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> Power-Supply
Circuits > APP 4322

Dec 10, 2008

Keywords: Battery Emulator, simulated load,
lithium-ion, li-ion, LI+, load testing, charger, fast
charge
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REFERENCE DESIGN 4322

INCLUDES: Tested
circuit Schematic Description Test
data

Simplified Lithium-Ion (Li+) Battery-Charger
Testing

By:

Alfredo
H. Saab, Applications Engineering Manager

Shasta
Thomas, Applications Engineer

Abstract: Because
the charging process for Li+ batteries can take an hour or
longer, testing a Li+ battery charger using its natural load
(i.e., a battery) is time consuming and inconvenient. This
application note presents a simple circuit for simulating the
behavior of a Li+ battery, thus providing a more convenient
method for testing Li+ battery chargers than using real
batteries.

A
similar version of this article was featured in Maxim's
Engineering Journal, vol. 64 (PDF, 1.99MB).

Introduction
Lithium-ion
(Li+) batteries are more delicate than other
battery chemistries and have little tolerance for abuse.
Consequently, Li+ battery chargers are complex
circuits, requiring highly accurate current and voltage
settings. If these accuracy requirements are not met, the
charger may fail to completely charge the battery, severely
reduce battery life, or otherwise degrade battery
performance.

Given the demands imposed on
Li+ chargers, it is critical that charger designs
be tested thoroughly and stepped through their entire
operating range. However, testing a Li+ charger
with its natural load (i.e., a Li+ battery) can
be time consuming and impractical in laboratory and production
environments. To simplify the process, this article presents a
battery-emulation circuit for accelerated, realistic testing
of Li+ battery chargers without actual
batteries.

CC-CV Charging
The Li+ battery-charging
process requires medium-accuracy constant-current (CC)
charging in a first phase, transitioning to high-accuracy
constant-voltage (CV) charging in a second
phase.

Figure 1 illustrates the V-I
characteristics of a modern CC-CV integrated
circuit (the MAX1737) used for a
Li+ battery charger. This type of IC is at the
heart of all Li+ battery chargers in consumer
products. The CC (between 2.6V and 4.2V battery voltage) and
the CV (4.2V) regions are clearly shown.

Figure
1. This V-I curve from the MAX1737 is typical for
Li+ cell chargers.

The region below
2.6V requires a different charging technique. If charging is
attempted on a battery discharged below 2.6V, the charger
applies a low-value ("conditioning current") charging current
until the battery reaches the 2.6V level. This is a safety
mechanism made necessary by the behavior of Li+
batteries when overdischarged. Forcing a fast-charge current
when VBATT < 2.6V can cause the battery to go
into an irreversible short-circuit condition.

The
transition point from the CC to the CV phase has a critical
tolerance of ±40mV. The reason for the narrow tolerance is
that a lower CV will not allow the battery to acquire its full
charge, and a higher one will reduce its useful
life.

Charge-process termination involves sensing that
the battery has reached its full charge and that the charger
must be disconnected or shut
down. This is accomplished by detecting, while in the CV
phase, the point where the charge current is reduced to a
fraction (usually < 10%) of the so-called fast-charge or
maximum charge current.

Testing Parameters for Li+
Chargers
Li+ battery-charger designs usually
have two basic building blocks: a digital block (control state
machine) and an analog block, composed of a well-regulated
current/voltage power supply with an accurate (better than 1%)
reference. A complete test of a Li+ charger
product (not just the IC) is a more involved and
time-consuming task than just verifying a few current or
voltage values.

Testing should step the charger through
its entire operating range: through the CC phase, up to the
transition from CC to CV, and on to charge termination. Recall
that the most realistic condition for such a test is to use
the natural load for the charger: a Li+ battery.
However, using a Li+ battery to test a
Li+ charger is time consuming because the
charging process can take an hour or more. The test time
varies widely, according to whether you combine a
higher-capacity battery with a slow charger, a lower-capacity
battery with a fast charger, or something in
between.

The charging process cannot, moreover, be
accelerated beyond a limit imposed by the battery's maximum
charge rate (the so-called fast-charge current) without
damaging the battery. For normal batteries used in consumer
products, this current is rarely specified above 1C (the
current needed to fully discharge the battery in one hour).
Therefore, the time required to carry the charger through the
full cycle will be longer than two hours, in most
cases.

If the test needs to be repeated, you must
discharge the battery in full—a process only slightly shorter
than charging. Or, you must have available a supply of
consistently discharged batteries.

An alternative to
load testing with a real battery is to test the charger using
a simulated but realistic load. This simulation should verify
the circuit's DC response and dynamic stability. Battery
simulation, however, is difficult to implement with the
standard loads used in power testing. Unlike most bench loads
for power-supply testing, batteries do not behave as
resistances or constant-current sinks. As noted above, testing
must also step the charger through its entire operating range.
The Li+ charger test circuit outlined below
satisfies all of these requirements.

Choosing a Battery-Modeled Load
Let us digress to
discuss two modeling approaches that should be considered, but
will then be discarded.

One approach to modeling a
battery load is to use a voltage source
capable of current sourcing (discharge) and sinking (charge)
in series with a resistor that represents the battery's
internal resistance. Because Li+ batteries demand
precision limits for voltage termination and charge current,
all Li+ chargers today are, in effect, regulated
power converters.

Moreover, because the stability of a
regulated power converter (the charger) depends on dynamic
properties in the attached load (the battery), you must choose
a load that closely resembles the characteristics of the
model. Otherwise, testing may only verify the V-I limits in
the charger itself.

Using a shunt voltage
regulator with a resistor in series to simulate the
battery's internal resistance may be adequate, if the test is
a one-time task and the simplest of battery models satisfies
the test requirements. This approach also offers the advantage
of being powered by the charger itself.

More rigorous
testing, however, requires a more elaborate model. This model
uses an internal voltage source whose value is a function of
the total electrical charge supplied to the battery during the
charging process.

The voltage between the terminals of
a battery being charged at constant current varies
continuously and with a positive slope. This behavior is
caused by the progressive reduction of depolarizing ions
accumulated around the battery's cathode during discharge and
other chemical processes internal to the battery. As a result,
the charger's operating point depends on the length of time
that it has been connected to the battery, as well as the
battery's past history. A load that simulates this more
complex model is harder to set up using the general-purpose
instruments found in most electronics labs.

When
charging circuits must be tested often, or when circuit
performance must be characterized in detail, a circuit that
closely simulates the battery under charge is a useful bench
accessory. The simulation should sweep continuously through
all DC operating points possible for the charger. The circuit
should also display the results so that operators can search
for problems, glitches, and oscillations. If the simulator
provides outputs for the battery voltage and signal, these
results can be presented directly as a scope shot.

The
test can be accelerated (from hours to tens of seconds) and
repeated as many times as necessary, making it much more
convenient than tests with a real battery. Accelerated tests
are not adequate, however, for determining the thermal effects
of power stress on the charger circuits. Therefore, you may
need to conduct additional tests over a longer period to
accommodate thermal time constants in the charger's power and
regulation circuits.

Building the Battery-Modeled Load
The circuit in
Figure 2 simulates a single-cell Li+
battery. Both the termination voltage and the fast-charge
current sourced during the charger's CC phase are commanded by
settings on the charger. The internal battery voltage is set
at 3V when the simulator is initialized to the fully
discharged condition, but that level can be raised to 4.3V for
testing an overcharge condition. The 3V initialization is
typical for the low-battery shutdown circuits used to
terminate the discharge of Li+ batteries. This
design is intended for use with standard, CC-CV type
Li+ battery chargers that terminate the charge at
4.2V. The design can easily be adjusted to accommodate
nonstandard levels of termination voltage and fully discharged
voltage.

Figure
2. By simulating the behavior of a single Li+
cell under charge, this circuit lets you test Li+
battery chargers without using real batteries.

The
charger under test drives the simulator with charging currents
as high as 3A, subject to a limit set by dissipation in the
power transistor.
The battery-voltage increase simulated by the Figure 2 circuit
is a function of all the charging current integrated by the
circuit from the moment the simulator is set to the fully
discharged state.

With the values shown and a 1A
charging current, the integrating time constant allows the
simulator to reach the charger's 4.2V limit in six to seven
seconds. This simulation of current range, internal
resistance, charge-termination voltage, and fully discharged
voltage is based on the specifications of a typical
Li+ cell—in this case, the Sony® US18650G3. The
simulated battery voltage does not include a simulation of
ambient-temperature effects.

The shunt voltage
regulator is designed around a MAX8515 shunt
regulator and a pair of bipolar power transistors. (This
regulator was selected for the accuracy of its internal
voltage reference.) The high-current TIP35 transistor is
attached to a heatsink
capable of dissipating about 25W.

One half of the MAX4163 dual operational
amplifier integrates the charge current, while the other
half amplifies and level shifts the current-measurement
signal. The operational amplifiers' high PSRR
and rail-to-rail
input and output ranges simplify the circuit design for
both functions. Note that the 0.100Ω current-sense resistor,
in series with the positive side of the battery simulator,
also serves as the battery's internal resistance.

The
simulator can be reset to the fully discharged state by an
external signal when operating in a system with automated
test-data acquisition. Alternately, it can be reset by a
pushbutton when the test setup is manually operated.

A
single-pole, single-throw switch
lets you choose from two modes of operation for the simulator.
In position A, the switch operates as an integrating charge
simulator as described. In position B, it assumes a set output
voltage and sinks current as necessary for spot-testing a
charger at a fixed DC operating point. For that purpose, the
"set" voltage can be manually adjusted between 2.75V and 5.75V
by the 50kΩ variable resistor. These set-voltage values refer
to the internal sinking source. The voltage actually measured
between the simulator terminals (VBATT) equals the
set voltage plus a drop caused by the sink current flowing in
the simulator's internal resistance (the 0.100Ω resistor). All
the power necessary for operating the simulator comes from the
battery charger's output.

Simulator Performance
Figure 3 shows the
typical V-I waveforms obtained while simulating the charging
of a Li+ battery up to 4.2V. Two test runs are
shown: one with an initial fast-charge current of 1A (traces B
and D), and one with a fast-charge current of 2A (traces A and
C). In both cases, the CC phase continues until the
termination voltage reaches 4.2V. After that point, current
decays exponentially while the simulated battery voltage
remains constant. The shorter time to termination for the 2A
run is just what you would expect after doubling the charging
current for a real battery. Notice, however, that doubling the
current does not halve the total charge time; it only halves
the time required to reach CV mode, as is the case with a real
battery.

Figure
3. Taken from the Figure 2 cell-simulator circuit, these
fast-charge waveforms show the behavior of a battery charger
delivering 1A during the CC phase (traces B and D) and then 2A
(traces A and C).

Figure 4 shows the V-I
curves obtained when sinking current at two different set
voltages: 3V and 4.1V. For both curves, the dynamic resistance
(indicated by slope) is simply the internal resistance
simulated by the 0.100Ω resistor.

Figure
4. The slope of these plots, which represent the Figure 2
circuit sinking current at 4.1V (top trace) and 3V (bottom
trace), shows the 0.1Ω internal resistance in both
cases.

Summary
Because the charging process for
Li+ batteries can take an hour or longer, testing
a Li+ battery charger using its natural load is
time consuming and frequently impractical. To speed
battery-charger testing, this article presents a simple
circuit for simulating the behavior of a Li+
battery. This circuit provides an efficient means of testing
Li+ chargers without using real
batteries.

References

Linden, D., and Reddy, T.B., Handbook of
Batteries, 3rd ed. (New York: McGraw-Hill, 2002).

Cromptom, T.R., Battery Reference Book, 3rd ed.
(Boston: Newnes, 2000).

Van Schalkwijk, W.A., and Scrosati, B., Advances in
Lithium
Ion Batteries (New York: Kluwer Academic, 2002).

MAX8515,
MAX4163, and
MAX1737 data
sheets.

A similar article appeared in the May 2008
issue of Power Electronics Technology.

Sony
is a registered trademark and registered service mark of
Kabushiki Kaisha TA Sony Corporation.

Related parts

MAX8515

Wide-Input 0.6V Shunt
Regulators for Isolated DC-DC Converters

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samples

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Download,
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APP 4322: Dec 10, 2008

AN4322, AN 4322, APP4322,
Appnote4322, Appnote 4322

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