Archive for July 9th, 2011

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.

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