Miscellaneous/Ask Isidor/Lithium Life 01




How Can the Life of lithium-based batteries be prolonged?
Battery research is focusing heavily on lithium chemistries, so much so that one could presume that all portable devices will be powered with Lithium-ion batteries in the future. In many ways, Lithium-ion is superior to nickel and lead-based chemistries and the applications for Lithium-ion batteries are growing as a result.

Lithium-ion has not yet fully matured and is being improved continuously. New metal and chemical combinations are being tried every six months to increase energy density and prolong service life. The improvements in longevity after each change will not be known for a few years.  

A Lithiumion battery provides 300 to 500 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 concern of memory when applying unscheduled charges.

Although Lithium-ion is memory-free in terms of performance deterioration, engineers often refer to “digital memory” on batteries with fuel gauges. 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. (Read more in ‘Choosing the right battery for portable computing’, Part Two.)

Temperature                                              40% charge level                                                       100% charge level     
                                                    (recommended storage charge level)                                         (typical user charge level)     
0°C                                                             98% after 1 year                                                    94% after 1 year     
25°C                                                           96% after 1 year                                                    80% after 1 year     
40°C                                                          85% after 1 year                                                     65% after 1 year     
      60°C                                                           5% after 1 year                                                   60% after 3 months     

Figure 1: Permanent capacity loss of Lithiumion 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 Lithiumion batteries. +

Aging of Lithium-ion is an issue that is often ignored. Lithium-based batteries have a lifetime of two to three years. The clock starts ticking as soon as the battery comes off the manufacturing line. The capacity loss manifests itself in increased internal resistance caused by oxidation. Eventually, the cell resistance will reach a point where the pack can no longer deliver the stored energy, although the battery may still contain ample charge.

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.

There are no remedies to restore Lithium-ion once worn out. A momentarily improvement in performance is noticeable when heating up the battery but the high internal resistance will revert to its former state with normal temperature.

If possible, store the battery in a cool place at about a 40% state-of-charge. This 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).

In spite of the high operating temperature and the harm inflicted to the battery during the use in a laptop, removing the battery when running on fixed power poses some risk to the laptop and manufacturers caution against it. There are issues of dust and moisture accumulating inside the battery casing that could cause damage to the unit. By not removing the battery, a replacement may be needed a little sooner but the battery manufacturers and dealers are happy to provide a new pack.

A large number of Lithium-ion batteries for cell phones are being discarded under the warranty return policy. Some failed batteries are sent to service centers or the manufacturer, where they are refurbished. Studies show that 80-90% of the returned batteries can be repaired and returned to service.
Some Lithium-ion batteries 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. The Cadex 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 Guidelines
·    Avoid frequent full discharges; recharge lithium-ion more often. Repetitive random charge does not harm the battery. There is no memory.
·    Although memory-free, apply a deliberate full discharge once every 30 charges to calibrate batteries with fuel gauge. Running down the battery                in the equipment does this. If ignored, the fuel gauge will become increasingly less accurate.
·    Keep the Lithiumion battery cool but do not freeze. Avoid a hot car. For prolonged storage, keep the battery at a 40% charge level.
·    Do not remove the battery from a laptop even though fixed power is used. Operating without a battery can inflict harm to the laptop  

Avoid purchasing spare Lithiumion batteries for later use. Observe manufacturing date. Do not buy old stock, even if sold at clearance prices.


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