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  • Rechargeable Batteries (strange)

    I've used a Sonicare electric (rechargeable) toothbrush for the last 3 or 4 years or so. Probably one of the best rechargeable devices I've met. But it has appeared to be on its last legs for the last year or so, only holding a charge for 2 or 3 days, maybe only a day sometimes.
    But recently I sort of gave up on it and just left it setting there uncharged for a week or two.

    Then I decided to give it another chance, and charged it overnight, which is the way I normally charged it. Lo and behold, the darned thing then ran for about 10 days!

    There has been several rechargeable battery discussions here in the past, and it's been pointed out that they tend to develop little metallic whiskers which can short them out. But I don't remember any mention of any sort of self-correcting. What'dya suppose is going on there?

    Incidentally, for those not familiar with these, the charger and toothbrush neither have any metallic contacts. It's all plastic to plastic contact between the two. So I guess the charging process apparently takes place thru some sort of induction current. (...that's a mystery to me too)

  • #2
    Nothing mysterious about how it charges. It's an air core transformer with one winding in the base of the brush and the other in the holder. As for the battery, I'm not sure what may have happened. Is it possible that the brush wasn't sitting properly in the holder until you moved it around?
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    • #3
      Brought some batteries in to be rebuilt a while ago, and while waiting a couple of the guys working there were talking and one of them said that often either dropping the battery or giving it a solid slam on a table is enough to disrupt the crystals or whatever else forms the memory.

      Tried it out on a couple batteries that I planned to have rebuilt the next month, and it seemed to work. Instead of only a 15 minutes to a half hour run time, they would hold hold a charge nearly as long as the rebuilts.

      Figured the guys were kind of stupid for discussing it with customers present, figure if it's a $40 to $60 rebuild, or a slam on the table I know what I'll go for, especially since it's a no-loss situation, if it works you save the money, if not you just pay the money you expected to pay anyway.

      Ken.

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      • #4
        Originally posted by Evan
        ..... Is it possible that the brush wasn't sitting properly in the holder until you moved it around?
        Not possible. The brush just drops straight down into the charger, and the shape, plus a molded plastic key ensures it can only fit in the proper way.

        One thought I'd had, is that maybe the charger itself has the problem, rather than the battery pak in the brush.

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        • #5
          Modern batteries no long have the memory effect as old NiCd batteries did. Most recent products use Ni-MH now. There should be a little sticker or imprint somewhere on the brush that says what it has in it for recycling reasons.

          Might have been voltage depression which is said to be cured by deep discharging the cells.

          http://www.repairfaq.org/ELE/F_NiCd_Memory.html

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          • #6
            It would appear that you can get a high probability of service improvement just by mailing the batteries to yourself. After a week on the road in a USPS truck they should bounce right back

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            • #7
              There's nothing on the toothbrush power unit indicating battery type. Might've been on the original packaging, which was tossed long ago.

              This was a xmas gift from daughter, which included two units - one for me, one for wife. Wife's gave up the ghost several months back. I took it apart ( destroying in the process) just out of curiosity. The batteries, and all other components were (epoxied?) in place, so the batteries were not replaceable.
              I kinda think I remember that they were NiMh.

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              • #8
                A device like that is a prime candidate for super capacitors instead of batteries. It never needs to run more than a few minutes and always is charged before use. I have a super cap that is the size of a dime in diameter and about 1/8 inch thick rated at .5 FARAD at 5 volts. Together with the supermagnet motors now I bet they could make a toothbrush without batteries. Capacitors have a very long life compared to batteries, maybe 10 to 20 years in that sort of use.
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                • #9
                  That's a lot of capacitance, Evan. Sounds like it would be fun to experiment with.

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                  • #10
                    It's a memory backup capacitor. They have a high internal resistance, around 1000 ohms. But if you put a bunch in parallel that wouldn't be a problem. It's probably what's inside the hand crank LED flashlights.
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                    • #11
                      I was thinking they'd be excellent for digital measuring instruments. The Asian variety I use drain the little batteries rather quickly because even when turned off they're still on. Something like this would be a quick recharge item, like instantly, and so last forever. I never seem to remember to pull the batteries from the calipers until the display starts flashing to let me know the battery is nearly dead.

                      As a result all my digital instruments are sitting in a box not likely to be used again soon

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                      • #12
                        I've used the same for a number of years. Great battery life, and the magnets in the brush head are amazingly powerful for their size. I assume some kinda NiMeFeMumboJumbo?

                        Doug

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                        • #13
                          It would appear that you can get a high probability of service improvement just by mailing the batteries to yourself. After a week on the road in a USPS truck they should bounce right back
                          That, of course assumes they actually arrive at your house

                          I had always heard that NiMH cells could be damaged by dropping. I guess the moral of the story is to make sure your "percussive maintenance" includes whacks that are hard enough to jar loose the metallic whiskers inside while not completely destroying the battery....of course what do you have to loose with a bad cell.

                          Shocking has long been used with NiCd cells. However, the mention of deep discharge is a new one on me. I do know that with most NiMH and NiCd cells, you run the risk of having a a cell "reverse" (reverse polarity) after reaching zero volts. In a pack this pretty much renders it toast as it will never be full voltage again unless the pack can be separated, the reversed cell found and fixed.

                          I have one of those smart chargers that I use for my amateur radio stuff. It will cycle packs if you want to, and it monitors so that it won't run it below a volt per cell to prevent just this problem.

                          Paul
                          Paul Carpenter
                          Mapleton, IL

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                          • #14
                            Just a W.A.G. here

                            For the original poster...
                            I had the same thing happen with a rechargeable tool (dustbuster?) What I think happened is that one of the cells in the series-connected pack retained charge better than the others so would top out while the others were still charging. Allowing it to sit gave time for some self-discharge to occur on the retentive cell so they were some what equalized when I connected it to the charger. This is just a guess; I don't know how the batteries are connected in your sonicare....

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                            • #15
                              supercapacitors wouldnt be so hot for a electric toothbrush or shaver if you wanted to take it with you on the road on a vacation. I have one of the linear panasonic chargers and I took it with me when I went down to Cali. After a week and a half it was only down to 60%. If it had supercaps I would have had to take the base station for it which is one more thing to bring and forget.

                              I have some supercaps from cap-xx. Pretty cool. They look like a little lipo cell. The difference is though that they have a very low ESR so they can output up to 30 amps. I was working on a design for a LED based infrared photoflash and was planning on using these to drive the LEDs (Special high power pulse rated led capable of 10 amps pulse peak.) Cap-xx sent me a tray of the supercaps as samples. They are based on carbon aerogel I believe.

                              http://www.cap-xx.com/

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