Two simple secondary battery circuits: Page 2 of 8

August 10, 2015 //By James Bryant
Two simple secondary battery circuits
James Bryant considers the issues related to two simple secondary battery circuits.
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.
 
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

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