- 3 - 3500 mAH Li-Ion cells (with tabs pre-installed) - $27.60
- 1 - protection circuit (PCB-LiS3A4) - $6.05
- 1 - 2.1 x 5.5mm 'Type M' barrel plug - $3.50
- red & black wire
- electrical tape
The finished pack fits easily into a KX2.
A few hints
- Order the cells with tabs attached. You'll cut off the free or excess part of all but two of the tabs, leaving the fixed portion as a solder point. It's much easier to solder to the tab material than to the battery terminals themselves.
- Orient the three cells with the polarity alternating from one cell to the next, as shown.
- Now rotate them in such a manner so that the existing tabs can be used to electrically connect the cells shown in white circles. Temporarily tape them together before soldering in order to stabilize them. Now solder the tabs indicated - you'll probably need a soldering gun rather than a soldering iron for this.
- You can now carefully remove the tape so that the protection circuit will fit as close to the batteries as possible.
- Set the battery aside and solder (with an iron now!) a short wire to each of the four pads on the protection circuit. The cut and fit the length of these wires to reach their assigned place on the battery pack.
- Solder a pair of wire (red & black) to the positive and negative ends of the battery pack and terminate these wires with a "Type M" DC connector to fit the KX2.
- Wrap everything in a layer of electrical tape. I put strips of tape over each end of the battery pack and the circuit board, then wrapped a layer of tape over everything.
Here is the resulting discharge curve at two different rates of discharge. The test shows a capacity of 2700mAH when discharged at 500mA. Charged at a lower rate, the pack would show a higher capacity and its capacity may well be 3500mAH after a few more charge/discharge cycles.
(In a future post I'll be comparing other 18650 cell options for internal KX2 batteries. The graphs will compare each battery to the others as well as to their own advertised capacity).