|About life expectancy of any rechargeable battery (was: HP 41 batteries)|
Message #10 Posted by Vieira, Luiz C. (Brazil) on 14 June 2010, 12:45 a.m.,
in response to message #1 by Peter Creyf
about four (maybe five) years ago I started using a pack of four Sanyo Cadnica 150mAh N-cells, and they are still working. I have at least five working HP41´s (halfnut and coconut units), and two CX kept without batteries (the internal clock is not a good friend of rechargeable batteries...), so using disposable batteries is not an option. I also have three 82242 IR modules, three card readers and one 82153A (optical wand), and all of these guys use battery power. Using rechargeables makes perfect sense, right?
Now for the facts. The current capacity in batteries is measured in Ampere X hours, and the nominal value (e.g. 150mAh in this case) means the maximum current in one hour till the nominal voltage in the battery terminal drops to 90% (I also found 85% in some literature). So, if a NiCad, 1.2Vcc, 150mAh battery keeps a 150mA load, after one hour its terminals might be measuring about 1,08 (or 1,02 if 85% is the factor). The 150mAh pack I have still keeps an HP41CV memory contents (no modules at all) for about three to four months before the battery indicator lits.
I ended up buying a brandless Chinese pack of four NiMH 400mAh, which lasts less than double the time of the four Sanyo. I was also given twelve 190mAh Sanyo Cadnica (three packs of four) that work almost the same time as the 400mAh Chinese units.
But the secret lies in the way you charge them, not the way you use them.
The average 1,000 charging cycles applies only if you respect the '10% nominal current' charge limit while charging (that's why the charging time is always 10 hours). Any fast charger will charge the batteries according to a non-linear charging curve, but chances are that fast charging will also reduce battery life or discharging characteristic. Also, many of the standard charges apply high current to the batteries, and most of them heat the batteries up somehow. I measured a regular charger right after getting my first 150mAh NiCads and it gave me almost 100mA charging current. It would kill the batteries very soon, I guess.
While measuring the charge current I noticed that the charger had about 3.8Vcc in its terminals with no batteries, so I measured its current and voltage in many circumstances and I realized it was just a passive current source. So I used a pair of NiCads in series (about 2.4Vcc) and added a small series resistor. Please, look at the pictures: (link to photobucket)
Assembly with two batteries
I used two-side coated PCB´s to accommodate the resistors
Assembly with batteries and resistors
As I did not open the charger to see the circuit, I simply tested with two different resistor values, measured their voltage drop with the two NiCads in series then I computed the correct one that would give me something closer to 15mA needed to charge the two NiCads (47 ohms) and another one closer to the 40mA for the two NiMH (27 ohms - should be smaller, about 23 ohms, but I decided to use a higher, commercially available value, hence a smaller current).
The batteries never heat up after their ten hours charging time (they do not even get warm to the touch) and their discharging periods did not significantly reduce so far.
Hope this helps a bit.
Edited: 14 June 2010, 1:59 a.m.