Today, portable applications have many unique battery requirements. They require a high-energy density to provide unlimited power (both transient and continuous). They need to be lightweight with small footprints. Batteries need to be safe for use and potential abuse and have an indefinite shelf life. And last but not least, they need to cost next to nothing. As Li-Ion or Li-Polymer batteries meet most of these requirements, they have become the batteries of choice for today's portable applications.
General Characteristics of the Rechargeable Li-Ion Batteries
Rechargeable Li-Ion batteries offer several advantages over other available batteries making them more suitable as an energy source for portable applications. They provide higher energy densities at up to 200 Wh/kg, 300-400 Wh/L. and higher cell voltages (4.1V for a cell with coke anode and 4.2V for graphite anode). Available in prismatic form factors, Li-Ion batteries also have a longer charge retention or shelf life and higher charge cycles.
The Li-Ion battery chemistry's higher energy density coupled with higher cell voltage enables manufacturing of smaller and lighter batteries ideal for applications where lighter and smaller energy sources are critical. However, for efficient usage of the cell's capacity and prolonged battery life- extremely tight controls of charging parameters are required.
The key to a battery's longevity is the selection of the charging parameters such as current, voltage and temperature. The accuracy of the applied voltage during the charge plays a significant role in the efficiency and the longevity of the cell. Exceeding the termination voltage leads to over-charging which, in a short term, increases the available energy from the cell but, in a long run, causes the cell to fail and can lead to safety concerns.
The effects of over-charging, which are cumulative, are illustrated in Figure 1.
Initial capacity increases by about 5% for each 1% increase in charge termination voltage. This apparent short-tm gain has serious consequences on the charge/discharge cycle of the battery. The reduction in charge cycle as a result of over-charging is illustrated in Figure 2.
Figure 1. Initial increase in capacity due to over-charging.
Figure 2. charge cycle reduction as a function of over-charging.
Under-charging on the other hand, even though it does not create safety concerns, significantly reduces the cell's capacity. The effect of under-charging on the cell's capacity is illustrated in Figure 3.
Figure 3. Effect of the termination voltage accuracy on pack capacity.
In general, charging Li-Ion cells is conceptually very simple. To understand the subtleties of charging Li-Ion cells, the equivalent circuit diagram for a generic Li-Ion cell is illustrated in Figure 4. The equivalent circuit behavior of the cell is generally considered to be a capacitor with a very large capacitance C with internal leakage RLeakage.
Figure 4 Li-Ion cell equivalent circuit.
The resistance and inductance between the leads and the cell itself is presented as Effective Serial Resistance (ESR) and Effective Serial Inductance (ESL). These parameters are a function of the cells mechanical construction and of the specific chemical content of the cell. The ESR associated with the battery is between 50 to 200 m-ohms and the ESL is in nano Henries (nH). The ESR creates special challenges for accurate cell voltage detection during the charge cycle as we will see later in this article.
Various charging methods are possible for Li-Ion batteries. The simplest Li-Ion battery chargers are generally referred to as Constant Voltage (CV) chargers (See Figure 5). These consist of a current-limited constant voltage source connected across the battery terminals. The current is limited to less than the battery capacity and its output voltage is regulated to the battery termination voltage of 4.1 V for cells with coke anode and a 4.2 V for graphite anode.
A depleted cell will draw as much current as there is available from the source to charge up. As the battery is charged, the voltage across it will increase and the charge current will taper off. The battery is considered fully charged when the charge current drops to below 0.1C. At charge completion, the charger must be totally off or removed since trickle charging Li-Ion batteries are not recommended. To prevent a defective battery from being indefinitely subjected to charge current, a back-up timer is used to terminate the charge cycle.
While CV charging is a relatively cost-efficient method, this method does require long battery charge times. Since the source voltage is kept constant, as the battery is charged, the charge current, thus the rate of charge, is rapidly reduced. The battery is then charged at much lower current rates than it is capable of.
Figure 5. Constant Voltage (CV) Li-Ion battery charger.
A faster approach is Constant Current/Constant Voltage charging (CV/CC) is seen in Figure 6. In a CC/CV charger-charging starts by applying a constant current equivalent to the battery capacity C. To prevent over charging during CC cycle, the voltage across the pack terminal is monitored. When the voltage reaches the specific termination voltage, the circuit is switched to operate as a constant voltage source. Even though the voltage across the pack terminal reaches the termination voltage, the actual cell voltage will be lower because of the voltage drop across the ESR.
During constant current charging, cells can be charged at high current rates approaching their termination voltage without any danger of the cell being exposed to higher voltages and over charging.
During the CC charging, the cell is charged to about 85% of its capacity. After completion of the CC cycle, the charger is switched over to the constant voltage cycle. During the CV cycle, the charge current is monitored for end of charge determination. As with CV chargers, the charge cycle is complete when the charge current tapers off to lower than 0.1C of the battery. A complete CC/CV charge profile is shown in Figure 7.
Figure 6. Constant Voltage/Constant Current (CVCC) Li-Ion battery charger.
Even though CC/CV charging requires significantly more complicated circuits to implement, using variations of the CC/CV charging method is dominant in charging Li-Ion batteries because they significantly reduce charge times.
So far we've assumed the battery that we're working with is a good battery. Which is not always the case. Batteries being charged may be defective and may not accept charge. Moreover, attempts to fast charge a defective battery may create unsafe conditions. The ideal charger must be able to detect all possible battery fault modes and charge accordingly. Another factor that we have deliberately ignored for simplicity's sake is battery temperature. It is not safe to charge a Li-Ion battery if its temperature is outside of a specified temperature range. So far, all the charger has had to do is keep track of the voltage regulation-or in case of the CVCC charger, keep track of the current and voltage. But as indicated above, increased charger efficiency and battery longevity with underlying safety concerns have given rise to the need for more intelligent charging operations
Figure 7. CC/CV charge cycle.
To prevent accidental exposure of a battery to adverse conditions, all Li-Ion battery packs contain somewhat sophisticated circuitry. Generally, protection involves protection of the cell from over discharge, over charge, excessive charge and discharge currents and from exposure to high voltages.
During the battery's charge or discharge cycle, if any of the parameters exceeds the limits set for a particular cell, the connection between the cell and battery terminal will open. Typically a device is reset after a delay, when adverse conditions are removed or the battery is pre-conditioned.
In addition to electronic protection, cells contain a mechanical secondary over-current protection device. A Polymeric Positive Temperature Coefficient (PPTC) over-current protection device is placed in serial between the pack and the cell terminal.
The PPTC device protects the circuit by going from a low resistance state to a high resistance state in response to over current. This change is the result of a rapid increase in the temperature of the device, caused by the generation of heat within the device by I2R heating.
A good charger design must be able to determine the suitability of the Li-Ion battery for fast charging safely and efficiently. The followings are some examples of chargers available to support portable applications.
End of Part One. Click here for Part 2.