6
lfp battery basics
ŸBattery 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 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.
ŸC- and E- 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 50Amps. Similarly, an E-rate
describes the discharge power. A 1E rate is the discharge power to discharge the
entire battery in 1 hour.
Battery Basics
ŸCell, modules, and packs – Electric vehicles have a high voltage battery pack
that consists of 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 connected
in either series or parallel. A battery pack is then assembled by connecting
modules together, again either in series or parallel.
A battery is a device that converts chemical energy into electrical energy and vice
versa. This summary provides an introduction to the terminology used to describe,
classify, and compare batteries for electric vehicles. It provides a basic
background, defines the variables used to characterize battery operating
conditions, and describes the manufacturer specifications used to characterize
battery nominal and maximum characteristics.
Guide to Understanding Battery Specifications
This section describes some of the variables used to describe the present condition
of a battery.
ŸState of Charge (SOC)(%) – 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.
Battery Condition
ŸOpen-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.
ŸState of Health (SOH)(%) – An expression of the present battery Health(Life
expection/Capacity) as a percentage of maximum life span. SOH is generally
calculated using current/voltage/cell resistance integration to determine the
change in battery life span expectation over time.
ŸDepth of Discharge (DOD) (%) – 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.
ŸInternal 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 thecharging energy is converted into heat.
ŸTerminal Voltage (V) – The voltage between the battery terminals with load
applied.Terminal voltage varies with SOC and discharge/charge current.
