TIMES HAVE CHANGED-

You'd probably be surprised at the number of devices containing some sort of rechargeable battery that you use on a regular basis. Cordless phones, mixers, radio-controlled gear, emergency flashlights, camcorders—and even vacuum cleaners—are just a few examples. The list goes on, but let's just say that rechargeable batteries are popular because of their convenience and reusability.Because you pay more for rechargeable cells, and depend on them for extensive use, it is imperative that you get maximum performance from them. But you can't do that unless you understand their physical nature.

January 1991, thats 19 years ago. What am I accomplishing by working with such ancient circuits? The software for the ancient micro controller an 8749 came on a 5 1/4" floppy for Petes sake. Useless you say.

But we still have, and probably have more bad NiCad battery packs laying around than could be imagined back in 1991. The principles of NiCad batteries and proper conditioning still hold true today. Of course today we simply throw them away, or some people responsibly recycle them, and then go down to Walmart and buy new ones made in China and good for 3 more months.

With our modern technology and the use of new embedded controllers as simple as the Basic Stamp or the Cstamp one can whip up this articles ideas on NiCad battery control and conditioning quite quickly.

As a bonus there are three great circuit designs embedded in this article. There is a great sample and hold circuit, a constant current power resistor circuit , and a bi level LM317 regulator circuit.

Another thing is that over the years as you work with your trade you tend to get in a groove with some principles and just don't think about them anymore, outside of the box I guess one could say. Such is for me on the NiCad battery having a memory and going bad because of it. I just accepted this as something that happened and was the nature of the beast. The author of this article says there is not such thing and "the secret to rechargeable battery life and performance is proper conditioning and use" and I agree.

The secret to rechargeable battery life and performance is proper conditioning and use. To condition a battery, you must discharge the internal cells to a pre determined voltage that is well below the operating level of most electronic equipment and that is beyond the "knee" of the discharge curve. Ni-Cd batteries exhibit a linear discharge rate over the majority of their discharge cycle. However, at some point just before full discharge, the voltage drops off sharply. That sharp downturn in voltage is called the "knee" (see Fig. 1).

Conditioning allows charged electrode material, that is not normally used, to be discharged or "exercised" to prevent premature battery-voltage droop, or kneeing. The premature voltage droop, or premature knee, is commonly mistaken for the battery malady known as "memory." Memory is virtually impossible to create during typical battery use. It takes laboratory-grade equipment and multiple precise charges and discharges to create the memory effect. The performance degradation you perceive as battery memory is actually due to the fact that not all of the charged electrode material in the cells of the battery is available for use by your equipment. That is if the battery has not been cycled, or discharged, sufficiently.

To properly facilitate the cycling process, the battery discharge current and end-point battery cutoff voltage must be carefully monitored to avoid damaging the cells. There are three ways to accomplish this: First, you purchase expensive laboratory-grade equipment specifically made for the purpose. Second, you can set aside a day and cycle the battery manually. But third, you can use the inexpensive, easy to build Battery Tool.

The Battery Tool is a microcontroller-based instrument that performs a controlled, user-determined, constant-current discharge on any type of rechargeable battery. Battery voltage can be as high as 18 volts, and the maximum discharge current can be set as high as 1.5 amperes..These maximums will accommodate most consumer batteries now in use. The Battery Tool monitors battery voltage and regulates the user-selected load current during the entire discharge cycle. Using the accompanying terminal program, you can determine such real-time parameters as 50% battery life voltage, 0% battery life voltage, battery capacity, knee voltage, and battery voltage under load. The Battery Tool also provides elapsed time and initial no-load battery voltage readings.

Since a history of battery performance is vital to determining when the battery is fully discharged or will not be able to provide useful service, the Battery Tool terminal program can save all of the above parameters to a disk file for retrieval and comparison later. The data collected during discharge can also be used to plot a typical battery discharge curve. If you're like most electronic experimenters, you have a gaggle of Ni-Cd's and chargers lying around. The Battery Tool can help determine if they are good or bad and, if they are good, what their capabilities are. Another plus for the Battery Tool is that you can build it for less than $100.

What does it all mean?

Figure 2 shows a completed Battery Tool test. Note that all of the parameters are included and saved in a file. The idea is to compile a history of battery performance. As the battery wears out, or if you accidentally abuse it, you can retrieve the history and determine just how much wear or damage has resulted. By using the Battery Tool, you'll never again have to guess about the condition of a particular rechargeable battery.

Let's talk about what all those real-time event readings tell you. The "test current" is the amperage drawn from the battery during the test. "Elapsed time" is the time it took to run the test. The "50% capacity voltage" represents both the average overall battery voltage during a test and the voltage at the point where half of the battery's useful charge is left. The 50% value is dependent upon the cutoff voltage you specify. Use your best judgment or, better yet, consult the manufacturer's recommendations when selecting your battery's cutoff voltage.

The "0% capacity voltage" is a calculated measurement that under load that would be read when all usable battery energy is depleted. The projected reading is based on conventional Ni-Cd battery formulas involving the 50% calculation. The zero-capacity condition should occur after the knee has formed.

The "knee voltage" defines the voltage point at which the characteristic knee will occur. The value is calculated by taking into account the 50% battery voltage versus time.

The "battery capacity" is just that. That is, if you were to look at your particular cells closely you would find a manufacturer's capacity rating or rated cell capacity. On a AA Ni-Cd cell that's usually between 450 to 550 milliampere-hours. That says under normal temperature and load conditions, the cell should be able to deliver the rated current for 1 hour. That may be true for new cells, but wear and misuse can reduce the performance figure. The Battery Tool gives you the real-world performance figures so you can most effectively use the chemical energy supplied by the battery. The Battery Tool calculates battery capacity every 60 seconds using the user-defined load current versus time.

The "battery no-load voltage" is the voltage measured with the battery at rest with no resistive load applied. Its only purpose is to give the user an indication of what the battery voltage is before loading.

Theory of operation

As shown in Fig. 3, the Battery Tool is based on IC2, an 8749H microcontroller, running at a clock speed of 10 MHz. The 8749H performs serial I/O, analog-to-digital processing, and battery-monitoring functions, as well as supplying the clock source for the analog-to-digital converter by executing a program contained in its internal EPROM.

I will now add in my own explanation in 2010 terminology as one would probably not obtain the software or be able to wire up the 8749 as shown. Also we have many embedded controllers that can do a much better job these days than the circuit used in this project. It was quite the project in 1991, nearly 20 years ago. Even a Basic Stamp or a Cstamp would do a great job these days. You can always consult the PDF for the full article and explanation.

 

The LM317 regulator(IC6) provides a reference voltage. If Q1 is turned on the regulator will supply +2.56V for a current measurement reference. If Q1 is off the LM317 will supply +5V for a voltage measurement reference. The potentiometers R2 and R3, resistor R1, and bypass capacitor C3 are used to setup the variable regulator LM317(IC6). These reference voltages are necessary for the use of the the 8 bit AD converter used, the ADC0809. If you use modern embedded controllers they have built in 16 bit AD converters eliminating some work. But getting two reference voltages this way with a LM317 is a very good thing to tuck away as a good circuit.

The next useful operation of this circuit is in the area of the 3900 comparator and Q2. The LM3900 (IC4) forms a low-drift ramp-and-hold circuit. This is necessary to draw a constant current from the Battery through R10 the 1R0 load resistor across the battery.R5 and R6 inputs are logic level control inputs. They are marked on the schematic as DOWN and UP . If R5 and R6 are both LOW the output of IC4-b is stable and the circuit in in a HOLD state. Applying a HIGH to R5 causes the output of IC4-b to rise. And conversly applying a high to R6 causes the output to become lower.

The MOSFET Q2 is simply functioning as a high wattage potentiometer whose wiper is the voltage supplied from IC4-b. As the voltage on the gate of Q2 increases the resistance between Q2's drain and source decreases. So if we control Q2 properly we are setting up a constant current load across the battery INDEPENDANT of the batteries voltage and operating temperature.

The three resistors R11, R12, and R13 form a divider for reading the voltage across the battery. The A/D converter used in this circuit, ADC0809 is an 8 bit converter. With the tap off of R13 one could read a max of +20 Volts DC. This would give a resolution of 0.0195 Volts per step.

 

What a wonderful circuit. It is a wonder to me sometimes when I see something like this of why I didn't think of doing this. Many times I have wanted to have a variable power resistor for a load across something I am testing. Into the junk box I go searching for a big power resistor. Just pop this little thing in there and I will have it made. Even that ramp and Hold circuit is kind of neat.

Revised 2013 by Larry Gentleman