Since visibility in the Dutch water is not that good, I am used to take a divelamp with me on every dive. The Ikelight 400. After some 100 dives it occured to me that my light became less bright after, say, 45 minutes. However it lasted more than an hour before it went out completely. So on a dreary sunday afternoon I decided to measure the current the NiCd battery of my divelamp supplies to the lamp. Next graph shows the current vs. time.
As you see the NiCd battery does not give up at once, but after a few stages of lower current supply. I used to charge my lamp after each dive, without completely discharging the battery first. So first thing coming up to my mind was that this must be the 'memory effect' everybody talks about: if you discharge your battery to, say, 50% and than recharge it, next discharge will only be to 50%! So I changed my habbit and completely discharged my battery before recharging it. After a couple of dives, I charged the battery and measured the current again. The curve is shown below.
So, apparently, the 'memory effect' vanished, as we might have expected, believing the stories about NiCd batteries. The battery supplies 3.4 Amps to the lamp for 80 minutes (1.33 hour) before dropping dead at once. Battery capacity is 1.33 hours times 3.4 Amps is 4.5 Ah. This corresponds to the specs of the battery. However, during diving, the problem of the lamp becoming faint persisted, sometimes after half an hour diving. So I decided to perform the measurement again. On the second measurement (Figure 2) I measured the current right after charging. I now measured a few hours after charging and another time, four days after recharging, corresponding to a common situation (I use to charge right after a dive, whereas the next dive can be a week later). The current graphs are shown below in Figure 3, together with the second measurement.
A more appropriate explanation for the curve is this: the battery consists of 4 separate cells placed in series. After charging the cells are charged fully. However, due to internal resistance the cells discharge. Apparently, not all cells discharge equally. During discharge through the lamp, the first current drop occurs when the most empty cell runs out of power. I verified this by measuring the voltage accross the individual cells after the first current drop: one cell appeared to be almost 0 volts, whereas the other 3 showed about 1.2 Volt. Since the first cell looses almost all of its power after a few days, it has probabely become defective. The reason might be overcharging (another bad habbit of mine is leaving the battery for days connected to the charger) or dropping the battery. I decided to replace the battery by 7 Ah NiMH cells. The discharge current is shown below together with the NiCd curve of Figure 2 for reference. The NiMH current is a bit less than the NiCd current, probabely due to contact resistance in the measurement setting.
The new NiMH battery supplies for 135 minutes (2.25 hours) about 3.2 amps. Capacity of the battery hence is 2.25 hours times 3.2 amps is 7.2 Ah. this corresponds to the battery specs of 7 Ah. This is quite a lot better than the NiCd battery (4.5 Ah).
