Circuit Enables Single NiMH Cell to Simulate a Lithium Battery

Posted on Feb 6, 2014

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

Circuit Enables Single NiMH Cell to Simulate a Lithium Battery
Click here to download the full size of the above Circuit.

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. 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 200 µA 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. The circuit of Figure 1...

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