Electronics Weekly Electronics Design & Components Tech News
Concept image of vehicle powered by electric energy with charge stored in battery.
The ability to operate electronics away from electrical mains has been a major driver for the success of portable electronics, from portable phones to power tools. As electrification takes hold in automotive systems, battery-powered operation is becoming a requirement for the cars of the future. Although lithium-ion and other technologies that are used widely today have increased the energy density of batteries and made these types of products possible, they need far more care than older chemistries in order to maximise the time between charges and overall product lifetime. It is not enough to simply attach a power supply to the terminals and remove it once the internal voltage has reached a sufficient level.
Challenges of battery technology
Lithium-ion batteries are particularly vulnerable to conditions such as overcharging. This not only damages the cells, but it can also be dangerous. Overcharging increases pressure inside the cells and can lead to thermal runaway, which can cause the battery to catch fire. As a result, overcharging protection is built into all off-the-shelf charging controllers.
Maintaining a long lifetime and high maximum charge call for a great deal of control over the process from start to finish. A device of this kind charges the battery in three successive phases: precharge; constant current; and constant voltage. The precharge state is used to bring the battery into a suitable state for full charging, which is important to avoid damage if the cells have been almost completely discharged. The process involves supplying a small current at first. The charger uses both measurements and a safety timer to check that this phase completes successfully and if not, flags an error to the host microcontroller. This can be achieved for portable systems.
The other two modes represent a compromise. Constant-current charging is the fastest method, but risks damage to the battery as it involves higher current levels that can cause overheating. Therefore, battery-charging controllers will include temperature-sensor inputs to restrict power if the heat becomes excessive. Typically, as the battery nears full charge, the charging controller will move to the constant-voltage mode. This avoids overheating and helps improve lifetime, but slows the charging process significantly.
Better control over charging delivers other benefits. It provides the ability to optimise the size and weight of the overall system, not just in terms of the battery itself, but power-delivery circuitry. Some charging controllers are highly integrated. For example, the MC34674 from NXP Semiconductors is a fully integrated charger for single-cell lithium-ion and lithium-polymer batteries optimised for travel applications that requires only an external LED for charge status indication, two decoupling capacitors and a thermistor circuit with a connection for detecting the battery temperature.
Chargers and voltage conversion for PD
The same need for high integration with few passive components is true of the DC/DC converters that will be used to condition power and relay it to the various devices in the system, which may have specific voltage and current requirements. The large number of power rails that may be needed in even comparatively small handheld devices, from the 1V of a digital system-on-a-chip (SoC) to 12V or higher for analogue I/O, puts a premium on compact implementations. By using fewer external passive components and by reducing conversion losses, the charger and regulator circuitry can be made smaller and operate without bulky heatsinks on the power transistors, delivering compact and efficient system designs.
One way in which the MAX77960B and MAX77961B help improve density is by acting as buck converters as well as chargers: supplying power to the portable system as long as the voltage delivered by the battery is higher than the desired output voltage.
Whether based on rechargeable or non-rechargeable chemistries the voltage from the battery will drop as the cells discharge. If that voltage falls below the operating range of the converter, any remaining charge in the battery will go unused, which will lead to lower battery autonomy times than expected. The use of dc-dc conversion using both buck and boost operation provides the ability to extract the maximum charge possible.
The ability to support low-energy shutdown modes is vital in IoT devices such as environmental sensors and the wearable products developed for monitoring health and wellbeing. These devices will sleep for long periods, waking for a fraction of a second before moving back into sleep mode again. With such low duty cycles, low quiescent current within the entire system, including that of the power-management circuitry, is vital. Products such as the MAX710ESE+ buck-boost converter (developed by Maxim, now owned by Analog Devices), for example, provides the ability to use only the linear regulator subsystem during such sleep modes to limit current consumption and avoid draining the battery during long periods of inactivity.
Battery monitoring and management
In many applications it is equally important to know how much charge remains and to control current flow from each of the cells in a pack. For automotive systems, the combination of these two elements can be crucial as drivers do not want to be stranded far from a charging point if the estimate is wrong or if a poor balance leads to insufficient voltage being delivered because several cells have run down excessively. In all systems, accurate charge estimation helps optimise charging time as well as capacity management. Some systems now exploit these measurements to slow charging and improve battery lifetime by comparing charge with expected usage. In many cases there are dedicated products that perform the battery management and monitoring.
The Renesas ISL94202, for example, is designed for batteries that contain up to eight cells and provides standalone pack control with no need for a supervisory microcontroller. The device automatically manages cell balancing within the pack and has a number of features to prevent overcurrent situations and other protection features. Another example is the TLE9012AQU from Infineon Technologies, which fulfils four main functions: cell voltage measurement; temperature measurement; cell balancing; and the provision of isolated communication to a central battery controller. This combination of features makes the device suitable for larger communications and automotive systems where multiple battery packs need to be used in parallel. To ensure accurate measurements of a cell’s state, each cell is monitored by a 16-bit-resolution ADC.
Today’s vehicle designs may incorporate several different battery subsystems to feed different parts of the vehicle, from the powertrain to the infotainment. Many teams are now planning to eliminate the low voltage battery used to supply the ancillary circuits and will use the higher voltage powertrain battery to also power the equipment in the cabin. This not only requires dc-dc converters that are optimised for the larger step down in power, but battery management systems that can handle a complex power topology. Battery management and support ICs, such as those offered by NXP, support isolated daisy chain communication to relay information to a central microcontroller and can perform current measurement synchronised across the different packs.
At the other end of the scale and to support the growing use of IoT sensors, energy harvesting is becoming an important method for topping up the power available to portable and battery-powered devices. Harvesting makes it possible to run security sensors and other low-energy devices for many years without changing the battery or connecting the device to mains electricity.
A common configuration is to combine a single-use battery such as a lithium coin cell with a rechargeable device, either a supercapacitor or small Li-ion battery. Harvesting technologies do not provide the charge in a convenient form and often the sources operate at high impedance. Therefore, careful circuit design is needed to minimise losses and to be able to harvest energy from solar cells and deliver charge to downstream circuitry. An integrated boost converter can start from input voltages as low as 225mV.
Bringing it all together
Battery management is now an important issue for a huge range of systems. The complexity of Li-ion and similar chemistries complicate the design of power delivery and consumption circuitry. However, the availability of high integration, specialised devices from major semiconductor providers makes the integration of batteries into systems as simple as possible. Design-in support from experienced engineers at distributors ensures teams receive the best advice on which devices to use for individual applications.
