Roff
Well-Known Member
I have hair clippers with a series stack of three 1.2V, 600maH AA cells. It seemed like it wasn't holding a charge, so like any good engineer, I opened it up and made some measurements.
The batteries were discharged when I opened it up, so I put the charger on it. It started charging at about 90ma, and after about 3 hours the rate was about 65ma. I then measured the open circuit voltage at 4.2V.
I disconnected the charger and turned on the clippers. The motor drew about 700ma, quickly dropping to about 600ma as the voltage plateau'ed at 3.6V, as is to be expected.
This is where it got strange. After only about 9 minutes of running, the voltage abruptly (within about 15 seconds) dropped to about 2.1V, and the current to about 540ma (realize that the motor is not a constant resistance load). It ran in that condition for a few minutes, until I turned it off. It was too slow to cut hair, and I suspect that under load it would have stopped as soon as the voltage started to drop.
The open circuit voltage quickly recovered to 3.6V, but dropped to 2.1V again almost immediately when I turned it back on.
It almost seems as if the voltage of one of the cells goes to zero abruptly, or maybe internal resistance goes up suddenly. Whatever the cause, I'm thinking that a new battery is in order. Any thoughts?
BTW, the charger is crude. It has a series 10ohm resistor, plus the output resistance of the wall wart. Are NiCads tolerant of overcharge? I found an Eveready document that implied that theirs are, so long as you keep the charge rate below 0.1 times the hour rating (which mine almost does), but I'm interested in personal the experiences of you guys.
The batteries were discharged when I opened it up, so I put the charger on it. It started charging at about 90ma, and after about 3 hours the rate was about 65ma. I then measured the open circuit voltage at 4.2V.
I disconnected the charger and turned on the clippers. The motor drew about 700ma, quickly dropping to about 600ma as the voltage plateau'ed at 3.6V, as is to be expected.
This is where it got strange. After only about 9 minutes of running, the voltage abruptly (within about 15 seconds) dropped to about 2.1V, and the current to about 540ma (realize that the motor is not a constant resistance load). It ran in that condition for a few minutes, until I turned it off. It was too slow to cut hair, and I suspect that under load it would have stopped as soon as the voltage started to drop.
The open circuit voltage quickly recovered to 3.6V, but dropped to 2.1V again almost immediately when I turned it back on.
It almost seems as if the voltage of one of the cells goes to zero abruptly, or maybe internal resistance goes up suddenly. Whatever the cause, I'm thinking that a new battery is in order. Any thoughts?
BTW, the charger is crude. It has a series 10ohm resistor, plus the output resistance of the wall wart. Are NiCads tolerant of overcharge? I found an Eveready document that implied that theirs are, so long as you keep the charge rate below 0.1 times the hour rating (which mine almost does), but I'm interested in personal the experiences of you guys.