Tuesday, November 24, 2015

Battery Simulator Primer

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.
When one of the cell is nearly full, charging current is diverted by the cell balancing circuit.

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.

Simulator power supply can sink and source, but conventional power can only source current.

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.

Using TS250 to test battery a charger.

Figure 4. TS250 Battery simulator and battery noise generator is used to test chargers


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.

1 comment:

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