BATTERY TECHNOLOGY
Simplifying multi-chemistry battery chargers
Archana Yarlagadda discusses a flexible battery charging system that can be applied to a range of voltages, battery chemistries, and battery charge profiles
Portable electronic devices, whether personal electronics, remote scientific instrumentation, or simple garage flashlights, all have one thing in common: batteries. These can be NiCd, NiMH, Li-Ion, or any other rechargeable battery chemistries.
When charging multi-chemistry batteries with
different cell capacities, the battery voltage can be higher or lower than the supply voltage at various stages of the charging. As a result the supply voltage needs to be either boosted or attenuated to match the battery voltage. For example, a supply voltage of 3.3 V needs to be attenuated
A battery charger has to determine the state of the battery (i.e. voltage, current and temperature) and control the charging current. The hardware to determine the state of the battery is common to the batteries. The battery voltage can be higher or lower than the input range of the microcontroller; thus, the voltage is usually measured by using a resistor divider circuit to attenuate the voltage. The current can be measured on the high-side (the current going into the battery), low-side (the current coming out of the battery) or, in case of the SEPIC convertor, by using a resistor in the secondary side of the inductor. The batteries usually have an embedded thermistor that provides an
Figure 1: Battery charging profile for Li-ion and NiMH batteries
down when a single cell NiMH battery (typically 1.25 V) is being charged. When a single cell Li-ion battery (4.1 V) is used, the input voltage needs to be amplified. To address such cases, the primary charge path is chosen to be Single Ended Primary Inductor Converter (SEPIC). This topology of switch-mode DC-DC conversion has the ability to both buck and boost a wide range of voltages to provide supply voltage flexibility. Two different rechargeable battery chemistries – Nickel-Metal Hydrate (NiMH) and Lithium-ion (Li-Ion) batteries – will be used as examples in this article. These two chemistries require different charge profiles but can both be readily serviced using the same flexible charging topology. The flexibility and simplicity in switching from one type of battery chemistry to another is implementation in software on a microcontroller. By designing the charging subsystem in a modular manner and encapsulating functions into various components, the same application can be implemented using different microcontrollers, depending upon system requirements. The use of components simplifies design, where the input and/or outputs are hardware and/or software. This approach allows developers to add battery charging as an additional feature to another main application, like motor control, accurate medical measurements, etc.
accurate state of the battery temperature. This is sometimes omitted in some commercial batteries to reduce cost. In such cases, an external
thermistor placed in contact with the battery can be used.
Based on these measured parameters, the charge current into the battery is determined and is controlled by the microcontroller. The main difference between different battery chemistries, from the battery charger perspective, is the charge profile. The charge profile of Li-ion and NiMH are given in Figure 1. In the profile shown in Figure 1,
the current is controlled by the microcontroller and the voltage and temperature are changes happening in the battery. The Li- ion battery uses a constant-current constant-voltage charge profile. Consider the nominal capacity of a battery to be denoted by “CA”. At startup, if the battery voltage is lower than the constant current threshold (Vrapid_start), the battery charger supplies a small amount of current (around 0.1 CA). This is the Pre-conditioning stage where the voltage in the battery gradually increases with this small charge current. When the voltage reaches the rapid charge threshold, the charge current is increased by the microcontroller to around 1 CA. This is the constant current stage that is maintained until the battery voltage reaches the
4 CIE Power Supplement April 2012
specified voltage (Vfull). The battery charger then enters the constant-voltage stage, where the charge current is decreased while the battery voltage is maintained at Vfull. When the current is decreased until the termination current, while maintaining the battery voltage, the battery charging is terminated. The current in the battery changes by a few °C during the whole charging process. If any of the battery conditions – voltage, current, or temperature – are outside the specified range for the corresponding battery charger stage, the battery charger shuts down the charging for protection.
The first two charger stages of an NiMH battery are similar to Li-Ion: Activation (with 0.2 CA) and Constant-Current (1 CA). The end of the constant-current stage in NiMH batteries is detected by a drop in the battery voltage (and drop in temperature) while the current is constant. After this drop in voltage, the NiMH charger profile enters a charge-top off stage where the current is reduced to a trickle charge level (around 0.05 CA). In this stage, a small amount of charge current is provided for a constant amount of time before charge termination.
Based on the charging requirements mentioned above, battery charging can be simplified to different levels using a state machine with predefined voltage, current, temperature, and time-out values. The state of the battery and the amount of current that needs to be provided for battery charging are controlled in the microcontroller state machine. A simplified state machine for charging both types of batteries is shown in Figure 2. This block diagram shows the different stages of charging.
Figure 2: Block diagram of different stages during charging Based on the battery chemistry chosen, the
microcontroller goes through the state machine of that particular battery and controls the charge current. The profile used for charging a battery can be pre-programming, pre-startup, or automatic decision. For the first two methods, the type of the battery is taken as an input from the user. In pre-programming, the type of battery charging required is chosen in the component
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