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Almost dead batteries power electronic circuits

The electrical energy of a battery cell is produced by a chemical reaction between its anode, cathode and electrolyte materials. It is worth noting that, in battery terminology, the positive terminal is the cathode, and the negative terminal is the anode.

When you operate your electric circuit, the current required by your circuit varies according to what you do with it. So, the typical current required is less than the maximum current. Theoretically batteries are supposed to have a voltage which doesn't depend on the current you draw from them, but this is only an approximation. In fact, all batteries have a negative V-I curve; if you increase the current taken from them, their voltage decreases. This effect is like having an equivalent serial resistor (ESR) in series with the battery. It is called the impedance. Usually it is a very small effect but the dead voltage sensor in your camera can be very sensitive; small effects can make a big difference.

Obviously all preppers are interested in getting off the grid, saving energy, or creating their own energy, but let’s say that you need to light up your campsite and all you have are several “dead” batteries. Did you know that a typically, the most 1.5V AA battery, is usually considered dead when it’s down to anywhere from 0.9V to 1.3V?

It all depends on the circuit that your device uses, but you can assume that it’s dead after about 20% of its voltage is used. So that means that almost 100% of the batteries out there that have been thrown away as dead, aren’t really dead. Of course, that doesn’t really mean much when those batteries can’t be used in the device that you want. Careful selection of components and circuit design can take you to the next level in extreme low-power design. However, a microcontroller lies at the heart of many embedded designs. The microcontroller normally is there to run a program, making the device operate as intended. Not to be overlooked, though, microcontroller manufacturers have added many features that aid in low power consumption.

In a battery-powered system, time is the critical parameter. Unlike ac-powered systems, where supply voltage varies within a specified range and the availability of rated current is unlimited in duration, a battery can only supply power for a finite length of time before it requires recharging or replacement. In addition, as the battery discharges, the greater the current drain, the greater the drop in battery voltage (Figure 1).

Figure 1

Cell discharge as a function of current discharge rate.

Cell discharge as a function of current discharge rate.

The key to designing an efficient battery-operated system then is (a) to maximize battery life by minimizing the current drawn by the circuit, especially the continuous “quiescent current”; and (b) if necessary, to maintain the voltage supplied to the load at a constant level during discharge by using some form of regulating circuit between the battery and the load. For example, an AA –type battery, with a capacity of 100 mA-hours powering a circuit that draws 1 mA, will operate for approximately 100 hours before recharging or replacement is required. If this quiescent current is reduced to 100 μA, the battery life ideally increases to about 1,000 hours. If we use the battery-powered circuitry with quiescent current up to 1 μA, the battery life would be increased to 10-11 years. That is the reason to use op-amps and dc-dc converters with the quiescent current as low as possible. For example, companies like TI and Linear Technology have such new devices with the quiescent current 400nA-2000nA.

Before designing a battery-operated system, it is important to understand the environment, requirements, and operating conditions under which the system will be used; this will allow the designer to determine what type of battery should be used (for example, primary or secondary), and how often the batteries would need to be replaced or recharged.

Extending Battery Life: Three ways to extend battery operation are: (1) Minimize the quiescent current if continuous operation is needed; (2) Pulse the load on and off so that the battery operates on a lower duty cycle; and (3) Power down the circuit when not in use.

3 comments on “Almost dead batteries power electronic circuits

  1. Victor Lorenzo
    October 15, 2017

    Some time ago I read about one little gadget, the Batteriser.

    It is covered by pattent US 20120121943 A1 (https://www.google.com/patents/US20120121943)

  2. D Feucht
    October 21, 2017

    The article began to talk about how “dead” batteries have a useful amount of voltage but then changed to minimization of battery current for life extension. The more basic goal of getting as much energy out of the battery as possible is aided by switching converters that can operate down to a low input voltage, lower than the load can operate from. This is a good topic in itself in that some switching converter circuits are better than others in operating efficiently from a low input voltage; the push-pull boost is one, though few understand how it works. It has a variation called a Weinberg converter.

  3. JasonR
    October 24, 2017

    Very good point. Low input voltage makes much more sense from a logical standpoint.

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