ALEX KARAPETIAN | ACOPIAN TECHNICAL CO. ASK almost any engineer about linear power supplies and the likely instinctive reaction will be, “Sorry, I can’t use them — they’re too inefficient.” Any possibility of using a linear supply usually ends right there; it is as though you’re asking them to go back to vacuum-tube AM radios.
Regarding charging rules, the lithium-ion and lithium-polymer batteries are not that much different. Figure 3 shows a complete charging cycle. A full charging process consists of 3 steps: PRE Charge, CC, and CV. This stage is referred to the condition that a lithium battery’s initial voltage is below 2.8V.
There are three methods to charging Li+ batteries: switch-mode, linear and pulse. Each method has its advantages and disadvantages. Switch-mode charging minimizes power dissipation over a wide range of AC adapter voltages, but consume more board space and add complexity compared to linear and pulse charging.
Li-Ion batteries have a much higher energy density allowing them to be smaller and weigh less than half of an equivalent Valve Regulated Sealed Lead-Acid (VRLA) battery currently used in UPS systems. Figure 1. Lithium-polymer battery system containing a Battery Management System (BMS), a charger and a large number of lithium-polymer cells.
A Li+ battery charger must limit the charging current and the battery's maximum voltage. Designers should consult the battery manufacturer to determine what's required to safely charge a particular battery. Other features are often added to improve the life of the batteries or the operation of the charger.
When designing a single-cell Lithium-Ion charger, record the allowed maximum charge current and voltage of the battery in use. Then determine the voltage and maximum charge current of the power supply you want to use for charging. Usually, this will be five volts and between 500 mA and 900 mA (USB 2.0 and USB 3.0).