For the past year or so, I've been using a Sonicare electric toothbrush. It's an amazing bit of technological kit that blasts plaque away with a brush head that vibrates at ultrasonic frequencies. I had used inexpensive battery toothbrushes for several years prior to acquiring the Sonicare, and I was fascinated to note several significant differences between it and its less expensive counterparts.
The first major difference is the lack of a motor. The other battery toothbrushes I have used have all had a small electric motor, which drives the the head with a simple mechanism that either causes the bristles to oscillate back and forth or spin around.
The Sonicare also oscillates, but there is no motor. The only physical connection between head and handle is the threaded collar that keeps the head in place. Instead of driving the head with the spinning shaft of a motor, this toothbrush uses an electromagnet that reverses polarity several thousand times a second, causing the head to vibrate several thousand times a second. Unlike traditional battery toothbrushes, which will run without a head attached, the Sonicare will not. Its head completes the mechanism.
A secondary advantage to this design is it allows the handle to be a completely sealed unit, ideal for operating in a wet environment like a bathroom--or a mouth. This sealed design continues at the bottom of the handle, where there are no electrical contacts despite the fact that the handle contains rechargeable batteries. Here again, magnetism comes into play as the batteries are charged by a process called magnetic induction. It's not the most efficient design, as it takes almost 24 hours to give the batteries a full charge, but it does facilitate a design where there are no metal contacts to allow leakage or oxidation.
As amazing as this design is, the batteries, two non-replaceable AA size nickle-cadmium (Ni-Cad) cells, are definitely its weak point. I was genuinely surprised the manufacturer, Phillips, opted for lower quality Ni-Cad cells when superior nickel metal hydride (NiMH) cells would have added, at most, a few pennies to the cost of the toothbrush.
The problem with Ni-Cad cells is twofold: They generally have a faster discharge rate than NiMH cells and they develop a 'memory' pattern in which they have a very short operational life between charges if they are charged at frequent intervals without being fully discharged first. To get the best life out of Ni-Cad batteries, they need to have deep cycle charges, which can be achieved by charging the batteries only when they are fully depleted. Fortunately, the Sonicare toothbrush is engineered to maximize battery life by alerting the user when when the batteries need charging by giving a beep and flashing an LED under the power button. While the instructions mention the low battery indicator feature, they don't state specifically that the battery should only be charged when this feature calls for a charge. On average, I have to charge my toothbrush about twice a month.
This is somewhat problematic given that many users will doubtless keep the toothbrush in its charging stand between uses. Over time, this can lead to premature battery failure, which spells the end of the toothbrush's operational life since the sealed handle can't be opened without destroying it.
I believe that NiMH batteries would be more suitable for this application because they hold a charge longer and have much less of a memory effect. I proved the superiority of NiMH batteries to myself a few years ago when the Ni-Cad batteries in my Remington electric razor died on me. Because the outer casing was held together with screws, I took it apart to find conventional AA-sized Ni-Cad batteries with solder tabs inside.
After searching around the Internet, I found a source for the batteries and saw they had both Ni-Cad and NiMH batteries available at virtually the same cost. I opted for the NiMH batteries, soldering them with a minimum of fuss. In the process, I extended the life of the device at about a tenth the cost of replacing it. My replacement batteries outperformed the originals in every way. Not only do they hold a charge longer, but six years later, they are still going strong, whereas the originals only lasted about five years.
Given what I know about these batteries, it boggles my mind that Phillips would choose an inferior battery for an otherwise highly advanced toothbrush. The only rationale that comes to my mind is one of built-in obsolescence in that it's not always in the manufacturer's best interest to let the consumer have too much of a good thing.
Update: 10 December 2011
I was in Target yesterday and saw a new entry level Sonicare model. This one uses conventional AA batteries and sells for the relative bargain price of $20, less than half the price of my Sonicare Essence. Because the batteries are replaceable, you can use either alkalines or rechargeables and you don't have to throw the handle away when the batteries wear out. If I were in the market for a Sonicare today, this model would be on the top of my list, along with some rechargeable NiMH batteries. The brush heads are still frightfully expensive, but at least one would potentially get a longer service life out of the handle.
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