ABC OF RECHARGEABLE BATTERIES （4）

Published:2011/8/9 1:59:00 Author:Phyllis From:SeekIC

By Gert Helles

Charging lithiumion and lithium-polymer cells

Whereas chargers for nickel-based batteries are current-limiting devices, chargers for lithiumion batteries limit both voltage and current. The first lithiumion cells called for a charge-voltage limit of 4.10 V/cell. Higher voltage means greater capacity, and cell voltages as high as 4.2 V have been achieved by adding chemical additives. Modern Li-ion cells are typically charged to 4.20 V with a tolerance of ±0.05 V/cell.

Full charge is achieved after the terminal voltage has reached the voltage threshold and the charging current has dropped below 0.03 C, which is approximately 3% of lcharge (Figure 6). The time for most chargers to achieve a full charge is about three hours, though some linear chargers claim to charge a Li+ battery in about one hour. Such chargers usually terminate the charge when the battery’s terminal voltage reaches 4.2 V. That kind of charge determination, however, charges the battery only to 70% of its capacity.




A higher charging current does not shorten the charge time by much. Higher current lets you reach the voltage peak earlier, but then the topping charge takes longer. As a rule of thumb, the topping charge will take twice as long as the initial charge.

Protection modes

Overvoltage: If the cell voltage sensed at Vdd exceeds the overvoltage threshold VQV for a period longer than the overvoltage delay tovd, the DS2720 shuts off the external charge FET and sets the OV flag in the protection register. The discharge path remains open during overvoltage. The charge FET is re-enabled (unless blocked by another protection condition) when the cell voltage falls below the charge-enable threshold Vce, or discharging causes Vdd - VpLs > VOC.

Undervoltage: If the cell voltage sensed at VQD drops below the undervoltage threshold Vyy for a period longer than the undervoltage delay tUVD, the DS2720 (see inset) shuts off the charge and discharge FETs, sets the UV flag in the protection register, and enters sleep mode. After the cell voltage rises above VuV and a charger is present, the IC turns on the charge and discharge FETs.

Short Circuit: If the cell voltage sensed at Vrjp drops below the depletion threshold V5C for a period of IsCDi the DS2720 shuts off the charge and discharge FETs and sets the DOC flag in the protection register. The current path through the charge and discharge FETs is not reestablished until the voltage on PLS rises above Vprj-Voc. The DS2720 provides a test current through internal resistor RJST (from Vrjo to PLS) to pull up PLS when VpD rises above W^Q. This test current allows the DS2720 to detect removal of the offending low-impedance load. In addition, it enables a recovery charge path through RJSJ from PLS to Vpp.

Overcurrent: If voltage across the protection FETs (VDD-VPLS) is greater than Voc for a period longer than toco fne DS2720 shuts off the external charge and discharge FETs and sets the DOC flag in the protection register. The current path is not reestablished until the voltage on PLS rises above VDD-Voc. The DS2720 provides a test current through internal resistor RJSJ (from Vprj to PLS) to detect removal of the offending low-impedance load.
Overtemperature. If the DS2720 temperature exceeds T^^x, the device immediately shuts off the external charge and discharge FETs. The FETs are not turned back on until two conditions are met: cell temperature drops below T^^X, and the host resets the OT bit.

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