Single NiMH battery simulates the circuit of a lithium battery

Introduction

Lithium-based (Li+) batteries are becoming common in portable equipment. They have desirable characteristics, but they are often in short supply. Lead times can be long unless you have a preferred-customer status with the battery manufacturer. A backup alternative to Li+ is therefore desirable, especially for smaller companies. This note describes a circuit that allows a nickel-metal hydride (NiMH) cell to be used in a circuit designed for a Li+ battery. It provides the same performance, size, and cost as a Li+ battery.

Nickel-metal hydride (NiMH) batteries are still widely used. They are much more economical than Li+ batteries and come in very small sizes (AAAA is now available).

Design Considerations for a NiMH Interface

A circuit that interfaces a NiMH battery with a lithium-optimized circuit should mimic the battery's terminal voltage, which declines as the battery discharges. The nominal terminal voltage for Li+ batteries (3.6V) is about three times that of NiMH batteries (1.2V). As a simple approach, therefore, you can force the output of an efficent step-up converter to equal battery voltage times 3: 6V/1.2V = 3.

Allowing the circuit to run constantly during shutdown, however, consumes unnecessary power. The NiMH circuit's equivalent leakage (its quiescent current) can be as a high as 200µA during shutdown, which is unacceptable. In fact, only a power-control capability is needed during shutdown.

Instead of maintaining 3x the NiMH voltage while in shutdown, you can run the proposed circuit in burst mode, which activates the step-up converter only when the circuit output drops below a defined threshold. When the output reaches the upper threshold, the step-up shuts down, allowing the output capacitor to discharge through the output load plus the NiMH circuit. Thus, the output voltage forms a sawtooth wave. If battery voltage falls below a lower limit, however, the circuit remains deactivated to protect the battery from depletion.