Disclaimer: If you make something similar showed in this blog and you or somebody or something gets hurt I am not responsible for that. Do it at your own risk.
As we all N900 owners know, battery life of the phone is not very good. After three years I finally got tired of sticking charger cable in every night to the phone and started to think how charging could be done differently. Few days went by and then I remembered: A while ago I saw in a supermarket a wireless charging kit for Nintendo Dsi. Could that kit be the key for N900 wireless charging?
I quickly googled the DSi battery and closer inspection showed that the battery voltage was the same in both devices, 3,7V. In the original DSi charging system there is a some kind of system that tells to the loading station when stop charging. (I guess system measures charging current or voltage and when certain level is achieved charging is stopped.) If I use DSi battery’s electronics I should be safe, the only thing that changes in the system is battery capacity. Also N900’s battery has its own protection system from overcharging so there is actually a double safety option. Feeding power back to N900 is another story but I think it can stand it.
Off to the supermarket we go.. Kit was still there and I quickly bought it. 15€ well spent.
One can see that the charging system uses only two wires, positive and negative. Third terminal of the battery is not used at all. That makes things easier, right?
I quickly disassembled the station. Manufacturer seems to added some weight to get more solid feel to it..
N900’s battery BL-5J. There are two small screws in the plastic head, covered with plastic, you have to do some digging. Also sticker pads needs to be removed before head comes out.
Some wires soldered. Be careful when soldering a Li-Ion battery terminals!
Fitting things together. On the right there is the coil and electronics from DSi’s battery. They are connected by two wire to the BL-5J battery positive and negative terminals.
First tests show that system is working and don’t get heated generally at all. Still, more testing needs to be done. It looks like charging is a bit slow, due the double electronics safety mechanism in the BL-5J and in the wireless charging system. (Maybe I should get more involved with some measurement equipments? :))
Because charging happens directly from battery’s terminals, N900 don’t look like charging, battery graph don’t do that rising thing what it normally do when charging. This should keep in mind, I don’t advise you to put any cables to N900’s micro USB port while this system is connected to you N900.. Maybe there should be a slide switch which disconnect the wireless charging electronics from the phone.
Also station is very picky about the position of the phone on top of it.
If I can get system reliable enough I might try to make some cover for coil’s electronics. This is a spare phone so I don’t have to carry it around and thus there is no hurry. In todo-list there is also a proper power feed for station, right now I feed the thing from PC’s USB port.