Batteries are devices that convert chemical energy into electrical energy. There are a deal of specifications in their specs, or datasheet. Sometimes, people may be confused by one certain term which battery suppliers are saying. In this and the next articles, we hope to explain and clarify all common battery specifications, so that you can understand each one.
Battery Basics
- Battery Classifications – Not all batteries are created equal, even 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 not both. Often the battery manufacturers will classify batteries using these categories. Other common classifications are high durability, meaning that the chemistry has been modified to provide higher battery life at the expense of power and energy.
- Primary and Secondary Cells – Although it may not sound like it, batteries for hybrid, plug-in, and electric vehicles are all secondary batteries. A primary battery is one that can not be recharged. A secondary battery is one that is rechargeable.
- Battery cells, modules, and packs – Complex applications like EV or some industrial vehicles have a high voltage battery pack that consists of individual modules and cells organized in series and parallel.
A battery cell is the smallest, packaged unit a battery can take. The voltage of battery cell varies according to different battery type. For a certain type battery, normally the voltage of battery cell is certain. For example, 3V for lead-acid battery cell and 3.2V for LiFePO4 battery cell.
A battery module consists of several cells generally connected in either series or parallel. For example, a 1P4S LiFePO4 battery module is made by 4 pieces LiFePO4 battery cells in series connection.
A battery pack is then assembled by connecting modules together, again either in series or parallel.
- C Rates – 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 batteries. 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 discharge the entire battery in 1 hour. For a battery with a capacity of 100 Amp-hrs, this equates to a discharge current of 100 Amps. A 5C rate for this battery would be 500 Amps, and a C/2 rate would be 50 Amps.
Battery Conditions
- SOC (State of Charge) (%) – An expression of the present battery capacity as a percentage of maximum capacity. SOC is generally calculated using current integration to determine the change in battery capacity over time.
- DOD (Depth of Discharge) (%) – 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.
- Terminal Voltage (V) – The voltage between the battery terminals with load applied. Terminal voltage varies with SOC and charge/discharge current.
- Open-circuit voltage (V) – The voltage between the battery terminals with no load applied. The open-circuit voltage depends on the battery state of charge, increasing with state of charge.
- Internal Resistance – The resistance within the battery, generally different for charging and discharging, 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.
In next article, we will explain the specifications you may see on battery technical specification sheets used to describe battery cells, modules, and packs.