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drill battery from laptop batteryI looooove woodworking, my heart always balances between that and electronics. I work on it to get better, so I get to guy tools. But I'm cheap, and I decided to buy a cheap drill powered battery (a Bosch if you care) with old dead 14.4v Ni-Mh batteries. I got changed the battery of my office laptop the other day and got to keep the old ones; they would still last 1h30 but after that they'd let me down. But it's enough for drilling a few holes!
So let me explain how I did it...
Bill of materialsFor the battery:
General principleIf the 4 liion batteries were in parallel, since they come from the same pack originally, one could assume it's safe to charge them in parallel. However one would have only ~4v output and one would sit on his drill because it would never turn...
So the liion cells are in serie, the 4 of them for an output that will be in the range [14.8 ~ 16.8]. That's more than the nominal of the drill, but it's just a motor, and it will just eat a +10% overvoltage, so nothing to worry much about. Plus it's a drill which you run by bursts, not continuous so we'll live with it.
When the drill is operated, circuit is closed and all work as a serial bank of batteries. Now problem is when to charge: you can't charge liion in serie (nope). So simple solution is to isolate each cell and charge them individually. In the above schema, to charge cell (1), you connect the point (A) to the (Bat+) of the charger, (B) to (Bat-) and you're in business! Electrons will flow between these points only, leaving the other celles untouched. Then to the next cell, plug (B) to (Bat+) this time, (C) to (Bat-) and repeat. That pluging thing is done by some MOSFET I guess on the charging board that is in the laptop battery. In my case, because I didn't thought of it preferred to use small relays I bought before. More seriously, with relays I get a physical connection and disconnection of the circuit. I felt more safe for now due to my current knowledge of Liions cells and MOSFETs.
Wiring: as mentionned, I use a 5 pin GX-12 connector you can easily find on eBay. And since each time I wonder: GX-12 Pin 1 = Positive (point A in above diagram) and Pin 5 is Negative (point E in the said above diagram).
Last part: when to stop? Easy. Those 2 USD chargers have leds, so just read their status. I used some optocoupler which is an overkill because we're operating on the same power source (USB) so no need to isolate, and now 2 weeks later as I type this I understand I should have used the config of the leds on that chip as a pullup: the pins 6 & 7 of the TP4056 are high impedance (not at same time), I should have soldered the resistor in place of the led and measure voltage there with my ATtiny26 (pullup so high if led should be off, low if led should be on)... anyway, now it's done and glued so I'll live with it an my over engineering of 30 JPY.
Source codeAll the source code on GitHub as usual.
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