How to prolong lithium-based batteries

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 lithium-ion 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.



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.
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.
2
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 full discharges; recharge lithium-ion more often. Repetitive random charge does
not harm the battery. There is no memory.
• Keep the lithium-ion battery cool but do not freeze. Avoid a hot car. For prolonged
storage, keep the battery at a 40% charge level.
• If your laptop runs without a battery and fixed power is used most of the time, remove
and store the battery in a cool place.
• Avoid purchasing spare lithium-ion batteries for later use. Observe manufacturing date.
Do not buy old stock, even if sold at clearance prices.