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Developing two simple secondary battery circuits

11 Aug 2015  | James Bryant

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Over the last couple of decades the use of rechargeable batteries has grown tremendously, and for most common applications there are ASICs [1] which control battery management with very little additional circuitry. In the last few weeks, however, I have encountered two issues where non-standard solutions are useful, and in each case a less well known feature [2] of many voltage references complicated the design. The issues are the measurement of battery capacity, and the charging of a lead-acid battery from a solar panel with minimal power dissipation in the control circuitry.

I built both of these at the same time, and when I tested the first prototypes the hysteresis of the comparators was much greater than it was designed to be. The circuit used is shown in figure 1 and consists of an ADR291 or ADR292Vage reference driving the non-inverting input of an AD822 or an AD8667 op-amp through a small resistor, Rin. The output of the op-amp goes to this input via a large resistor, Rfb, and a 1N4148 diode to provide hysteresis. This has an accurate threshold as the voltage on the inverting input goes low since the feedback through Rfb only happens when the op-amp output is greater than the reference voltage and the diode conducts. The hysteresis of this circuit is Rin(V+—Vref—Vdiode)/(Rin + Rfb) where V+ is the op-amp positive supply voltage and Vdiode is the voltage drop in the 1N4148 diode.


Figure 1: Simple threshold circuit—which does not work with some references!


When I designed this circuit I assumed that if I used 1MΩ as Rfb the current (approx 10µA) flowing into the reference output would flow in the resistor chain R1 + R2 + R3 (see the simplified schematic of these references in figure 2) and the reference accuracy would be unaffected. In fact measurements on a number of ADR292 devices show that they will not tolerate reverse currents above about 5µA without their output voltage rising by about 40%. This caused me to abandon my original design and to write this article. The final designs are described below.


Figure 2: A simplified schematic for the ADR291/2.


Of course if I had used a voltage reference with an output stage capable of sinking as well as sourcing current there would have been no problem.
Rechargeable battery capacity measurement
The measurement of battery capacity has become important because of blatant overstatement of battery capacities by manufacturers and retailers. I recently needed to power an item of medical electronics (a CPAP machine) for three days without access to an electricity supply. Calculation suggested that three 18650 Li-Ion cells with 3AH capacity would do this, but when I bought three with alleged capacity of 3.6AH from a well-known retailer and tested them they lasted less than one day. At about the same time my sister complained to me that the battery life of her cordless phone had dropped over five years from seven days to two, but that when she had replaced its pair of 800mAH NiMH AAA cells with new ones bought from the shop that had sold her the phone the life with the new batteries had further dropped to little over a day.

Some experiments seemed a good idea. Several battery chargers have battery capacity measurement as a feature but when I used one the results from consecutive tests of the same battery varied by more than 25%. It was at this point that I built the battery capacity measurement circuit shown in figure 3.

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