I'm pondering a little project to measure the capacity of a rechargeable battery. I did a quick search and one of manuka's projects came up http://picaxe.orconhosting.net.nz/bikeamph.jpg
Here is the problem. You go along and buy a rechargeable battery and it has 2600mAh printed on the side. But is it really 2600mAh? I've got some that are more like 400mAh, even when new. But heck, why not print 2600mAh on the side anyway, as the customer will never know! The best guide to the true capacity seems to be weight - proper ones (eg Duracell rechargeables) are a lot heavier.
Then there is the problem of older batteries. The capacity of a pack is the capacity of the lowest cell.
So - a picaxe 08M, a 10 ohm resistor and a battery holder. A flashing led to indicate it is discharging. V=IR and R is constant so V can be used to measure the current.
Discharge down to 1V and then hold that value. (I'm pretty sure you can discharge a single cell right down to 0V. The problem with reverse voltage only occurs with multiple cells. And it simplifies the circuit not to have a shutoff circuit. Though I suppose you could use a relay or mosfet or transistor).
The biggest problem might be a display. Displaying 2200mAH is 4 digits. Maybe 4 7 seg displays with a daisy chain of HC595s?
Or - there is the active thread at the moment with a LCD display only using 2/3 of the memory space of an 08M.
Or - I wonder if you could just output a voltage on pin 2 of an 08M using pwmout and make the voltage proportional to the aH measured? eg 2200mAh is 2.2V. Run it into an analog voltmeter (or a DVM, which may actually be cheaper than a voltmeter).
So at the very simplest level, an 08M, 100R resistor, reset button into 1 pin, pwmout of one pin, readadc on one pin, and flashing led on one pin to indicate it is discharging. I think that is all it needs.
Hmm - what about flashing the led a different number of times to indicate a number, eg 11 flashes then a pause is 1100mAh. Then you don't need a display.
Any other ideas?
Here is the problem. You go along and buy a rechargeable battery and it has 2600mAh printed on the side. But is it really 2600mAh? I've got some that are more like 400mAh, even when new. But heck, why not print 2600mAh on the side anyway, as the customer will never know! The best guide to the true capacity seems to be weight - proper ones (eg Duracell rechargeables) are a lot heavier.
Then there is the problem of older batteries. The capacity of a pack is the capacity of the lowest cell.
So - a picaxe 08M, a 10 ohm resistor and a battery holder. A flashing led to indicate it is discharging. V=IR and R is constant so V can be used to measure the current.
Discharge down to 1V and then hold that value. (I'm pretty sure you can discharge a single cell right down to 0V. The problem with reverse voltage only occurs with multiple cells. And it simplifies the circuit not to have a shutoff circuit. Though I suppose you could use a relay or mosfet or transistor).
The biggest problem might be a display. Displaying 2200mAH is 4 digits. Maybe 4 7 seg displays with a daisy chain of HC595s?
Or - there is the active thread at the moment with a LCD display only using 2/3 of the memory space of an 08M.
Or - I wonder if you could just output a voltage on pin 2 of an 08M using pwmout and make the voltage proportional to the aH measured? eg 2200mAh is 2.2V. Run it into an analog voltmeter (or a DVM, which may actually be cheaper than a voltmeter).
So at the very simplest level, an 08M, 100R resistor, reset button into 1 pin, pwmout of one pin, readadc on one pin, and flashing led on one pin to indicate it is discharging. I think that is all it needs.
Hmm - what about flashing the led a different number of times to indicate a number, eg 11 flashes then a pause is 1100mAh. Then you don't need a display.
Any other ideas?
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