Introduction
Almost all mobile devices such cell
phones have battery and battery charger built-in. During product development,
there is a need for battery simulator to properly testing these mobile
devices, especially the charging circuitry. Recent advances in rechargeable battery
technologies such as those lithium-ion and lithium iron phosphate high energy
density batteries are being used in many mobile devices such as cell phones and
tablets. These battery-powered portable devices required thorough bench testing
during the development phase. As will be discussed later, battery simulator is
an important piece of test equipment for lab bench testing.
What is a Battery Simulator?
A battery simulator is an
electronics device that emulates many real battery characteristics. These
characteristics are voltage, power, current, ESR, and more. It provides the
necessary voltage, power, and current to the mobile system like a real battery
does. The mobile device will not know the difference between a real battery and
a simulator. When charged, a battery simulator will sink current; when not
charge, it will sources current to the mobile system load. Battery simulator
can seamlessly transition between source and sink current without glitch even
at high transitional speed. It is a very convenient bench testing tool for test
battery-powered devices and system.
When a real battery is being charged,
its voltage is slowly rise and may take hours to arrive at its final voltage. A
battery simulator on the other hand, its output voltage can easily be
controlled by a knob or a computer program, and can reach the final voltage in
seconds. Battery simulator is ideal for testing battery chargers.
Why Using Battery Simulator
The purpose of a battery simulator is to
replace the battery in a mobile device with an emulated “battery”. Usually a
battery simulator is replacing the battery for testing purpose. The mobile
device operation and performance are generally not impacted by it. It enables
engineers conveniently and thoroughly tests these portable systems both during product
developments and mass production. Below are some examples why battery simulators
are important test equipment.
Charger Testing
Most mobile electronic devices have
built-in charger circuitry that recharges the battery. During product
development and mass production testing, the charger circuit must be thoroughly
tested to ensure its reliability and safely chargers batteries. A typical
battery charging cycle, from dead battery to full, may take up to several hours
complete. If the battery charger is tested with a real battery, it will take
hours. A battery simulator can quickly adjust its output voltage from 0V (dead
battery) to 4.2V (fully charged) in just seconds. It enables test engineers to
quickly test a battery charger. Similarly it may take several days to discharge
a real battery in a mobile device, but a simulator can emulate the battery
being depleted in a matter of seconds. All of the while test engineers can
observe how the system and charger behave and measure the system to make sure
it meets all of the specifications without waiting for hours. Battery simulator
can simulate an overcharged battery or a wrong battery type. If a malfunction
is found in the system, a battery simulator is an indispensable piece of test equipment
for debugging, because engineer can easily simulate the battery voltage from
high to low and back-and-forth to find where the malfunction occurred. Battery
simulator is very important for charger circuit testing.
Battery ESR and Temperature Simulation
Advance battery simulator can even
emulate dynamic battery ESR (electric static resistance). Battery ESR is generally dynamic and frequency dependent. In addition, ESR changes with battery size and capacity, state of charge, battery type, temperature, age, among others. An
advance emulator or simulator can easily change these characteristic to simulate various
type of battery cells and capacity, in addition to simulating battery temperature. An advance simulator can artificially changes the age of that battery, battery
temperature, battery size, just to name a few.
Testing Cell Balancing
Another important application of simulator is to emulate a series-connected battery cells inside a battery pack. Many
medium-power mobile devices employ multiple cells connected in series within a pack. Often time these batteries are lithium-based such as lithium-iron-phosphate (LiFePO4). Common configurations are 2, 3, 4, and 6 cells in
series. Series connecting these batteries increases the system input voltage and
deliver power more efficiently. Due to battery voltage variations from one cell to the
next, not all cells in a pack are at the same voltage. In rear cases,
one or two of the batteries are damaged or defected. When charged a series
connected battery cells, some cells will be over-charged and others
are under charged. Without cell balancing, system design engineers often
intentionally undercharge the battery cells to avoid potential overcharge a
battery cell. Therefore cell balancing is often needed to obtain the
most optimal battery energy as well as keeping the battery pack safe.
Understand Battery Cell Balancing
Battery cell balancing is a process that
ensures each cell in a pack is charged to their optimal capacity. It also
ensures batteries are neither overcharge nor undercharge. This is usually
achieved by using a balancing circuit is to divert charges away from
fully-charged batteries to the uncharged batteries. As shown in Figure 1,
when one of the cells is closer to fully charged, the cell balancing
circuitry diverts some of the charging current away from that cell while maintaining high
charging current to the two undercharged cells. As the battery is approaching
fully-charged, charging current is continued to reduce. The process continues
until all three batteries are full-charged.
Figure 1. Simplified battery cell balancing concept showing the top battery is nearly full and reducing charging current.
Stress Test Cell Balancing Circuitry
The balancing circuitry as well as the
charger itself must be properly tested during product development. To
efficiently test the cell-balancing circuit, one or more battery simulators are
needed. Design engineers can simulate one or more battery is being out of balance
to stress test how the battery cell balancing circuits reacts. As shown in
figure 2, each simulator/emulator can adjust its voltage independently. By doing so, it
is quite easy to adjust each emulated battery voltage to mimic a battery is
either full, undercharged, overcharged, dead-battery, or damaged. Test
engineers can easily simulate any combinations of the above mentioned battery cell conditions (undercharged, overcharged, fully charged, or damaged) to stress
test both the cell balancing and the charger circuitry. In conclusion battery simulators
are ideal for testing battery charger and battery cell balance circuitry.
Figure 2. Three simulators are used to
test the charger and cell balancing circuitries.
How does a Battery Simulator Work?
In a nutshell, a battery simulator is a special power supply that can sink and
source current. A real battery can do the same. This type of power supply is
sometimes called two-quadrant or four-quadrant (if voltage can be negative) power
supply. In the contrary, a typical lab power supply can only source current,
but cannot sink current. Therefore a bench power supply cannot be use as a
simulator. The battery simulator circuit and its ability to sink and source
current is depicted in Figure 3B. A conventional source only power supply is
depicted with a single transistor for sourcing current. A simulator has two
power transistors: one for sourcing and one for sinking current. It can quickly
transitions between sink and source current without any glitch.
How to Use a Battery Simulator
The purpose of a battery simulator is to
replace a real battery for fast and efficiently test battery chargers and mobile
systems. The TS250 and the TS200 are designed to simulate a battery. They
can sink and source current as a real battery does. They can also be used to
simulate battery noise such as ripples and voltage transient for system
testing.
As shown in Figure 4, the TS200/TS50
battery simulator output is connected to a mobile system charger’s battery
terminals. They are taken place of a battery. Use the DC OFFSET knob to adjust
the emulated battery voltage, at the same time monitor the charging current.
Adjust the battery simulator output voltage from low to high and back to
observe the charger behavior.
Figure 4. TS250 Battery simulator and
battery noise generator is used to test chargers
Conclusion
Battery simulator is a very important
equipment for testing mobile devices with a battery charger circuitry. Common
batteries used for today’ portable systems are lithium iron phosphate (LiFePO),
lithium ion (Li+), lithium polymer, nickel metal hydride (NiMH), nickle cadmium
(NiCd), and lead-acid. Most battery simulators can emulate these batteries for
testing battery charger and system. Battery simulator is ideal for both bench
testing during product development and fast testing during mass production.
