by David Gunderson, Micro Power Electronics, Inc.
The battery charger control designer is faced with a fundamental choice; to use a part out of the rich selection of dedicated charge control ICs available from many vendors, or to use a programmable microcontroller. Because battery charge control is a slow process, inexpensive microcontrollers with embedded A/D, signal conditioning, and pulse width modulation (PWM) modules can be used to directly control the charger’s power conversion circuits. The microcontroller can also be used for charger to Battery Management System (BMS) communication and interaction (e.g. smart charger), flexible user interfaces (e.g. charge status displays), battery conditioning control, and other flexible features. However, microcontroller circuits and firmware are normally more expensive to design & test and often cost more to produce than chargers based on dedicated charge control ICs. This article will present the tradeoffs to consider when making a choice between microcontrollers and dedicated charge control ICs for battery charge control.
A. Charging the Battery – a requirements summary
Most recent battery charger design activity is for Lithium chemistry batteries. Li-Ion, Li-Polymer, Li-Iron-Phosphate (LiFePO4) and related cell types have the best volumetric and weight energy density of any of the commercially available rechargeable cells. This makes them highly desirable when used in portable power applications, including Electrical Vehicles (EV), portable computing and communication devices (smart phones, PDA, Tablet and laptop computers), military computer-assistant warrior systems, and medical parameter monitors. Nickel chemistry batteries are still used, but are rapidly being replaced by Li chemistry types.
Charging Li chemistry cells requires that the charger control both charge current and battery voltage. During the initial part of charge, a constant current (CC mode) is sourced into the cells until the battery voltage rises to a specific value (called the float voltage). Once the float voltage is reached, the charger’s output voltage is maintained at the float value (CV mode) until the charge current decreases to a fixed, low value. Once the low current is reached, the charger is turned off Figure 1. Unlike Ni and Lead chemistry batteries, Li chemistry batteries are usually not trickle charged after charge termination. Maintaining a low current after charge termination can actually damage some Li cell types.
Figure 1. Li-Ion Single-cell CC/CV Charge Profile
Note that a close estimate of charge time of a Li chemistry battery when the standard CC/CV algorithm is used can be derived with the following equation:
Charge time (hrs) = 1.3 * (battery capacity in Ah)/(CC mode charge current)
With proper design and intelligent tuning of the CC/CV mode algorithm, one can do a bit better than this, but it’s a good starting point. You can also do much worse if the CC to CV mode transition is made too early due to poor design or inaccurate battery voltage measurement.
So, the minimum requirements for a Li chemistry battery charger are means to control both the current into the battery and the voltage at the battery charge terminals. In addition, for safety purposes, most Li chemistry battery chargers can disable charge if the battery temperature is too high or too low. Lastly, in many cases, the charger can reduced the charge current when the battery voltage is low to safely recover an over-discharged battery.
Note that the standard shorthand for the cell configuration in a Li chemistry battery is “nSmP”, meaning “n” cells in series and “m” cells in parallel. When designing a charger, the series number is very critical because it determines the battery voltage. The parallel number determines battery capacity and only is used to calculate charge time at a specific charge current.
Lastly, note that battery charger conversion efficiency is becoming a major issue due to regulations that are being put in place by the USA DOE and similar regulatory agencies in other countries. As these new regulations are put into effect, high efficiency will become a primary converter type selection criteria.
B. Dedicated Charge Control IC summary
All dedicated charge control ICs convert a DC voltage input (typically from an AC/DC power supply) to the required current and voltage for battery charging.
Most dedicated charge ICs for Li chemistry batteries support the requirements outlined in the section above; i.e. CC mode and CV mode control, battery temperature enable/disable, and reduced current low-voltage battery recovery. Examples are the large selections from Texas Instruments (~160 parts listed), Linear Technology (~60 parts listed), Maxim (~70 parts listed), and Intersil (~50 parts listed). Other companies offering more limited selections of charger ICs include Fairchild, Analog Devices, Freescale, Micrel, ON Semiconductor, and Torex Semiconductor.
When selecting a dedicated charge control IC, you normally start with the battery chemistry, the number of serially connected cells (or max battery voltage), the desired charge current, and whether charge enable/disable on temperature is required. Also important is the max and min input DC voltage and whether the power source is a USB interface. Most of the IC vendors have parametric selection tools on their web sites to narrow your choices once these selections are made.
Almost all dedicated charge control ICs implement buck-type converters, where the input voltage is higher than the maximum battery voltage. A very few ICs support buck/boost type voltage conversion. The headroom required between the minimum input voltage and the maximum battery voltage is an important selection consideration.
There are two broad types of dedicated charge control ICs; Linear converters and switched-mode converters. Linear converters are usually limited to less than 1A charge current and situations where the input and output voltages are similar. Otherwise, the power loss in the converter becomes unmanageable without expensive heat removal (heatsinks, fans and such). However, linear converters are cheap, small, and easy to design. Figure 2.
Figure 2. Linear Converter
Switchmode converters are more complex to design and implement, but can support an almost unlimited range of input/output voltages and charge current. Modern switchmode converters run at such high switching frequencies that very small external inductors and ceramic capacitors can be used, making the circuit small and relatively simple. Sometimes a switchmode converter is used, even when a linear type could be selected, because better conversion efficiency is required than can be had with a linear type. Figure 3.
Figure 3. Switchmode Converter
So, why not just select a dedicated charge control IC in all cases instead of doing the expensive embedded software (firmware) development and circuit design required for using a microcontroller in a battery charge control application? Read on for more.
C. Using a Microcontroller for Charge Control
Many microcontrollers (microprocessors with embedded program memory, RAM, and interface modules) have the built-in A/D, signal conditioning, and pulse width modulation (PWM) control required for a battery charge control design. Examples are the PSOC line from Cypress, the MPS430 line from TI, PIC processors from Micro Chip, Atmel AVR processors, and many others.
One can design a battery charge controller using a cheap, relatively low-power microcontroller because charge control, unlike general purpose power supply control, is very slow due to the battery’s electrochemical nature. Nothing much happens in a battery in less than a few hundred milliseconds other than protection trips (and battery charges should be designed to never trip the protection!). As a result, a software-implemented control loop works well for battery charge control. The CC/CV protocol charge control required for Li-Ion battery charging can be implemented in a few hundred lines of C.
The only hardware support circuits required are voltage and current measurement amplifiers, an A/D, a PWM output, and a few general purpose I/O ports and much of this is built into available microcontrollers (you can do all of this inside a PSOC). If communication with the battery’s fuel gauge is required, an I2C (or SMbus) interface is usually available in these processors as well.
All of the vendors mentioned above publish extensive app notes on how to use their products as battery charge controllers. Some even have evaluation systems for this application that can get you started with circuit and firmware design.
In most cases, microcontroller-based charge controllers are more costly to design and produce than designs that use dedicated controllers. Why go to all the cost and trouble? Read on for more.
D. Charging Single-Cell (1S) Li-Ion batteries – the Simple Case
Charging a battery with a single series cell (1SxP) requires the simplest charge control design. A large selection of dedicated charge control ICs that handle up to 3 amps charge current, with internal switching MOSFETs, and which require very few external parts are available.
In addition, battery charging using the +5VDC (500mA max) source distributed on a USB interface is becoming common. This is almost always for 1S batteries and a good selection of dedicated linear and switchmode controller ICs for this application are available.
Single-cell charging algorithms normally do not require communication between the battery and the charger. As a result, most single-cell battery chargers are implemented with dedicated charge control ICs.
Examples of this sort of charger are cell phone chargers, shaver chargers, and charger docks for smart phones and tablet computers. The core voltages of these portable devices are low enough that a single Li chemistry cell can supply their ~3V minimum input voltage from the battery. Many of these devices can be charged off USB bus power.
An exception to this is situations where a multi-bay charger is best for the application. These include medical and military applications where several batteries are always on charge at a central site. Microcontrollers can often control more than one battery charger bay since the control algorithm required is slow. This can be a production cost advantage, but also causes the firmware to be more complex and difficult to design and test. We at Micro Power have designed chargers with up to 4 charge bays controlled by one inexpensive PSOC microcontroller.
E. Charging 2S to 4S Li-Ion batteries – Rising Complexity
When the portable device requires higher than Li chemistry single cell voltage, a battery with 2 to 4 cells in series is required. Charging these batteries is a more complex design problem because of cell balancing and CC/CV algorithm tuning requirements.
Higher S-count batteries must be charged such that the maximum cell voltage (not the battery voltage) is less than the specified float voltage. If the charger continues to push current into the battery when one or more cell voltages is too high, cell damage can result, reducing the life of the battery and even causing a safety issue in the most extreme case. Batteries can be designed with internal cell balancing circuits that either shunt current around some of the cells or push additional current into selected cells to keep the cells in balance. However, it’s sometimes necessary for the charger to participate in balancing, and to do this, the charger must communicate with the battery management system (BMS). Dedicated charge control ICs do not typically support this sort of interactive charge control, so a microcontroller must be used.
In addition, to optimize charge time, the charge control algorithm should be tuned for battery temperature, internal battery voltage, and other parameters known only by the BMS. For example, to optimize charge time, the charger should stay in CC mode for as long as possible. However, sometimes the battery charge current path contains an anti-reverse diode, preventing the charger from measuring the actual cell stack voltage. The BMS can measure the cell stack voltage internal to the battery and communicate that to the charger, which can use that more accurate voltage in the CC to CV mode transition algorithm, and keep the battery in CC mode longer. This can reduce the charge time by a significant amount.
Also, a charger for a more complex battery usually has some sort of status display (LED bar graph, LCD, etc). A microcontroller is usually required to implement this since the dedicated charge controllers have very simple status display support.
Lastly, high-end chargers for the complex batteries found in military and medical applications sometimes contain microcomputer systems for storing and communicating information about individual batteries (typically via a USB I/F to a PC). This information can be used for preventative maintenance and battery fleet status reporting.
F. Charging High Voltage Batteries – System Design Required
To reduce current requirements, batteries for electric vehicles, large system back power, and other high power requirement applications are built from very high series count cell stacks. In addition, EV battery systems also support regenerative braking systems, active cooling/heating, and other advanced battery management systems. As you might expect, high S-count batteries require complex cell balancing circuits and algorithms. These complex, high voltage batteries require that the BMS and charging systems be completely integrated.
Integrating the charger with the BMS into a single, system usually requires computer control for much of the BMS function since dedicated charge control ICs are not flexible enough.
Battery fleet management is common in these complex systems, so the charger/BMS system must acquire and maintain information about battery health and history.
It’s likely that the distributed energy storage in EV and household and business backup power systems will be used for power grid peak load management when the nation-wide smart-grid becomes a reality. This will require that the charging system also be synched to the grid inverter so that the battery can source as well as consume power to/from the grid. Robust communication through the charger with the BMS will be a requirement for these integrated systems so that the smart grid can maintain information about battery status and capabilities.
All this shifts the charger away from being a simple current-controlled voltage converter to be a subsystem in a complex, computer controlled energy management system.
G. Making the decision
In summary, making a decision on the type of charge controller to use for a specific application goes as follows:
- If the battery is a 1S Lithium chemistry type and the charge current is < 500mA, or charging off USB bus power is required use a dedicated, linear-type or a minimum function switchmode charge controller. An example of this is the TI bq24100 series.
- If the battery is a 1S to 3S, single-bay Lithium chemistry type and the charge current is < 3A, normally use a dedicated, switchmode charge controller such as a TI bq24105 or bq24170. However, if the application requires charger to battery communication, advanced user interface, or communication with a host computer, consider use of a microcontroller.
- If the battery is a 1S to 3S, multi-bay Lithium chemistry type and the charge current is < 3A, trade off the costs for a switchmode charge controller with a microcontroller controlling more than one bay.
- If > 3A charge current is required, or the battery is > 3S, a switchmode converter using a microcontroller is almost always required because communication between the charger and BMS is usually needed for safety and optimum charge time.
- No matter what the S-count, if battery history and status recording and communication is required, a microcontroller (or even a more complex microcomputer) system must be used in the charger.
Texas Instruments: http://www.ti.com/ Navigate to the Power Management page, then the Battery Charge Management page
Linear Technology: http://www.linear.com/products/battery_management
Analog Devices: http://www.analog.com/en/power-management/battery-management/products/index.html#Battery-Chargers
ON Semiconductor: http://www.onsemi.com/PowerSolutions/parametrics.do?id=106
Torex Semiconductor: http://www.torex.co.jp/english/products/pro15/index.php
Micro Power: http://www.micro-power.com/
Cypress PSOC: http://www.cypress.com/?id=1353