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Lithium-ion Battery Packs
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How to extend the life of lithium ion battery packs:
These days, battery research is targeting heavily on lithium ion battery pack chemistries, so much so that one could assume that all portable electronic devices will be powered with lithium-ion batteries in the future. In many examples, lithium-ion is superior to nickel and lead-based chemistries and the applications for lithium-ion batteries are growing as a result.
Lithium-ion battery packs have not yet fully matured and are being improved continuously. New metal and chemical combinations are being tried every few months to increase energy density and prolong service life. The improvements in longevity after each change will not be known for a few years.
On average, the lithium-ion battery pack provides anywhere between 500-1000 discharge/charge cycles. The battery prefers a partial rather than a full discharge. Frequent full discharges should be avoided when possible. Instead, charge the battery more often or use a larger battery. There is no issues of memory when charging whenever it is convenient.
Although lithium-ion battery packs have no memory effect with regards to performance deterioration, batteries with fuel gauges occassionally exhibit what engineers refer to as "digital memory". The reason is because short discharges with subsequent recharges do not provide the periodic calibration needed to synchronize the fuel gauge with the battery's state-of-charge. A deliberate full discharge and recharge every 30 charges corrects this problem. Letting the battery run down to the cut-off point in the equipment will do this. If ignored, the fuel gauge will become increasingly less accurate over time.
The aging of lithium-ion battery packs is an issue that is often ignored. Depending on the state-of-charge and temperature during storage, lithium-based batteries have a typical life span of 2-3 years (lasting longer if they are partially charged and kept cool). The clock of the battery starts ticking as soon as it comes off the manufacturing line. The capacity loss manifests itself in increased internal resistance caused by oxidation. Eventually, the lithium-ion cell resistance will reach a point where the pack can no longer deliver the stored energy, although the battery may still contain ample charge. Increasing internal resistance is common to cobalt-based lithium-ion which is a chemistry that is found in laptops and cell phones. The lower energy dense manganese-based lithium-ion, also known as spinel, maintains the internal resistance through its life but loses capacity due to chemical decompositions.
The speed by which lithium-ion ages is governed by temperature and state-of-charge. Figure 1 illustrates the capacity loss as a function of these two parameters.
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Figure 1: Permanent capacity loss of lithium-ion as a function of temperature and charge level.
High charge levels and elevated temperatures hasten permanent capacity loss. Improvements in chemistry have increased the storage performance of lithium-ion batteries.
There are no remedies to restore lithium-ion once worn out. A momentary improvement in performance is noticeable when heating up the battery. This lowers the internal resistance but the condition reverts back to its former state when the temperature drops.
If possible, store the battery in a cool place at about a 40% state-of-charge. Some reserve charge is needed to keep the battery and its protection circuit operational during prolonged storage. The most harmful combination is full charge at high temperature. This is the case when placing a cell phone or spare battery in a hot car. Running a laptop computer on the mains has a similar temperature problem. While the battery is kept fully charged, the inside temperature during operation rises to 45°C (113°F).
Removing the battery from the laptop when running on fixed power protects the battery from heat but some battery and laptop manufacturers caution against it. They say that dust and moisture accumulating inside the battery casing could damage the laptop. The dealers will be happy to provide you with a new pack when a replacement is needed a little sooner.
The question is often asked, should the laptop be disconnected from the main when not in use? With lithium-ion it does not matter. Once the battery is fully charged, no further charge is applied. It is recommended, however, to turn the laptop off overnight because heat harms the battery.
Some lithium-ion battery packs fail due to excessive low discharge. If discharged below 2.5 volts per cell, the internal safety circuit opens and the battery appears dead. A charge with the original charger is no longer possible. Some battery analyzers feature a boost function that reactivates the protection circuit of a failed battery and enables a recharge. However, if the cell voltage has fallen below 1.5V/cell and has remained in that state for a few days, a recharge should be avoided because of safety concerns. To prevent failure, never store the battery fully discharged. Apply some charge before storage, and then charge fully before use.
Simple Recommendations
- Avoid frequent full discharges because this puts additional strain on the battery. Several partial discharges with frequent recharges are better for lithium-ion than one deep one. Recharging a partially charged lithium-ion does not cause harm because there is no memory. (In this respect, lithium-ion differs from nickel-based batteries.) Short battery life in a laptop is mainly cause by heat rather than charge / discharge patterns.
- Batteries with a fuel gauge should be calibrated by applying a deliberate full discharge once every 30 charges. Running the pack down in the equipment does this. If ignored, the fuel gauge will become increasingly less accurate and in some cases cut off the device prematurely.
- Keep the lithium-ion battery pack cool. Avoid a hot car. For prolonged storage, keep the battery at a 40% charge level.
- Consider removing the internal laptop battery from a laptop when running on fixed power or off of an external laptop battery. (Some laptop manufacturers are concerned about dust and moisture accumulating inside the battery casing.)
- If you have a spare lithium-ion battery pack, use one to the fullest and keep the other cool by placing it in the refrigerator. Do not freeze the battery. For best results, store the battery at 40% state-of-charge.
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