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.
My original intention had been to build a smart charger with integral capacity measurement. But I already had a smart charger which worked very well as a charger. If