
When the red light flashes, it determines that the battery is faulty. Sometimes the red light flashes a few times, then cuts off. Then wakes up and flashes, then cuts off. This is the unit actually shorting out inside. I believe if this condition is left untreated, the shorting will destroy the battery and even the pager. It must either be solid red, or flash consistently. I had a pager that eventually got so sick I had to send him to the pager doctor across the county. My loner/replacement pager is brand new, and I've treated him well so he treats me well.
My routine consists of:
1) Poking the pins inside of the charger base
2) Turning the volume on the charger, down to about 7% or less
3) Grasp the charger base with left hand, in a manner that essentially squeezes the top and bottom halves together, around the pager socket
4) Blow on the holes at the bottom of the pager. The pager seems to enjoy this part.
5) Shove pager into his base, while saying a prayer
6) Charging light flashes, then goes solid red.
7) I win, Motorola.