I use these toothbrushes: http://www.amazon.com/Oral-B-Advanced-Toothbrush-Vibrating-Bristles/dp/B00336EUV8
They tend to run out of battery quite quickly. I'm happy to keep buying more because regular toothbrushes just don't get the job done in comparison.
However I've stumbled upon a neat little trick.
I've been taking them apart to insert 10440 size Lithium batteries in place of AAA cells.
As you can imagine this gives a boost in power which is quite amazing. I'm reasonably certain that it's not too much power, I'll ask my dentist about it next time I see her, but I am pretty much hooked on the extra power. More gum stimulation is healthy right?
Anyway, the real issue is that after a short amount of time (a few days of normal usage) the motor eventually gets more and more sluggish. It would start out going quite fast, then gradually slow down. Pausing for a while, then restarting sees it regain some power.
I can of course do some more controlled tests to better determine what's going on but the behavior seems like a temperature dependent effect to me. When I perform this modification to a "new" (i.e. its alkaline cell it came with has been exhausted and I put my lithium cell in it) motor it runs nice and powerful for a few sessions of brushing before this pattern of rapid loss in performance starts to set in. I check the voltage of the battery and it is above 3.8V (not depleted).
Whether it's the battery or a degrading motor that is limiting the vibrating power, I think I should be able treat it by limiting the current in the circuit. As it is, there is no circuitry for regulating current: the battery is directly connected to the tiny DC motor.
What modifications could I make?
I have very little space to work with so it seems like I'll only be able to insert one or two small components (diode, resistor), and the circuit must remain small.
My hope is that a well adjusted modification can keep the current supplied by a lithium cell to within a range that does not degrade the motor. This way I can make the circuit once within the assembly and I can transfer that into new toothbrushes (because these need to be replaced as they are worn out).
I'll be back to post measurements of current, battery internal resistance, and maybe temperature when I get around to doing this.
Best Answer
The motor runs faster b/c you're applying a higher voltage to it; the motor has to turn faster to generate the back emf to balance the battery voltage. Now, if you limit current, and the motor runs more slowly, it will again be generating less than the battery voltage, so the remainder of the voltage will be the drop across the current limiting device or circuit. The voltage drop across the current limiter, times the current through it, represents power that is wasted.
I assume your idea is that you still want the motor to run faster than the original device, but slower than the direct lithium-to-motor configuration. But you see, the concept of limiting the current here is kind of counter productive: You added a higher voltage battery, and inserted an extra element, and then burn off power to bring the motor speed down from what it would be w/o the current limiter. The more direct solution would be to choose a battery that's not so much higher than what the motor was intended to see. I.e., instead of jumping from 1.5V to 3V, maybe try to hit something like 1.8 or 2V. Of course, batteries don't come in arbitrary voltages like that, but the point is, don't overkill it with 3V, then try to kludge it back to where a lower battery voltage would get you to directly.
Probably the best solution would be to modify the motor itself. If you rotate the brush assembly relative to the magnets, the motor's speed/torque characteristics can be changed. It requires mechanical finesse, but you might be able to mod the motor to work well at 3V. Or you might be able to outright replace the motor.