原创 [博客大赛]Replacing a burnt Li-ion battery charger IC

When electronic devices are broken down, it is good to repair. Repairing the broken devices is not only environmentally friendly, but the process can train our problem-solving skills.
Especially I like the feeling of "Lost and found" (失而復得). When something is out of order, its user or master will know how important is that thing and will treasure it much more than before after it has been repaired.

Helping people in need is good - "Helping others is the basis of happiness" (助人為快樂之本). Sometimes I help my friends to repair their stuffs, like phones, cameras, etc. Although in most of the cases the repair is free of charge, I do not care about that (the reason is that my repair skills are very poor, the device may explode some time after ).

This time the stuff to be repaired was an Android cordless phone for home use.

Fig. The Android cordless phone to repair
 

The symptom was that the battery cannot be charged up, because one day my friend connected a universal adaptor to the phone with reversed pin configuration...

Fig. The universal adaptor, the polarities can be easily swapped

Opening the case, it was found that an IC has been burnt... Most likely it was a battery charger IC. Since the IC was burnt, the marking on it could not be seen anymore...

It had a DFN package. The dimension of it was 2.5mm x 2.5mm... We searched the Internet and could hardly found such a battery charger IC. It could tailor-made.

So I unsoldered it and used multimeter to check the circuits. The circuit was approximately the one shown below:

Fig. The approximate layout of the charging circuit

From previou' experiences, the charging IC should have a "EN" pin. Charging is enabled when the pin is pulled high. So it was guessed pin 9 is the "EN" pin...
And that the IC should have a charge indicator output "\CHG", pin 4 should be the pin.

Since I only had a MAX1811, single cell Li-ion charger IC, on my hand, I tried to solder the IC with wires.

Fig. MAX1811 Li+ charger
 

The single cell for the phone was a Panasonic's 18650-size battery with capacity 2450mAh. It had a built-in protection circuit inside, so I was less afraid of explosion, etc.

Fig. The battery

It is said the typical charging current for a Li-ion battery would be Capacity/3. So I set the SELI pin to be HIGH - 500mA and the SELV to be HIGH (4.2V, for Li Polymer battery).

Fig. Inside after repair

Fig. The new battery charger circuit

Fig. The phone was repaired successfully.

Although the new circuit was quite ugly, it worked... And I had ensured every soldering point was good enough and not to be short-circuited easily.

To my surprise, the battery could be charged up!!
The phone could detect the status of charging!