See if this helps clear up some of the points about Lithium-Ion batteries:
Use and Misuse of the Lithium-Ion Battery
There is a great deal of confusing information being stated about the Lithium-Ion battery used in portable devices. On one level it is stated that the battery should be deep-discharged to shutdown regularly so as to calibrate the battery. In reality, the Lithium-Ion battery cannot be calibrated. Unlike the old NiCad battery, the Lithium-Ion battery has no memory. The only calibration that can be done is of the charge reporting circuitry.
Depth of Discharge
An independent group calling itself the Battery University which is sponsored by an instrumentation company not associated with the battery industry has conducted numerous tests of Lithium-based batteries. Their article How to Prolong Lithium-based Batteries provides results of tests performed over a range of discharge/charge behavior, temperature effects, and so on. They found strong evidence that deep discharging to shutdown followed by charging results in a significant reduction in the battery’s cycle life. Discharge to 50% remaining charge and then recharging gives a cycle life of about four times that of repeated deep discharging and recharging.
Charging Behavior
Well designed portable devices have charging circuitry that prevents overcharging of a battery. When the battery charge reaches 100% the circuitry ceases to charge the battery and switches to powering the portable device. When the battery charge trickles down to a predetermined value, typically 93-95% charge, the circuitry switches to charging mode and tops up the battery. This will continue as long as the device remains on an AC power source.
Leaving a Lithium-based battery on an AC power source permanently is not good for the battery. The Battery University studies found that maintaining a high cell voltage for the life of the battery is detrimental. However, that can be remedied by periodically taking the device off the AC power source and discharging the battery partially and then reconnecting the power source. Since this is about a portable device it should be used on the battery periodically, or why would the user have bought a portable instead of a fixed desktop device.
Battery Cycle Life
Battery life, that is cycle life, is also misunderstood. The projected cycle life of a Lithium-Ion battery is quoted as some where in the range of 350-1,000 charge cycles. The cycle life is not the number of cycles at which the battery will fail, but, rather, the point at which the battery charge capacity drops to 80% of the as-built capacity. Also keep in mind that the cycle life prediction is statistical and follows the Normal, or Gaussian, bell shaped curve. There will be a number of batteries that last much longer than the predicted cycle life, and a number that last much shorter than the predicted cycle life. But the vast majority should reach the predicted cycle life if properly handled during their lifetime.
The charge cycle, by the way, is counted as a full discharge/charge cycle. Discharge to 50% and then charge counts as one-half of a cycle. Discharge to 25% and charge counts as three-quarters of a cycle. And so on. So the behavior of discharging to 50% and then recharging as a regular practice will mean that it takes 2,000 such cycles to equal the predicted 1,000 full charge cycles as the life of the battery.
Checking Battery Status
The status of the battery is very easy to check without the need to resort to third-party applications. Simply click the apple at the left side of the menu bar, About This Mac, More Info, System Report, Hardware, and Power. The most interesting information is: the Charge Remaining, the Full Charge Capacity, the Cycle Count, the Condition, the Amperage, and the Voltage.
Dividing the Charge Remaining by the Full Charge Capacity will be equal to, after rounding off, the reported battery charge at the upper right of the menu bar.
The Cycle Count, as noted above, is the number of full charge cycles the battery has had during its lifetime.
One of the most important pieces of information is the Battery Condition: anything other than Normal is cause for concern. The statement Service Battery can sometimes be cleared by an SMC reset of a Mac portable computer, but should be an indicator that the battery needs to be watched. A message such as Replace Battery is clear indication of a failure of the battery.
Note that the Voltage and Amperage are given in milli-volts and milli-amps, so the value is a factor of one-thousand larger than usually thought of. A negative Amperage indicates the computer is being powered by the battery and not the AC power source. A normal voltage for a Mac portable computer will be in the neighborhood of 12 volts. The actual value will vary for different models of Mac portables, ranging from 11 volts, or in some cases a little lower, to 12.5 to 13 volts. The main point is to see that the voltage is not excessively low, indicating the failure of a battery cell.
