DESIGN IDEAS L Battery Conditioner Extends the
Life of Li-Ion Batteries
Introduction
Li-Ion batteries naturally age, with an expected lifetime of about three years, but that life can be cut very short—to under a year—if the batteries are mis-handled. It turns out that the batteries are typically abused in applications where intelligent conditioning would otherwise significantly extend the battery lifetime. The LTC4099 battery charger and power manager contains an I2C controlled battery conditioner that maximizes battery operating life, while also optimizing battery run time and charging speed (see Figure 1). The Underlying Aging Process in Li-Ion Batteries Modern Li-Ion batteries are con-structed of a graphite battery cathode, cobalt, manganese or iron phosphate battery anode and an electrolyte that transports the lithium ions.
黑猎蝽The electrolyte may be a gel, a polymer (Li-Ion/Polymer batteries) or a hybrid of a gel and a polymer.
In practice, no suitable polymer has
been found that transports lithium
ions effectively at room temperature.
Most ‘pouch’ Li-Ion/Polymer batteries
are in fact hybrid batteries contain-
ing a combination of polymer and gel
electrolytes.
The charge process involves lithium
ions moving out of the battery cath-
ode material, through the electrolyte
and into the battery anode material.
Discharging is the reverse process.
Both terminals either release or
absorb lithium ions, depending on
whether the battery is being charged燃烧炉
or discharged.
The lithium ions do not bond with
the terminals, but rather enter the
terminals much like water enters a
sponge; this process is called “in-
tercalation.” So, as is often the case
with charge-based devices such as
electrolytic capacitors, the resulting
charge storage is a function of both
the materials used and the physical
structure of the material. In the case
of the electrolytic capacitor, the foil is
etched to increase its surface area. In
空调节能器the case of the Li-Ion battery the termi- nals must have a sponge-like physical
makeup to accept the lithium ions.
The choice of battery anode material
(cobalt, manganese or iron phosphate)
determines the capacity, safety and
aging properties of the battery. In
particular, cobalt provides superior
capacity and aging characteristics,
but it is relatively unsafe compared to
the other materials. Metallic lithium
is flammable and the cobalt battery
anode tends to form metallic lithium
during the discharge process. If several
safety measures fail or are defeated,
the resulting metallic lithium can fuel
a “vent with flame” event.
触摸按键Consequently, most modern Li-Ion
batteries use a manganese or iron
phosphate-based battery anode. The
price for increased safety is slightly re-
duced capacity and increased aging.
Aging is caused by corrosion, usu-
ally oxidation, of the battery anode
by the electrolyte. This reduces both
the effectiveness of the electrolyte in
lithium-ion transport and the sponge-
like lithium-ion absorption capability
of the battery anode. Battery aging
results an increase of the battery series
resistance (BSR) and reduced capacity,
as the battery anode is progressively
less able to absorb lithium ions.
The aging process begins from the
moment the battery is manufactured
and cannot be stopped. However, bat-
tery handling plays an important role
in how quickly aging progresses.
Conditions that Affect
the Aging Process
The corrosion of the battery anode is
a chemical process and this chemical
process has an activation energy prob-
ability distribution function (PDF). The
by George H. Barbehenn Figure 1. The LTC4099 with I2C controlled battery conditioner
OVERVOL TAGE
PROTECTION
NTC
L DESIGN IDEAS
Figure 3. Battery discharge current vs voltage for the LTC4099 battery conditioning function
BATTERY VOL TAGE (V)
B A T T E R Y
C U R R E N T (m A )
60
90120
300150
Figure 2. Yearly capacity loss vs temperature and SoC for Li-Ion batteries
TEMPERATURE (°C)
C A P A C I T Y L O S S (%)
20
30
40
100
gprs水表
50
activation energy can come from heat or the terminal voltage. The more acti-vation energy available from these two sources the greater the chemical reac-tion rate and the faster the aging.Li-Ion batteries that are used in the automotive environment must last 10 to 15 years. So, suppliers of automotive Li-Ion batteries do not rec-ommend charging the batteries above 3.8V. This does not allow the use of the full capacity of the battery, but is low enough on the activation energy PDF to keep corrosion to a minimum. The iron phosphate battery anode has a shallower discharge curve, thus retaining more capacity at 3.8V.
Battery manufacturers typically store batteries at 15°C (59°F) and a 40% state of charge (SoC), to minimize aging. Ideally, storage would take place at 4% or 5% SoC, but it must never reach 0%, or the battery may be damaged. Typically, a battery pack protection IC prevents a battery from reaching 0% SoC. But pack protection cannot prevent self-discharge and the pack protection IC itself consumes some current. Although Li-Ion batter-ies have less self-discharge than most other secondary batteries, the storage time is somewhat open-ended. So, 40% SoC represents a compromise between minimizing aging and preventing dam-age while in storage (see Figure 2).In portable applications, the reduc-tion in capacity from such a reduced SoC strategy is viewed negativ
ely in marketing specifications. But it is sufficient to detect the combination
of high ambient heat and high bat-tery SoC to implement an algorithm that minimizes aging while ensuring maximum capacity availability to the user.
Battery Conditioner Avoids Conditions that Accelerate Aging The LTC4099 has a built-in battery conditioner that can be enabled or
disabled (default) via the I 2C interface. If the battery conditioner is enabled and the LTC4099 detects that the battery temperature is higher than ~60°C, it gently discharges the battery to minimize the effects of aging. The LTC4099 NTC temperature measure-ment is always on and available to monitor the battery temperature. This circuit is a micropower circuit, draw-ing only 50nA while still providing full functionality.
The amount of current used to dis-charge the battery follows the curve shown in Figure 3, reaching zero when the battery terminal voltage is ~3.85V. If the temperature of the battery pack drops below ~40°C and a source of
energy is available, the LTC4099 once again charges the battery. Thus, the battery is protected from the worst-case battery aging conditions.Conclusion
Although the aging of Li-Ion batteries cannot be stopped, the LTC4099’s battery conditioner ensures maximum battery life by preventing the battery-killing conditions of simultaneous high voltage and high temperature. Further, the micropower, always on NTC moni-toring circuit ensures that the battery is protected from life-threatening conditions at all times. L lines used by the LTC2175, and allows it to be packaged in a space saving 7mm × 8mm QFN package.
The dual version of the LTC2262 is the LTC2268. It dissipates 299mW of total power, or 150mW per ADC. It also has LVDS serial output lines that re-duce space, and allow the LTC2268 to be in a 6mm × 6mm QFN package.The dual and quad versions of LTC2262 are available in 12- and 14-bit versions, in speed grades from 25Msps up to 125Msps. A complete list of the variant is shown in Table 1.
Each device shares the excellent AC performance of the LTC2262, and features better than 90dB of chan-nel-to-channel isolation. The serial outputs of the multiple channel parts mitigate the effect of digital feedback, producing a clean output spectrum. In sum, the performance of LTC2262 is not sacrificed when migrating into multiple channel parts.
Conclusion The LTC2262 ultralow-power ADC simplifies design with a unique combi-nation of featur吸波
es. Digital noise can be reduced by using DDR LVDS signaling, alternate bit polarity mode, or the data randomizer. The number of data lines needed to transmit 14 bits of data can be reduced to seven with DDR CMOS signaling, which simplifies layout. The LTC2262 is part of a pin-compatible family of 12-bit and 14-bit ADCs with sample rates from 25Msps to 150Msps, with power consumption ranging from
35mW at 25Msps up to 149mW at 150Msps while maintaining excellent AC performance characteristics. L
LTC2262, continued from page 25
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