Nuts and Volts - August 2017

8 August 2017

In this column, Kristen answers questions about all aspects of electronics, including computer hardware, software, circuits, electronic theory, troubleshooting, and anything else of interest to the hobbyist. Feel free to participate with your questions, comments, or suggestions. Send all questions and comments to: Q&A@nutsvolts.com.

n WITH KRISTEN A. McINTYRE

More on NiCd Chargers

QI am an Ophthalmologist and a long time Nuts & Volts reader. I use several hand diagnostic ophthalmic instruments that are powered by rechargeable 3. 5 volt NiCd batteries that are well charged constantly by a transformer, a resistor, and a diode. They are under a constant voltage charge of 3. 5

VDC in a well charger. The batteries are made up of 3-1/2 size AA cells in series. Is there a better charger for the 3.5V NiCds since they remain on constant charge?

John Haley

AThis is similar to another recent question I got about NiCd vs. NiMH charging. Just to repeat a bit of what I’ve written before: “For a NiCd, as you mention, a constant current is generally adequate, as long as the current is kept relatively low. Charging at a rate like 1C (or the capacity of the battery in one hour) can cause overheating, but lower rates are fine.” Please keep in mind that everything I’m about to say only applies to NiCd batteries, and not to nickel metal hydride (or NiMH) batteries. Your simple charger is probably doing a reasonable job when the cells are healthy. The resistor limits the current and the transformer provides an upper bound on the voltage, given the characteristics on how voltage is induced in transformer windings (actually a consequence of Faraday’s Law of induction).

Figure 1 reproduces this simple circuit. The peak current is approximately expressed by (Vp - 0.6 - 3Vb)/R = I, where Vp is the peak voltage from the transformer and Vb is the cell voltage; typically 1.2V. The approximation results from the diode drop voltage of ~0.6.

We don’t know the transformer voltage or the value of the limiting resistor, but we can say with certainty that if the battery voltage decreases, then the current flowing into the batteries will increase. So, how could the battery voltage decrease beyond discharge?

NiCd cells are prone to internal shorts resulting from chemical filaments growing inside; particularly if they are stored without charging. If one of the three cells you have shorts internally, the current through the remaining cells increases. The current equation changes to (Vp - 0.6

- 2Vb)/R = I. Notice the factor of 2 instead of 3 on Vb. This might or might not be a really significant change, depending on the resistor and transformer voltage. There is the potential to overheat the remaining batteries, perhaps damaging them.

What can we do better? The easiest approach is to avoid charging the batteries in series. Instead, use three separate resistors to limit the current to the cells individually, making it so that one cell’s failure wouldn’t significantly affect the next (Figure 2). This could be impractical, though, because the batteries have to be reconfigured from their normal use topology for charging. One other approach might be to use a current source that would deliver roughly the same current even