Nuts and Volts - June 2017

June 2017 7

A quick tour shows that Q1 is the temperature sensor. This should be physically attached to the device that you want to be cooled. R2 controls how much gain there is for the thermal sensing. Q3 and Q4 form a threshold comparator, and R3 sets the comparison voltage. D1 is the voltage reference.

In this way, you can set the point at which the circuit starts to react. Q3 is also a common-emitter amplifier for the thermal signal, and that controls the current source made by Q2: a Darlington pair in a single package. The collector of Q2 goes to the fan, controlling its speed.

NiCd to NiMH

QNow that Ni-Cad batteries are becoming an endangered species, is there any way to convert their chargers to work on the newer and ecologically sounder Ni-Metal Hydride batteries? I read that the Ni-Metal batteries have very different charging requirements rather than the constant current chargers for Ni-Cads.

I have several bench chargers, not to mention a multitude of wall warts that I would like to modify.

Michael Herman La Quinta, CA

ANickel-cadmium or NiCd batteries are certainly becoming more scarce. It’s unfortunate in some sense because they have some significant advantages over nickel-metal hydride (or NiMH) batteries. While NiCd batteries exhibit lower energy density, they generally have lower self-discharge rates. There are some NiMH batteries that are specifically designed for low self-discharge, however.

NiCd batteries are also very robust. They can handle high discharge currents for longer, and can generally take a lot of abuse. I have a NiCd battery pack on a Handi-Talkie that is probably about 25 years old and still going strong.

I use that Handi-Talkie on a weekly basis. I always worry about how warm the battery pack gets when I charge it, The biggest downside of NiCd batteries is the toxicity of the cadmium making up the negative electrode. Cadmium is a metal that is toxic to most higher lifeforms.

It is also more expensive than alternatives. Because of its toxicity, the EU has completely banned them for consumer use, though they still find applications in commercial and medical gear because of their desirable characteristics.

Unfortunately, the charging requirements for NiCd batteries are different than NiMH. 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. Regardless, this is different from NiMH charging methods.

NiMH batteries generally are charged with two different methods: delta-V and delta-T. For delta-V charging, the voltage is monitored while a constant current is applied. When the voltage drops slightly, this is an indication that the battery is fully charged (see Figure 2). Note that NiCd batteries can be charged this way as well, but it really depends on the charger, and the voltage drop characteristic is slightly different. It’s not uncommon for a NiCd charger to just continue to apply current even after reaching full charge. NiCd batteries will not be damaged by the heat generated from this overcharging. NiMH batteries, however, will eventually be damaged.

For delta-T charging, the battery temperature is monitored. As a NiMH cell is charged, energy is stored in the chemical state within the battery. As full charge is reached, the excess energy not going into the chemistry is released as heat instead. By monitoring the temperature, the full charge state can be detected and charging terminated. Many chargers for NiMH batteries use both methods to avoid overcharging. As above, overcharging is not good for that chemistry.

The wind up is that NiCd chargers generally do not have the necessary sensing to charge NiMH batteries properly. There are many chargers for sale that will charge both types (Maha Powerex makes several), but I think it would be difficult to repurpose NiCd chargers for NiMH. In general, it’s a good idea to use chargers appropriate for the battery chemistry involved. As I’ve mentioned before in this column, improper battery charging can cause heating, outgassing of flammable or explosive gasses, and even fires. Please be careful.

Clean Automotive Power

QMany hobbyists (like myself) are playing with circuits used in automobile applications. The voltage in a 12V automotive environment is particularly dirty. I would like to see an article on how dirty the automotive electrical voltage supply is and why, and I’d also like to see ways of cleaning and stabilizing the 12V power in an automobile for circuit applications.