Written by Ismael Jones
Category: Technical Info
Published: 23 November 2006
prolong lithium-based batteries (BU34)
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 300-500 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, batteries with fuel gauges exhibit what engineers
refer to as "digital memory". Here is the reason: 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.)
Aging of lithium-ion is an issue that is often ignored. A
lithium-ion battery in use typically lasts between 2-3 years. The
capacity loss manifests itself in increased internal resistance
caused by oxidation. Eventually, the cell resistance reaches a
point where the pack can no longer deliver the stored energy
although the battery may still have ample charge. For this reason,
an aged battery can be kept longer in applications that draw low
current as opposed to a function that demands heavy loads.
Increasing internal resistance with cycle life and age is typical
for cobalt-based lithium-ion, a system that is used for cell
phones, cameras and laptops because of high energy density. 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. Spinel is primarily
used for power tools.
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
High charge levels and elevated temperatures hasten permanent
capacity loss. Improvements in chemistry have increased the storage
performance of lithium-ion batteries.
The mentioning of limited service life on lithium-ion has caused
concern in the battery industry and I will need to add some
clarifications. Let me explain:
If someone asks how long we humans live, we would soon find out
that the longevity varies according to life style and living
conditions that exist in different countries. Similar conditions
exist with the batteries, lithium-ion in particular. Since
BatteryUniversity bases its information on the feedback from users
as opposed to scientific information derived from a research lab,
longevity results may differ from manufacturer' specifications.
Let's briefly look at the various living conditions of the
The worst condition is keeping a fully charged battery at elevated
temperatures, which is the case with running laptop batteries. If
used on main power, the battery inside a laptop will only last for
12-18 months. I must hasten to explain that the pack does not die
suddenly but begins with reduced run-times.
The voltage level to which the cells are charged also plays an
important role to longevity. For safety reasons, most lithium-ion
cannot exceed 4.20 volts per cell. While a higher voltage boosts
capacity, the disadvantage is lower cycle life. Figure 2 shows the
cycle life as a function of charge voltage.
||Figure 2: Effects
on cycle life at different float charge levels(Choi et
Higher charge voltages boost capacity but lower cycle
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 momentarily but
the condition reverts back to its former state when the temperature
drops. Cold temperature will increase the internal resistance.
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.
Avoid keeping the battery at full charge and 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. With the concern of the battery
overheating and causing fire, a spokesperson for the U.S. Consumer
Product Safety Commission advises to eject the battery of affected
laptops and to run the machines on a power cord. It should be noted
that on a power outage, unsaved works will be lost.
The question is often asked, should the laptop be disconnected from
the main when not in use? Under normal circumstances, it should not
matter with lithium-ion. Once the battery is fully charged, no
further charge is applied. However, there is always the concern is
malfunction of the AC adapter, the laptop or the battery.
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. Some battery analyzers (Cadex)
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 months, 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
All personal computers (and some other electronic devices) contain
a battery for memory back up. This battery is commonly a small
non-rechargeable lithium cell, which provides a small current when
the device is turned off. The PC uses the battery to retain certain
information when the power is off. These are the BIOS settings,
current date and time, as well as resource assignment for Plug and
Play systems. Storage does shorten the service life of the backup
battery to a few years. Some say 1-2 years. By keeping the computer
connected to the main, albeit turned off, a battery on the PC
motherboards should be good for 5-7 years. A PC should give the
advanced warning when battery gets low. A dead back-up battery will
wipe out the volatile memory and erase certain settings. After
battery is replaced, the PC should again be operational.
Generally speaking, batteries live longer if treated in a gentle
manner. High charge voltages, excessive charge rate and extreme
load conditions will have a negative effect and shorten the battery
life. This also applies to high current rate lithium-ion batteries.
Not only is it better to charge lithium-ion battery at a slower
charge rate, high discharge rates also contribute the extra wear
and tear. Figure 3 shows the cycle life as a function of charge and
discharge rates. Observe the good laboratory performance if the
battery is charged and discharged at 1C. (A 0.5C charge and
discharge would further improve this rating.)
Battery experts agree
that the life of lithium-ion depends on other factors than charge
and discharge rates. Even though incremental improvements can be
achieved with careful use of the battery, our environment and the
services required are not always conducive to achieve optimal
battery life. The longevity of a battery is often a direct result
of the environmental stresses applied.
||Figure 3: Longevity
of lithium-ion as a function of charge and discharge
A moderate charge and discharge puts less stress on the battery,
resulting in a longer cycle life.
- 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 fuel gauge (laptops) 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 cool. Avoid a hot car. For
prolonged storage, keep the battery at a 40% charge level.
- Consider removing the battery from a laptop when running on
fixed power. (Some laptop manufacturers are concerned about dust
and moisture accumulating inside the battery casing.)
- Avoid purchasing spare lithium-ion batteries for later use.
Observe manufacturing dates. Do not buy old stock, even if sold at
- If you have a spare lithium-ion battery, 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%
2003, Last edited: September 2006
About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc.,
in Vancouver BC. Mr. Buchmann has a background in radio
communications and has studied the behavior of rechargeable
batteries in practical, everyday applications for two decades.
Award winning author of many articles and books on batteries, Mr.
Buchmann has delivered technical papers around the world.
Cadex Electronics is a manufacturer of advanced battery chargers,
battery analyzers and PC software. For product information please