Engineers who have applied nickel-metal-hydride (NiMH) batteries use a constant current to charge them. They detect the end-of-charge point in an alkaline-battery system by monitoring for a temperature rise in the battery. But lithium-ion batteries charge at a constant voltage with a current limit.
"You can look at charging in two ways," explained Dr. Graham Archdale, lithium-ion sales manager for Saft. "First, you charge at a constant voltage and limit the current. When the battery reaches the charging voltage, the current will tail off. Then when the current drops to x, which depends on the size of the battery, you can say the battery has a full charge. Second, you can charge the battery at a constant voltage – again limited to a maximum current – and when the battery reaches that voltage, you start a timer and turn off the charger after a set time, say three hours."
"Unlike some batteries, lithium-ion batteries do not take a trickle charge," said Isidor Buchmann, CEO and founder at Cadex, a manufacturer of battery chargers and battery-test systems. Below 4.2V you have a fairly high charge current, but once you reach 4.2V, the current drops because the battery has absorbed as much energy as it can and it goes into saturation mode. At that point, a charger considers the battery fully charged.
"The charging process should work that way, but price competition causes some people to take shortcuts they shouldn't," said Buchmann. "They may charge a battery to 4.15V and maintain the charge current at a reduced level, but never turn the current off. The battery manufacturers do not recommend that process." If the charger never gets a battery to 4.2V, the battery may reach only 90 to 95 percent of its energy capacity.
In some cases, a product may discharge a lithium-ion battery below the voltage its manufacturer specifies as a cut-off point. "In that case, the safety circuit can go to sleep," noted Buchmann. "Our chargers detect this condition and apply a low current so the control circuits can wake up and start to accept the charge. Then the charger increases the current so the battery goes through a normal charge cycle."
"When customers ask what charger we recommend, we remind them they cannot mix battery types and chargers," said Julius Cirin, vice president of corporate marketing and technology at Ultralife Batteries. "When engineers move from NiMH to lithium-ion batteries, sometimes they want to use the same charger, but they cannot. We provide guidance on our Web site, and our engineers help customers, too."
According to Ralph Wise, director of technology, at Ultralife Batteries, engineers who use lithium-ion batteries generally know how to design a charger circuit. "If they ask for assistance we recommend they go to the companies from which we buy ICs for our own chargers. The IC-vendor can offer application assistance and reference designs."
Engineers should understand that a battery pack's internal circuits do not directly control charging, but the circuits do prevent an overcharge and too deep a discharge. If the internal circuits detect a limit, they shut down the battery.
"On large battery packs, we are most concerned about internal heating, which occurs when a user tries to charge a pack at too high a temperature, say on the front seat of a vehicle in a hot climate," said Wise. In that case, the battery will shut down. So, the internal circuits monitor conditions and do not allow charging outside specific limits. During normal operation, that protection remains transparent to the user. "We would recommend the user cool the battery and then charge it," said Wise. "Engineers must put that type of information in their product-use guides."
--Jon Titus
For further reading
Deng, Qi and Henry Lee, "Select the right Li-ion and Li-polymer battery charger," Portable Design, July 2006. www.portabledesign.com
