Smoke from Imax B6 charger

This is the history about an Imax B6 charger who was used to charge a large 15Ah Li-ion battery. Because of the large battery capacity, the charging current was set to the maximum the charger is capable: 5 amps.

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.

There are two mosfet transistors in bridge configuration. The N-channel one, just under the LCD is a IRF7811W. The burned one was the P-channel transistor, a Fairchild Si4835, a 8-SOIC P-channel mosfet used for switching applications.

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.

After the transistor replacement, the charger worked again nicely having no problem to charge again the battery at 5 amps current. Of course any medium or high power P-channel mosfet will do: just find a way to fit it inside the charger with adequate dissipation.

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.