On its website, Sonicare specifically states that the batteries in its electric toothbrushes cannot be replaced. This can be dismal news for those who spent $100 or more on their Sonicare electric toothbrush. Before you start shopping for a brand new Sonicare toothbrush, you have nothing to lose by at least attempting to replace the rechargeable battery in your current toothbrush. Despite Sonicare's protestations, the adventurous and electrically inclined do-it-yourselfer can often successfully replace the battery in a Sonicare toothbrush.
Take your Sonicare toothbrush off of its charger base and set it on a soft towel. Turn the toothbrush on and allow it to run until the internal rechargeable battery is depleted of any remaining charge.
Use a flathead screwdriver to pop out the trapezoid-shaped rubber piece located on the back of your Sonicare toothbrush, where the brush head attaches to the toothbrush handle. Set the rubber piece aside until you are ready to reassemble your toothbrush.
Remove the entire replaceable brush head piece from your Sonicare toothbrush. Separate the brush head piece with the toothbrush bristles from the cuff piece that screws the replaceable brush head onto your toothbrush handle. Replace the cuff piece onto the Sonicare toothbrush handle and turn it clockwise until the housing loosens. Pull the internal toothbrush components free and set the housing aside.
Examine the toothbrush's PC board to locate the two metallic soldering points that hold the battery securely in place and provide the electrical circuit connection between the PC board and the battery. Place a piece of desoldering braid over each of the soldering points. Touch the desoldering braid with a hot soldering iron; the desoldering braid will wick up the soldering points for easy removal.
Remove the rechargeable NiCad battery from your Sonicare toothbrush. If necessary, use the flathead screwdriver to help pry it free. Recycle the old NiCad battery at recycling facilities in your area.
Solder two 2 inch wires to the top of your toothbrush's PC board, in the same locations as the soldering points you removed in Step 4. Once attached, thread the wires through the board so that they can be accessed from the battery compartment on the reserve side PC board.
Rough up the positive and negative terminals on a rechargeable AAA battery with a pocket knife to help the solder better adhere. Be gentle when roughing up the terminals to avoid damaging your rechargeable battery.
Set your soldering iron to a low heat setting so you can solder the 2 inch wires to your new rechargeable battery. Solder the positive wire to the positive battery terminal. Repeat the process, soldering the negative wire to the negative battery terminal. Wait for the soldering points to dry before inserting the battery into the battery compartment.
Reassemble your Sonicare toothbrush to complete the battery replacement process. Slide the internal components back into the toothbrush housing and then replace the trapezoid-shaped rubber piece and the replaceable brush head. Set the Sonicare toothbrush in its charger base to charge a bit before use.