What are the battery selection criteria for low energy projects?

A battery is an electrochemical device that can store energy in the form of chemical energy. This translates into electrical energy when the battery is connected in a circuit due to the flow of electrons due to the specific placement of chemicals. For more information about how batteries work, you can read the article we have already covered: Introduction to batteries and their types.

A battery is not an ideal finite power source. The energy stored in a fully charged battery cannot be fully supplied to the digital circuit because the amount of energy a battery can supply depends on the current drawn from the battery itself. In other words, the greater the discharge current, the greater the battery's energy waste.

For example, alkaline batteries generally have a lifespan of seven to 10 years. BR and CR lithium batteries can last about 10 to 15 years, and lithium thionyl chloride cells can last more than 20 years.

General terms
Before we proceed further, we must know the following terms.

• Nominal voltage: Voltage of a fully charged cell at the positive and negative terminals of the battery.
• Power/Battery Capacity: The energy stored in a battery is called battery capacity.
• Energy Density: Energy density is the measure of how much energy a battery contains in proportion to its weight. The higher the energy density of the battery, the more expensive the battery technology.
• Self-discharge rate: Batteries don't last forever. Even if they are not used, electrochemical reactions still occur, slowly discharging the battery naturally. This process is called self-discharge rate.
• Shelf Life: Battery life is the period of time a battery can remain in storage without losing its capacity.
• Battery life: This is the run time with a fully charged battery in mAh.

Battery life calculation
Battery life can be calculated using the battery input current and the circuit charging current. Battery life is inversely proportional to charging current, which means it will be high when the charging current is low and will be low if the charging current is high.

Battery capacity can be measured by the following formula:
Battery Life = Battery Capacity (mAh) / Charge Current (mA) * 0.9

*The 0.9 factor takes into account (temperatures, aging, etc.) external factors that may affect battery life.

For example, a 2500mAh (2.5Ah) battery with a device that consumes 500mA (0.5A) you have:
2.5Ah/0.5A * 0.9 = 4.5 hours

Battery Ratings
There are two classifications of batteries:

Primary battery. Primary batteries are commonly known as dry cells. A primary battery is a convenient source of power for electronics and portable devices. They are the “immediately discarded when discharged” type. These batteries are basically the DC power source. The main features of primary batteries are that they are cheap, lightweight, easy to use, and require no maintenance.

Examples are zinc-carbon cells, depolarized metal-air batteries, and alkaline zinc-manganese dioxide cells. The most common primary batteries are 1.5V alkaline batteries (AA, AAA, AAAA, C, D, 9V).

Based on chemical compositions, the types of primary batteries are:

Based on the nomenclature, the types of primary batteries are as follows.

AA batteries
The AA battery is a standard size single cell cylindrical dry battery . It is an extremely common battery and is produced by several brands such as Toshiba, Duracell, etc. It has a dimension of 50.5 mm in length and 14.5 mm in diameter.

AAA batteries
AAA batteries are smaller in size compared to AA, but the capacity of an AA battery is much larger than that of a AAA battery. It has a dimension of 44.5 mm in length and 10.5 mm in diameter.

AAAA batteries
The AAAA battery weighs 43% less, is 40% smaller and is 20% thinner than the AAA battery. The AAAA battery is 42.5mm long and 8.3mm in diameter. These batteries are also rated LR8D425 by IEC and 25A by ANSI/NEDA.

C Batteries

C batteries are a widely used dry battery type that provides a reliable, long-lasting charge for devices with medium to high power consumption needs. Measures 500mm x 26.3mm.

D Batteries

D batteries are large disposable cylindrical cells used for applications that consume occasional power. These are larger than C dry cells. The standard size of a D battery has an approximate length ranging from 58.0 to 61.50 mm.

9V Batteries

This battery has a rectangular shape and has plug-in connectors on the top of the battery. The most common 9V battery in this range is known as the PP3 battery.

CR123A Batteries

This is a cylindrical cell battery widely used for a variety of applications, from medical devices to military-grade technology. They are commonly called the 123 battery. It is 34mm tall and 17mm in diameter.

23A Batteries

An A23 battery is cylindrical and approximately two-thirds the length of a AAA battery. Its dimensions are 28.2 mm in length and 10.0 mm in diameter. An A23 battery is basically an 8-cell device with a nominal voltage of 12 V.

CR2032 Batteries

This is a coin cell battery – also called a button cell – that uses lithium chemistry. Compared to normal AA and AAA batteries, CR2032 lithium batteries are known to produce more efficient and stable power.

Secondary battery
A secondary battery is electrically rechargeable. The most common secondary battery is the lead-acid battery used in automobiles. Examples are nickel-cadmium, nickel-metal hydride and lithium batteries. .

Acid lead
This battery is composed of lead dioxide at the anode and a lead matrix or sponge at the cathode. The electrolyte can be liquid (sulfuric acid and distilled water) or paste or gel with a pressure regulating valve.

Nickel Cadmium
Nickel-Cadmium is a battery composed of a cadmium anode and a nickel hydroxide cathode. The electrolyte is composed of potassium hydroxide. The charging technique for this battery is constant current, as there is no direct relationship between voltage and charge level.

Nickel Metal Hydride
This is a battery made up of a nickel hydroxide anode and a metal hydride cathode. It is a modification or improvement of the nickel-cadmium battery. The main characteristics can be seen in the table below.

Sodium Sulfide

This type of battery with new technology has a sulfur anode and sodium cathode. The electrolyte is an aluminum oxide ceramic compound used as a separator and as an electrolyte.

Lithium-Ion
Lithium-ion batteries are based on compounds with lithium on both electrodes, usually with graphite on the cathode and lithium on the anode. The charge-discharge process is based on the insertion-disinsertion of lithium ions, thus generating the conversion of chemical energy into electrical energy.

Lithium Polymer
The Li-Po battery is an improvement or modification of the lithium-ion battery, but its main characteristic is that the electrolyte is a solid polymer, allowing the creation of tiny batteries because the solid electrolyte takes up less space than the liquid electrolyte.

Battery consideration
Let's look at an example. We have three cases:

1st case: Select a battery for the TV remote control that should work for 1 year and consume much less energy.

So in this case, let's say the IR LED on the remote control should have a battery life of 1 year. Let's assume a person clicks a remote control 100 times a day and each click takes 100 ms. Total remote usage time will be 10,000 ms per day. Let's take the 100 mAh battery.

Battery Life = Battery Capacity (mAh) / Charge Current (mA) * 0.9
Battery Life = 100(mAh) / 20(mA) * 0.9
Battery life = 4.5 hours = 16,200,000 ms
Total number of days = 16,200,000 ms / 10,000 ms = 1,620 days
Number of years the battery can operate = 1620/365 = 4.4 years

Now, even a 100 mAh battery can last more than four years. Therefore, we can consider AA, AAA or AAAA batteries with a capacity less than or equal to 100mAh.

2nd case: Select a battery for cardiac ECG detection so that it can be moved with the patient and is always on to show the patient's vital signs.

Let's look at an example of an IoT device that measures biological signals. It consumes 77mA at 3.8V. It should run continuously for about six hours before discharging.

Battery Life = Battery Capacity (mAh) / Charge Current (mA) * 0.9
Battery Life = 100(mAh) / 20(mA) * 0.9
6 hours = Battery capacity / 77mA * 0.9
Battery capacity = 513mAh

In this case, we will select secondary batteries, such as lithium-ion batteries and nickel-cadmium batteries, which can be used and whose capacity must be greater than 513mAh.

3rd case: We want to select a battery that works for a long time, about 10 years and that consumes moderate energy.

Let's take as an example a fire alarm whose standby load is 250mA, alarm load of 750mA and standby period of 24 hours with 30 minutes in alarm condition:
Capacity = ((Standby Charge Current x Standby Charge Current Time) + 1.75 x (Alarm Charge Current x Alarm Charge Current Time)) x1.25
Capacity = 1.25((24 x 0.25) + 1.75(0.75 x 0.5))
= 1.25((6 + 0.66))
= 8.32 Ah

From this result, at least 9Ah batteries would be needed to maintain this system for the required period.

For this case, we will select primary batteries whose useful life should be around 10 to 15 years and have a capacity of 9Ah. Therefore, we can choose any lithium thionyl chloride composition battery whose power requirement meets our requirements.

Key considerations

We must always know the peak current (this is the maximum amount of current that the output is capable of supplying for brief periods of time) of the device, as the average current consumption does not affect the battery. However, inrush current can have an adverse effect on actual battery capacity, especially if there is not enough time between periods of high current discharge (example: MCU awake) to let the battery rest (example: MCU sleep) and recovering. .

To avoid these periods of high current discharge, you must use a capacitor large enough to supply current to the device when the MCU is active. While the device is at rest, the battery charges the capacitor, and when the device becomes active and requires a high rate of discharge, this capacitor will supply current to the device. This way, the battery would not go through periods of high discharge rate and the efficiency of the battery could be improved.

In conclusion, if the power requirement is low or battery usage is infrequent over a long period of time, a primary battery will be used. If battery usage is frequent (if the primary and secondary batteries are capable of meeting power requirements), the choice will depend on user preference.