Beard Trimmer: NiCd Rejuvenation

Strictly speaking, I do not have a beard: I simply do not shave (*). There being no money in selling Trimmers for the Non-shaving, a while back I bought a battery operated Beard Trimmer. The NiCd cells lasted for the predictable few years and recently gave up the ghost entirely: an overnight charged produced a weak buzz with no cutting action to speak of.

The case uses one-time snap-together latches, which makes dismantling it a challenge. Start by removing all the gimcrackery on the business end, then pry out the two latches holding the it-was-white-once cutting length adjustment ring.  With that out of the way, undo the two latches inside the top and work your way down, prying the case halves apart in the way the overlap flange doesn’t like, so as to force the latches loose.

This picture shows the six latches, three on each side. The ones just to the right of the blue impeller require the most cursing:

Beard trimmer case and innards
Beard trimmer case and innards

The circuit board snaps out, with the two PCB contact areas clamped down by springy contacts leading to the motor.

Beard trimmer - battery charger PCB
Beard trimmer - battery charger PCB

The two NiCd cells boast of their High Energy, but they’re only 600 mAh. That’s actually too much for this high-drain, short-run application, as they don’t completely discharge. They’re held in place on the right end with a blob of hot melt glue:

Beard trimmer - NiCd cells
Beard trimmer - NiCd cells

I unsoldered the cells and gave ’em a brute-force overnight charge at C/10 = 60 mA, then ran a discharge test (clicky for more dots):

Beard Trimmer - NiCd Discharge Test
Beard Trimmer - NiCd Discharge Test

Lookee that! The cells still deliver their rated capacity, even though they no longer worked with the stock charger. I repeated the slow-charge and discharge trick, which produced a perfectly overlapping trace.

Flushed with success, I unleashed the built-in charger overnight, then produced a third overlapping trace.

So they suffered from voltage depression, most likely due to never being completely discharged and then being overcharged far too often. That’s cured by a complete discharge and recharge, which worked perfectly.

I hack back the overgrowth when it gets bushy and recharge the trimmer when it seems to be getting weak, which used to take a week or two. That’s a bad way to maintain a NiCd battery, particularly as the PCB applies a very low load to keep its computronium running, but I have better things to do than babysit a beard trimmer. Honest.

Anyhow, assembly is in the reverse order and it’s perfectly happy again.

I probably won’t change my evil ways, so the next time I’m sure the battery will be really and truly dead.

(*)  Not shaving adds about ten minutes a day to my life, which I regard as a fair tradeoff over the course of several decades. It also added a decade to my apparent age, Back In The Day when that mattered. Now it seems to knock off a decade, which isn’t entirely a Bad Thing.

8 thoughts on “Beard Trimmer: NiCd Rejuvenation

  1. Aren’t there laws against riding a recumbent and not having a beard? I’d be careful with that beard trimmer… before you know it, you trim a little too much and get ticketed by the ‘bent police…. :-)

    If it were mine, I’d have installed some new NiCd cells or perhaps NiMH. I fear it’s just a (short) matter of time before those old cells fail again. For example, In the past, I tried to fix internally shorted NiCd cells (short caused by internal growth of dendrites) with a zap from a 12V battery. That fixed the short…. for a few weeks. Then the problem came back again. Another zap would solve it again… for a while.

    BTW, I always thought that voltage depression from overcharging was relatively small, about 50mV. I.e. nominal voltage gets reduced from 1.25V to 1.20 V. Then again, there are few topics where so much myth and dis-information exists than rechargeable batteries. Or chain lubrication. Or bicycle helmets. Or…. err…. never mind….

    1. riding a recumbent and not having a beard?

      My ladies have been riding outlaw for some years, but I fear the hammer would come down if I tried going hairless.

      We will not discuss the subject of Aerobellies in this forum.

      I’d have installed some new NiCd cells

      I was astonished that one charge-and-discharge cycle restored almost exactly the rated capacity; I fully expected to harvest some parts and junk the trimmer. Now that I have a benchmark, we’ll see how it fares…

  2. Unless I am greatly mistaken, this is the same model of Norelco trimmer that I, too, disassembled a couple of years ago due to battery troubles.

    The thing had taken to going from full charge to total discharge in a single use. So I popped in a new set of NiCds, only to find a very modest improvement. Seems like a problem either in the charger or in the state-of-charge sensor.

    IIRC, the charging was controlled by a small embedded uC that was sending strings of pulsatile charging jolts. Probably it looked at the voltage recovery post-jolt to determine the charge state; and no doubt the designer thought this would be a better way to charge than a steady trickle charge.

    But it No Function Good, if you know what I mean.

    Next time I open it up, I’ll try a couple of full discharge/recharge cycles and see if that helps. Thanks for the idea.

    1. a small embedded uC

      These days you can’t tell if it’s a microcontroller or an op amp, unless you reverse-engineer the surrounding parts… and sometimes not even then.

      After two weeks of giving it a buzz whenever I thought about it, the thing is stone dead. NiCd cells aren’t noted for their charge-holding ability and I didn’t measure the current drain through the logic circuitry, but that seems rather short. It’s time for a trim again, so we’ll see what another charge-discharge cycle does for it.

      1. Must be something in the air. Mine went stone dead after being unplugged for a week, and simply refuses to restart.

        I opened it up and found that one NiCd cell was fully charged, the other was registering a short circuit. After some electronic CPR, though, I was able to recharge it.

        The **next** time this happens, I’m just going to rip out the silly MSP430 CPU board and replace it with (1) an SPST toggle switch and (2) a 60 mA current source to trickly charge tha NiCds.

        1. I opened it up

          Right around the third or fourth time I do that to something, the thought crosses my mind that maybe, just maybe, I should replace the something. [grin]

          That notwithstanding, I’ve marked the location of those pesky snap latches on various gadgets I’ve fixed to make opening the case easier the next time around…

  3. Well, it died again. Same symptoms — one good NiCd battery, one shorted NiCd battery. So I did a battery-ectomy, and jammed in a 2.5V LDO regulator so the thing can just run off the wall wart.

    Here are two pix of what I have to feel is orc engineering at its finest…

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