Just how good are these Nokia BL-5C used battery packs lying around in your bottom drawer for hobby purposes?
I found two in my electronics dump. I decided to measure the main discharge characteristics of them including their current capacity and the energy stored in them.
Read on for the details.
So I opted for a discharge current of max 60 mA, that even my 0.25W resistors will handle (see below). This is well below the typical max 230 mA discharge current of the battery pack (according to this site), although I couldn't get a reliable factory figure in this respect. This later figure seems a bit high to me to take it as a typical discharge current, given that these Nokia phones went on forever with a single charge of these 1020 mAh batteries compared to the current smartphones. Therefore, they didn't deplete the battery too quickly (certainly not within 4-5 hours), so it's probably not a very good idea to use a very high discharge current with these packs. Using this setup a BL5C battery - even one in a top shape - would deplete within a day or so (17 hours max).
Method of Measurement
I glued Molex connector contacts onto a piece of ABS plastic. This was then secured by a G clamp to connect to the battery.
The total power dissipated by the two resistors altogether is a maximum of 4^2/66=0.24W. Since the voltage splits evenly in the circuit, each resistor will dissipate only the half of this power. Therefore the resistors stay well within their specifications, dissipating about 0.12W maximum power each.
Results and analysis
Uncertainties of the measurement
The time measurement had an error of 0.3%, when compared to a time server which was compensated for, therefore the uncertainty of the time measurement falls into the same negligible category as the difference in temperature, therefore it was ignored. So overall, the energy measurement had the same 2.8% error at 95% confidence.
Since the temperature coefficient of the resistors were very low (this was also confirmed with a measurement), we can ignore the resistance increase due to the slight heating. Therefore R(t)=R. The resistance change due to this was not measurable using the DMM, the calculated change in the resistance values due to heating is less than 10 mOhm. Whereas the absolute uncertainty of the resistance value is 330 mOhm.
POwer and energy
To sum up the assumptions explained above, the energy in the battery was calculated using the following formulae. The summing was done over the whole time period of the measurement, with n time periods in total.
Replacement battery results
Original - older battery results
However, in both cases the capacity seems to be "missing" from the beginning of the curve. These batteries can be charged up above 4 volts normally, but these used ones I had either didn't take the charge, or the phone I used didn't charge them up to the highest voltage, despite showing them to be at full charge. Still it seems that they're more than adequate for many hobby purposes.