Battery Emulation Circuit Speeds Charger Testing

May 1, 2008 12:00 PM
By Alfredo H. Saab, Applications Engineering Manager, and Shasta Thomas, Applications Engineer, Maxi

Testing a Li-ion battery charger using its natural load (i.e., a battery) is time consuming because the charging process can take an hour or more. The test time varies widely, depending on whether you combine a fast-charge battery with a slow charger, a slow-charge battery with a fast charger, or something in between.

In any case, the charging process cannot be accelerated beyond a limit imposed by the battery's maximum charge rate. This is the so-called fast-charge current, and exceeding this limit may damage the battery. For normal batteries of the sort used in consumer products, this current is rarely specified above 1C — that's the current needed to fully discharge the battery in one hour. So the time required to carry the charger through the full cycle will be longer than two hours, in most cases.

If there is a need to repeat the test, you must discharge the battery in full — a process that is shorter than charging but not by much. Another option is to keep a supply of consistently discharged batteries on hand. A more convenient alternative to load testing with a real battery is to test the charger using a simulated but realistic load.

However, battery simulation, which should verify the charger circuit's dc response and dynamic stability, is difficult to implement with the standard bench loads used in power testing. That's because batteries — unlike most of the bench loads — do not behave as resistances or constant-current sinks. In addition, testing should step the charger through its entire operating range: through the transition from constant current to constant voltage (CC-CV) and on to charge termination. A dedicated charger test circuit satisfies all these requirements for battery simulation.

Complex Charge Requirements

By virtue of the battery they are designed to charge, Li-ion battery chargers are more complex and accurate electronic power systems than chargers used for other battery types. A Li-ion battery needs a different type of charging process than other battery technologies, the CC in a first phase transitioning to the CV in the second phase.

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

Li-ion is also more delicate than other battery chemistries, with little tolerance for abuse. In addition, this chemistry demands high accuracy for the battery-charger current and voltage settings. Failure to provide the required accuracy can result in a severe reduction in battery life, failure to reach a complete charge and other degradations in battery performance.

Fig. 1 illustrates the V-I characteristic of a modern CC-CV integrated circuit (a MAX1737) used for a Li-ion battery charger. This type of IC is the component at the heart of all Li-ion battery chargers for consumer products. In Fig. 1, the CC and CV regions are clearly shown. In the first region, battery voltage ranges between 2.6 V and 4 V. In the CV region, the battery voltage remains at 4.2 V.

The region below 2.6 V is different. If charging is attempted on a battery discharged below 2.6 V, the charger reduces the charging current to a low value (conditioning current) until the battery reaches the 2.6-V level. This is a safety mechanism made necessary by the behavior of Li-ion batteries when overdischarged. In other words, forcing a fast-charge current when V_BATT is less than 2.6 V can cause the battery to go into an irreversible short-circuit condition.

IC-based Li-ion battery-charger designs usually have two basic building blocks: a digital block (control state machine) and an analog block composed of a well-regulated current/voltage power supply and an accurate reference (better than 1%). A complete test of a Li-ion charger product (not just the IC) may be required at either the design/prototype phase or when verifying or troubleshooting production units. This testing is more involved and time consuming than just verifying some current or voltage values.

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