Simplified Lithium-Ion (Li+) Battery-Charger Testing

Introduction

Lithium-ion (Li+) batteries are more delicate than other battery chemistries and have little tolerance for abuse. Consequently, Li+ battery chargers are complex circuits, requiring highly accurate current and voltage settings. If these accuracy requirements are not met, the charger may fail to completely charge the battery, severely reduce battery life, or otherwise degrade battery performance. Given the demands imposed on Li+ chargers, it is critical that charger designs be tested thoroughly and stepped through their entire operating range. However, testing a Li+ charger with its natural load (i.e., a Li+ battery) can be time consuming and impractical in laboratory and production environments. To simplify the process, this article presents a battery-emulation circuit for accelerated, realistic testing of Li+ battery chargers without actual batteries.

CC-CV Charging

The Li+ battery-charging process requires medium-accuracy constant-current (CC) charging in a first phase, transitioning to high-accuracy constant-voltage (CV) charging in a second phase. Figure 1 illustrates the V-I characteristics of a modern CC-CV integrated circuit (the MAX1737) used for a Li+ battery charger. This type of IC is at the heart of all Li+ battery chargers in consumer products. The CC (between 2.6V and 4.2V battery voltage) and the CV (4.2V) regions are clearly shown.

Figure 1. This V-I curve from the MAX1737 is typical for Li+ cell chargers

The region below 2.6V requires a different charging technique. If charging is attempted on a battery discharged below 2.6V, the charger applies a low-value ("conditioning current") charging current until the battery reaches the 2.6V level. This is a safety mechanism made necessary by the behavior of Li+ batteries when overdischarged. Forcing a fast-charge current when VBATT < 2.6V can cause the battery to go into an irreversible short-circuit condition.

The transition point from the CC to the CV phase has a critical tolerance of ±40mV. The reason for the narrow tolerance is that a lower CV will not allow the battery to acquire its full charge, and a higher one will reduce its useful life.

Charge-process termination involves sensing that the battery has reached its full charge and that the charger must be disconnected or shut down. This is accomplished by detecting, while in the CV phase, the point where the charge current is reduced to a fraction (usually < 10%) of the so-called fast-charge or maximum charge current.