The charger was able to charge the battery nicely four or five times, but the next time, the charger went into fire and smoke.
After opening the charger, a small burned ic-like transistor was found. It was not possible to read any mark in the small transistor, so the only way to know what transistor model was, was to open a second charger.
After quick search in the junkbox I didn't find any Si4835, but I found a lot of P-channel mosfets, some of them with greater specifications than the Si4835, so I replaced the burned Si4835 with a D-Pack IRFR5505 transistor. Because packages are different a small modification was made to Imax B6 PCB: I removed some varnish over a large copper track (next to the diode) connected to the drain of the mosfet that was used as electrical connection and thermal heatsink. To wire the gate terminal an external resistor was used instead the original SMD one, acting also as a wire jumper. Source was soldered directly.
And talking about adequate dissipation... The IMAX B6 charger use the bottom cover as heatsink for a pair of TO-220 transistors. Check the transistors lay flat over the bottom cover. Many times they are not, causing severe overheating and finally, transistor failure.
Just spent five minutes to disassemble the charger and check for the transistors. It’s worth it.