18500 battery

USB powered 18500 battery switch mode charger circuit design

Charge NiMH batteries via USB

Figure 1. USB-powered single-cell NiMH battery switch-mode charger

Although Li+ batteries have ostensibly taken over the portable world, NiMH batteries have not been completely abandoned. Surprisingly, although there is still a large gap in energy per unit weight, its energy per unit volume is only about 15% lower than that of Li+ batteries. The biggest disadvantage of NiMH batteries is the high self-discharge rate. Hybrid NiMH batteries have solved this problem to a great extent. For example, SANYO® Eneloop® batteries can still retain 85% of their power after one year of static use. The appeal of NiMH batteries lies in their low cost, high safety, and ease of user replacement. At least standard batteries have these advantages.

The portable device shown in Figure 1 is powered by a AA NiMH battery and is charged via USB. The charger switching frequency is approximately 150kHz and the battery charging current is 1.1A (typical AA NiMH battery at approximately 0.5°C). Since the buck converter converts 5V, 500mA into 1.5V, 1.1A when charging the battery, the current supplied to the battery by the circuit (1.1A) is greater than the current obtained from the USB interface (500mA). It should be noted that since charging termination cannot be correctly determined at low charging rates, only 500mA or higher power ports can be used for charging. So, when enumeration determines that only 100mA is available, charging should not be activated. The system stops charging by turning off Q2 on TMR, leaving the timer resistor floating.

Another particularly useful function of this charger is to detect the battery impedance to determine whether the connected alkaline battery or a faulty battery is connected. Charging will be prohibited when this condition is detected. This allows users to insert alkaline batteries in an emergency without worrying about accidental charging.

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