Week 3 Challenge

A battery pack is a set of any number of (preferably) identical batteries or individual battery cells.[1] They may be configured in a series, parallel or a mixture of both to deliver the desired voltage, capacity, or power density. The term battery pack is often used in reference to radio-controlled hobby toys and battery electric vehicles.

Components of battery packs include the individual batteries or cells, and the interconnects which provide electrical conductivity between them. Rechargeable battery packs often contain a temperature sensor, which the battery charger uses to detect the end of charging. Interconnects are also found in batteries as they are the part which connects each cell, though batteries are most often only arranged in series strings.

When a pack contains groups of cells in parallel there are differing wiring configurations which take into consideration the electrical balance of the circuit. Battery regulators are sometimes used to keep the voltage of each individual cell below its maximum value during charging so as to allow the weaker batteries to become fully charged, bringing the whole pack back into balance. Active balancing can also be performed by battery balancer devices which can shuttle energy from strong cells to weaker ones in real time for better balance. A well-balanced pack lasts longer and delivers better performance.

For an inline package, cells are selected and stacked with solder in between them. The cells are pressed together and a current pulse generates heat to solder them together and to weld all connections internal to the cell.

An advantage of a battery pack is the ease with which it can be swapped into or out of a device. This allows multiple packs to deliver extended runtimes, freeing up the device for continued use while charging the removed pack separately.

Another advantage is the flexibility of their design and implementation, allowing the use of cheaper high-production cells or batteries to be combined into a pack for nearly any application.

Battery Basics

• Cell, modules, and packs – Hybrid and 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.

• 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 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.

• C- 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 50 Amps. Similarly, an E-rate describes the discharge power. A 1E rate is the discharge

power to discharge the entire battery in 1 hour.

• Secondary and Primary 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 Condition

This section describes some of the variables used to describe the present condition of a battery.

• State 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.

At the end of product life, batteries can be removed and recycled separately, reducing the total volume of hazardous waste.

Packs are often simpler for end users to repair or tamper with than a sealed non-serviceable battery or cell. Though some might consider this an advantage it is important to take safety precautions when servicing a battery pack as they pose a danger as potential chemical, electrical, and fire risks.

• Depth 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.

• Terminal Voltage (V) – The voltage between the battery terminals with load applied.

Terminal voltage varies with SOC and discharge/charge 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.