Lithium-polymer (LiPo) batteries have a higher power density than most other battery technologies. They also deliver a higher nominal voltage per cell: 3.7 V compared to 1.2 to 2 V for other cells. But that creates a problem if you want to replace an older 6-V battery with a LiPo battery. The voltage from one cell is too low, and the voltage from two cells (7.4 V nominal) is sometimes too high.
For example, I purchased a 2 million-candlepower cordless spotlight from a discount store. The device produced a very bright and well formed beam, but its small 6-V lead-acid battery deteriorated quickly. A two-cell, 2800-ma-h LiPo battery for a radio-control model airplane was available, so I designed and built the circuit described here to power the spotlight from the 7.4-V LiPo battery.
The circuit converts dc voltage within a range of 6.4 to 8.4 V from the LiPo battery to a switched-dc waveform with a nominally constant 6.25-V rms value to power the 6-V, 3-A (18-W) incandescent bulb (Fig. 1). At about 6.3 V, near the end of the LiPo battery’s charge, the circuit stops switching and directly connects the battery to the bulb. When the battery falls to 6 V, the circuit disconnects it from the bulb to prevent deep discharge.
The circuit offers high efficiency and uses low-cost, mature components. It is important to note that this circuit is suitable only for loads that are happy with an unfiltered switched dc waveform, such as lamps, relays, some motors, and small heaters. It is not a regulated and filtered power supply.
Comparator U1a, one of four in the LM339, is connected as an astable multivibrator that free-runs at approximately 750 Hz. The peak-to-peak voltage across C1 decreases as the LiPo battery voltage falls during discharge. Comparator U1b compares the value of the sawtooth-like waveform across C1 to a 4.49-V reference voltage provided by U3, a TL431 adjustable shunt regulator (Fig. 2). The output of U1b is connected to the gate of the N-channel MOSFET (T1), which drives the load.
For LiPo battery voltages above about 6.4 V, U1b produces a rectangular waveform with a duty cycle that depends on the value of the battery voltage. The duty cycle increases as the battery voltage falls. When the battery voltage falls to about 6.3 V, the peak voltage across C1 is a little below the 4.49-V reference voltage, so the output of U1b stays high. This keeps the MOSFET on, which powers the load continuously (100% duty cycle).
Comparator U1c compares the battery voltage (from voltage divider R11/R12) to the 4.49-V reference voltage. The output of U1c is connected to the output of U1b (outputs are open-collector) to pull the MOSFET gate low (thus disconnecting the load) when the battery voltage falls to about 6 V.
The interesting aspect of this circuit is the relationship of the duty cycle to the rms value of the switched dc output voltage. The duty cycle increases as battery voltage decreases, which tends to keep the output nominally constant as battery voltage falls. The carefully selected values shown in Figure 1 result in a nominal output voltage of 6.25 V rms with a variation of only 0.25 V rms (4%) as the two-cell LiPo battery falls from 8.4 V to 6 V.
Figure 3 is a plot of the rms output voltage versus the battery voltage. These are calculated values. The cordless spotlight shines brightly, with almost no perceptible change in brightness as the battery discharges.