How to 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.
1: Permanent capacity loss of lithium-ion as a function of temperature and charge
| || |
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 lithium-ion battery.
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.
2: Effects on cycle life at different float charge levels
(Choi et al., 2002)
Higher charge voltages boost capacity but lower cycle life.
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 before use.
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.
of high-power lithium-ion
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.)
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.
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.
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 clearance prices.
- 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% state-of-charge.
Created: February 2003, Last edited: September
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.
is a manufacturer of advanced battery chargers, battery analyzers and PC software.
For product information please visit www.cadex.com.
for a printer-friendly version
Copyright 2003 - 2005 Isidor Buchmann