| Electronics > Repair |
| Best way to fix hair clipper that kills 18650 batteries? |
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| jklasdf:
Hi RoGeorge, there isn't actually a dedicated charger IC on the board (it board is not fully cost optimized, but definitely still built down to a cost). U2 is a general-purpose 8-bit microcontroller from Holtek, and these 8-bit microcontrollers seem to have some kind of proprietary I2C-like programming interface to reprogram the flash memory (likely need some custom initialization sequence at power on to get the microcontroller to use the pins as a programming input). In the actual circuit, the ICPDA and ICPCK pins used for reprogramming (U2 pins 5 and 9) are used as outputs to drive the LED display, and I think it is unlikely that they programmed the microcontroller to talk SMBus on those pins. The charging circuit consists of Q1 and Q3, which is controlled by a PWM pin on the microcontroller. The charging circuit is not ideal in that when Q1 is switched on fully, the voltage drop across Q1 and schottky diode D1 is not enough to prevent too much voltage from the 5V input from being applied to the battery. The DW01 battery protection IC (U3) should limit any over-voltage on the battery from charging though (assuming it responds fast enough), and I guess they can keep the PWM duty cycle low enough that hopefully the battery isn't too overstressed? I haven't actually measured while charging to see if the battery continuously sees pulses over 4.2V while charging. If so, that might also contribute to killing the batteries. |
| jklasdf:
I decided to make some measurements for how much current the motor actually draws. The following is for supplying power to the motor directly: * 4.0V: 1.4 to 1.5A (power draw of 5.6 to 6W) * 3.5V: 1.3 to 1.4A (power draw of 4.6 to 4.9W) * 3.0V: 1.2 to 1.3A (power draw of 3.6 to 3.9W) There is a very noticeable difference in the motor speed at 4V compared to 3.5V compared to 3V, and I guess I can kind of see why they paralleled both halves of dual-FET Q5 together, instead of using them in series for proper protection of the battery. Doing so makes the motor run slightly faster. I can't see any obvious reason why I can't change dual-FET Q5 to be in series (would require hacking up the board a little), with one of the FETs controlled by the DW01 overdischarge (DO) pin. The current is not really that high. Here's a random TSSOP8 8205A dual-FET datasheet I found (not sure if it's the actual one used) which is rated for 3.5A continuous. Even if the on-resistance is worse (say 100mohm) it should only be a couple hundred milli-volts of extra drop for the two individual FETs in series instead of parallel. The motor speed difference would probably be noticeable, but should still work? Typical application circuit from DW01 protection IC with dual-FET, with OD actually being used for over-discharge protection: |
| jklasdf:
I decided to measure the actual idle current draw, and it is slightly higher than I was expecting. * 4.0V: 550uA * 3.5V: 450uA * 3.0V: 340uA So say maybe 500uA on average when idle, which is roughly double what I had guessed before. If the battery is say only 1500mAh (instead of the 2500mAh stated), that would give 3000 hours, which is 125 days (should still last a few months if the battery were fully charged before hand, but not ideal, especially since the battery charge indicator is wrong and overestimates the % charge). |
| mobby_6kl:
That's an impressive amount of effort for hair clippers :) I wouldn't worry about that idle power. It should be 0, but really running the motor for 1 minute will use several orders of magnitude more energy than idling for weeks. For charging, I'd just stick in one of those 1S charging boards from Aliexpress, bypassing any of the existing circuitry. This would also make your life easier if you ever have to travel with this as you could just use USB. I've had a cheapo unit from Lidl where I had to replace the battery and since the charger input was 5v, also replaced their weird jack with USB. I eventually got nicer Philips clippers as a gift which now doesn't last quite as long as it did. There's no obvious way to open it though so I haven't messed with it so far. Probably a bunch of plastic clips that'll get broken instantly. |
| janoc:
Re using a protected battery - protected battery protects you from a fire when you attempt to charge or use a battery damaged by overdischarging. However, it will not protect the battery from dying! Most protection circuits shut the battery off at around 2.5V, some even as low as at 2V. If you keep discharging the battery that low, it will have a very short and unhappy life. The protection circuits are meant to be the last resort measure, not to replace a proper battery charge monitoring/low charge shut-off. If you want to fix this piece of junk, either install a switch or you will likely need to replace the board with something better designed. |
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