The cordless Dremel Multipro rotary tool uses a rechargeable battery so you work anywhere without having an AC power source. Unfortunately, like most power tools, the battery pack is filled with cheap nickel cadmium cells that don't last very long on a charge and will start to wear out after a couple years. Since the pack uses standard AA size cells, you can replace them with high-capacity nickel metal hydride cells for a battery that lasts 3 times longer.
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Step 1: Opening the Pack
The battery pack is held closed by four small plastic tabs on the back. Push them out of the way with a small screwdriver and the blue part will come off.
Now remove the two groups of NiCD cells. They are 700mAh AA's, with little tabs holding them together. Each group is 3.6V, so the tool connect them in either series or parallel to select high or low speed.
Step 2: NiMH Cells
You'll need a total of 6 nickel metal hydride AA cells, with solder tabs attached. You can buy these from a number of online stores for a little under $2 each. Just make sure you get the ones with tabs so you can solder them together.
Total price for all 6 cells from various stores:
http://www.batteryjunction.com/onepcsaa20ni.html 2300mAh, $10.80
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=222 2200mAh, $9.96
http://www.all-battery.com/index.asp?PageAction=VIEWPROD&ProdID=1336 2300mAh, $10.92
Step 3: Solder Into Packs
Separate your new batteries into two groups. Arrange them the same way as the old pack, in series. To make sure you have the polarity right, you can check the groove on the end of each NiCD cell that indicates the positive end, look at the +/- markings on the outside of the pack enclosure, or check the old pack with a voltmeter.
If your cells have short tabs like mine, you'll need to splice them together with a small piece of wire.
Once you have them connected in the right order, solder the tabs together and check the total voltage to make sure everything conducts.
After they're soldered, wrap some tape around each pack to hold them together and cut off the bottom tabs.
Step 4: Assemble and Test the Pack
Place the new cells into the plastic pack enclosure. Before you close everything up, plug the pack into the Dremel and test it on both speeds. It won't stay in very well by itself, so you'll have to hold it in to keep electrical contact.
Once you're sure everything works, snap the blue plastic part back in to close the battery pack. You should be able to charge it with the original charger, although it will take much longer because of the higher capacity cells.