Batteries and Fuel Cells
Honey, where is my power cord?
By, Thong Huynh, application engineering director, Analog Devices A
more common modern-day question would probably be “Honey, where is my battery charger?”. The reduction in cost and improvement in
battery performance, especially Li-Ion based, at the turn of the century has fueled a steady growth of battery-powered energy storage and portable equipment. Also, supercapacitors (aka ultracapacitors) are increasingly finding usage in a variety of applications due to their unique characteristics. The lead-acid battery, a 150-year-old technology, is still popularly used in cars, wheelchairs, scooters, golf carts, and uninterruptable power supply (UPS) systems. These energy storage devices must be recharged once their energy has been depleted. Industrial systems such as emergency lighting with battery backup, UPS backup power, and HVAC use a 24 VDC power source—that is, a 24 V battery is used to back up these systems. The 24 VDC power source, however, can rise to 60 V peak voltage during transient conditions, according to IEC 61131-2 and IEC 60664-1 standards.
In either situation, the equipment requires charger solutions that can accommodate higher battery voltage and withstand higher input voltage during transient events.
Charger basics
There are many charger topologies. The linear charger drops the voltage difference between the power source and the battery through a power switch. This type of charger is the least efficient, since it dissipates a lot of power across the power switch when the voltage difference between the power source and the battery is large. The boost charger boosts the voltage from the power source to the battery voltage. This topology requires the power source voltage to be lower than the battery voltage. The buck charger steps down the voltage from the power source and requires the power source voltage to be higher than the battery voltage. The buck-boost charger can charge the battery with a power source voltage that is either higher or lower voltage than the
22 April 2022
Figure 1. Generic synchronous rectification buck charger.
battery voltage. This topology requires four power switches (compared to two for the buck) and generally is not as efficient.
The synchronous rectification buck charger is the most efficient and is the focus of this article. Figure 1 shows a generic synchronous rectification buck charger circuit. Most buck chargers today operate at a relatively low voltage. Many are rated at only 28 V input with some at 40 V. Allowing ±10 per cent input voltage regulation and a 2 V drop across the buck charger, a 28 V-rated charger can only practically charge a 5S Li-Ion battery stack (maximum). We will examine a new family of 60 V input charger ICs that allow higher voltage charging—up to 52 V battery voltage (or a 12-cell Li-Ion stack)—and that can withstand a 65 V input voltage transient. The standby current on a charger should be low to save energy. Energy Star assigns five stars to mobile phone chargers and other small chargers that draw 30 mW or less on standby. One star goes to chargers with 300 mW or more, and there are other ratings for everything else in between. Energy Star aims to reduce current consumption of personal chargers that are mostly left plugged in when not in use. There are over one billion such chargers connected to the grid globally at any given time.
Components in Electronics
Even though the lead-acid battery, Li-Ion- based battery, and supercapacitor are all energy storage devices, they have very distinct charging/discharging characteristics. We will examine these characteristics and discuss a charging solution for each of them. A good battery charger provides battery performance and durability, especially when charging under adverse conditions.
Lead-acid battery charger The lead-acid battery must be charged slowly. Typical charge time is eight to 16 hours. The
battery must always be stored in a charged state, and a periodic fully saturated charge is essential to prevent sulfation. It is common practice to charge lead-acid batteries to 70 per cent in about eight hours, and another eight hours to do the all-important absorption charge. A partial charge is fine provided the lead-acid occasionally receives a fully saturated charge to prevent sulfation. Leaving the battery on float charge for a prolonged time does not cause damage.
Finding the ideal charge voltage limit is critical. A high voltage (above 2.45 V/cell)
Figure 2. High voltage lead-acid battery charger controller.
www.cieonline.co.uk
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