I have now two original Nokia BL-5J 1230 mAh batteries connected parallel (2460 mAh?) inside N900, under Mugen cover.

It was not difficult, but it is not easy either. Took me ~5 hours to make everything. One has to be very careful not to shortcut (+) and (-) poles when doing this and getting the cables (soldered) to the right places may be difficult.

1) I disassembled the 1st BL-5J (look at the pic on the previous page of this thread)
First carefully take out the tape, so you can reattach it later.
The whole battery case under the tape is (+) pole and the (-)-pole is in the middle of the battery. The battery was almost empty (3.68V) , just in case....

2) Soldered a blue (-) cable (has to tolerate 1A) to the middle "pin" under the circuit board.

3) Soldered red cable to the plus connector, yet not destroying the female pin connector.

4) Made two holes to the edge-plastic, so the cables can come out but the cover still can be screwed back to the battery, and yet the battery fits inside N900 battery hole.

5) Assembled the 1st battery back together, the two screws and the original "tape"-cover.

6) Charged both battery 1 and battery 2 to full. Both were 4.18V.
Now it was safe to connect them parallel because both had the same voltage and hopefully potential.

7) Soldered blue cable to (-)-pin and red cable to (+)-pin of the 2nd battery. The 2nd battery lies on top of the first one rotated 180 degrees.

8) Made the parallel connected batteries as a one package by taping them together tightly. One has to make sure there won't be shortcuts even when one drops the phone, back cover and the battery-pack flies apart from the phone.

9) Additionally, I disassembled the Mugen back cover to make two holes in it, where one can store extra microSD-cards. Glued it back together.

I estimate, I can have several (4 - 6) extra microSD-cards inside the two holes now. Also, maybe this microSD-card-USB2-adapter will fit inside, which would be useful to transfer quickly lots of files between any PC and N900. If I would be satisfied to have lens cover always "open", even more space would be usable and I could fit this tiny Bluetooth2-USB2-dongle inside also.

(I used to have microSD-USB-adapter, Bluetooth2-USB2-dongle and extra microSD-card all inside the old Mugen cover, but that system was stolen last summer. But the new Mugen cover can be downgraded back to that if one wants. Then one just have to tape the white patch and the magnet to the phone itself to make camera and microSD-card mounting work.)


10) Put the battery pack to the N900, closed back cover, booted the phone and started the test how many hours it will take the consume the battery pack empty (to 3.6V).

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