All matter on Earth is made up from molecules and they basically consist of atoms. These atoms are made of electrons, protons and neutrons. And electricity is about the flow of free electrons attracted to protons and repelled by other electrons.
A battery is a device that uses these properties to convert chemical energy into electrical energy and vice versa. This article provides and introduction to the terminology used to describe, classify, and compare batteries as they are used today in electronics, cars and even small aircraft.
With this basic background, we look at the variables used to characterize battery operating conditions and describes the manufacturer specifications used to characterize battery characteristics.
This article touches on some basic battery stuff we as an aircraft builder should be at least a little bit familiar with as we see more and more batteries in electronics devices in our aircraft too.
Vehicles or devices using batteries, normally have a high voltage battery pack that constructed of several individual modules and cells organized in series and parallel. A cell is the smallest, packaged form a battery can take and is generally on the order of one to six volts. A module consists of several cells generally combined in either a series or parallel configuration. A battery pack is then assembled by connecting modules together, again either in series or parallel.
Connecting in series increases the voltage of the pack and in parallel increases its current capability. Either way, power goes up . With a higher voltage the current is lower at the same power level. This reduces wiring size and weight.
Like humans, not all batteries are created equal, thats even true with batteries of the same chemistry. The main trade-off in battery development is between power and energy: batteries can be either 'high power' or 'high energy', but you can not have both at the same time. Often manufacturers will classify batteries using these categories. Other common classifications are high durability and cycle life, meaning that the chemistry has been modified to accommodate either durability of charge/ discharge cycles.
It is obvious that a battery with high power, high density, high recharge cycles or durability and low cost will never exist. Depending on the application, the manufacturer will choose a suitable chemistry.
In describing batteries, discharge current is often expressed as a C-rate in order to normalize against battery capacity, which is often very different between battery chemistries. A C-rate is a measure of the rate at which a battery is discharged relative to its maximum capacity. A 1C rate means that the discharge current will theoretically empty the entire battery in 1 hour.
Battery cells are either primary (non-chargeable) or secondary, re-chargeable. Easy enough.
Below a description of some of the variables used to show the current condition of any battery.
An expression of the present battery capacity as a percentage of maximum capacity. The SoC is generally calculated using current integration to determine the change in battery capacity over time.
The percentage of battery capacity that has been discharged expressed as a percentage of maximum capacity. A discharge to at least 80% DoD is referred to as a deep discharge.
The voltage between the battery terminals with a load applied. The terminal voltage varies with the state of charge and discharge/charge current.
The voltage between the battery terminals with no load applied. The open circuit voltage depends on the battery state of charge, increasing with that and is non-linear.
The resistance within the battery, which is generally different when charging and discharging, is also dependent on the battery state of charge. As internal resistance increases, the battery efficiency decreases and thermal stability is reduced as more of the charging energy is converted into heat. Maximum Internal Resistance is also known as the resistance within the battery, which is generally different for charging and discharging.