What to Do When Your Electric Shaver Battery Is No Longer Available?

During my travel to Japan in 2011 I bought a very slick electric shaver Panasonic ES4853P. It worked marvelously for many years until its Ni-Cd battery lost most of its capacity. However, I still got quite a lot of mileage by keeping its charger connected at all times, including during shaving. "Change the battery" is the obvious advice, but this very special battery is no longer available anywhere. "Well, pull out battery and just use the charger" is another obvious advice (the two elements are connected in parallel, after all) but the charger is unable to produce sufficiently high starting current to kick the shaver's DC motor. You see, quite a lot of current is required to overcome motor's initial friction. But once the motor is moving the current demand is well within charger supply. Normally, the starting current is supplied by shaver's battery, but when it's gone there is no other source...

1. Supercapacitor 1 F (yes, one farad)
2. Diode
3. Resistor 2.6 ohm
4. Connecting wires

I just had the perfect part for the job: a 1 F (one farad) supercapacitor able to withstand 5.5V DC. At first I connected it straight, in place of the battery. It worked! The supercap was charging very quickly and provided the shaver's motor with enough kick to start. But the motor was getting too much initial current - it was revving like crazy for the first 3 seconds. "Hmm, the motor could burn" was my concern. The obvious idea was to limit the supercapacitors discharge current by adding a resistor. After some experimentation I've determined that 2.6 ohm was more/less optimal. However, this resulted in slow charging times... What to do? Well, yet another obvious idea :) was to connect a diode in parallel with the resistor, as shown in the attached circuit. Fast charging and slow(er) discharging. The shaver now works perfectly!

Years ago, I have used similar strategy to save a lot of money. We bought a fancy 12.4 US gallon trash can with lid activated by a motion sensor; pictured here. Quite convenient to use. I don't remember who made it and there is no manufacturer's name on it. It was battery operated requiring as many as 6 AA batteries. Well, these batteries lasted only 3 (!) weeks! We were spending a small fortune on batteries. :( I bought a 9 V DC charger, drilled a hole in the lid for the socket, and added an ordinary 33 mF electrolytic capacitor instead of batteries. No resistors, no diodes. It's much cheaper to operate and without the hassle of replacing batteries all the time. It's been working fine for years!