Disclaimer: My father used to say:
"The three things man will never understand are batteries, economics, and the opposite sex."
I got about 4 years on my DeWalt NiCds. Seems I do something with them every month. The 2nd generation batteries and charger from DeWalt added a 3rd terminal on the battery. Typically in NiCds this goes to a temperature sensing device. A "rapid charger" for NiCds is usually a charger and battery combination that makes sure the battery doesn't get too hot when charging. A "normal" charger just limits the charge current to keep this from happening. As the batteries get older, their internal resistance goes up resulting in a bit less power and higher temperatures when charging. So the temp sensor helps a bit on battery life.
Lithium Ion batteries require smarter electronics to charge. You can't just pump current into them. They also require a temperature sensors. This is no longer a big technical problem, and there are many chips that do Lithium charger stuff. The stories about Lithium batteries catching fire are what engineers call the learning curve. In hybrid cars, the battery cells are in small clusters with a little chip to monitor them so it's OK. The chip can also take the cluster out of the game so that a single cell failure doesn't mean the entire battery pack has to be changed out. This is not the case in hand tools.
The lifetime of both types of batteries is more a function of how many times they have been charged and discharged than anything else. Leaving them sitting around uncharged for long periods of time (months) also decreases the operating life. Higher output currents also affect life, so the battery in a flash light will see longer life than the battery running a saw. Much like the construction differences in lead-acid batteries, such as a deep discharge vs. car starting battery, the construction of the battery can affect both performance and life. With Lithium batteries, there is interplay between the way the charger works and the battery construction. While most batteries (both types) have typical life's of 300-400 charge cycles, some manufactures claim charges in excess of 1000 cycles. Some of the Apple computer products with non user serviceable batteries claim to be in this league. In hybrid cars, this higher number of cycles is very important, otherwise you would need new batteries in 2 years. Discharge rate complicates battery lifetime. Operating temperature (either due to ambient or high discharge rate) complicates this. Partial charge cycles complicate things, and pretty soon here I'll be refering you back to my Dad's statment
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Finally, the life of batteries requires some condition or threshold of performance that must be crossed in order to declare the battery dead. For example, when time you get using the battery drops by 25% from a new one, it's dead (even though it still can be used). The new LED lights that tout 50,000 hours of life have a similar threshold, such as "at 70% of the brightness of a new led, the led is declared dead" (even though it still puts out light).
It may be hard to believe, but some marketing types have been know to tweak the thresholds so they can claim a longer battery life.
As always with me, TMI and too long post. Want a short and sweet? try this:
Li battery life: 400 charges or 4 years, which ever comes 1st.
Disagree with me on this? Talk to the Old Man- trust me, he's got all of eternity to discuss this one. :laughing:
Pete
