Internal Battery Design

The wireless revolution, the prolific use of PDAs, MP3s, MP4s, Laptops, Cell Phones, Smartphones, DVD players, and other portable devices have increased the need for smart and high capacity portable batteries.

Portable batteries however are not typical in design. Indeed the battery that powers your portable device is what is known as a smart battery and as such the internal system design of a smart battery is more complex then most people realize.

To begin with high powered portable devices require an electrical current. There are two types of electrical current (direct current flow and alternating current flow). Direct current means that the flow of charge is in one direction. A battery produces direct current (DC) because there is no way to change the + and – you see on the battery.

In order to create direct electrical current electrons must be caused to break away from atoms to create an electron flow. Why? The answer is because electricity is a property of certain subatomic particles (protons, electrons, and neutrons) which couples to electromagnetic fields and causes attractive and repulsive forces between them; by doing so an electrical flow is created, and this is where electricity comes from. Let’s explain!

Scientists have found ways to create large numbers of positive atoms and free negative electrons (in other words they have found ways to separate electrons from atoms). Since overpopulated proton (positive) atoms want electrons (negative) so they can be balanced, these positive atoms have a strong attraction for electrons. The manufactured disequilibrium creates a state of continuous flow of electrons to atoms with an overpopulation of protons (positive atoms). When electrons move from one atom to another atom a current of electron flow (which is how we get electricity) is created.

This current can then be captured, stored, and used to power a potable device. In a portable battery the creation of electricity begins with a chemical reaction. To cause electrons to break away from atoms a chemical reaction must occur. In PDA batteries for example lithium ion or lithium polymer is used. Lithium is used due in large part to its superior energy density in terms of power per unit of weight and space.

General Characteristics of Lithium

Name: lithium
Symbol: Li
Atomic number: 3
Atomic weight: [ 6.941 (2)] g m r
CAS Registry ID: 7439-93-2
Group number: 1
Group name: Alkali metal
Period number: 2
Block: s-block
Standard state: solid at 298 K
color: silvery white/grey
Classification: Metallic

Lithium is used, amongst many other uses, as a battery anode material (due to its high electrochemical potential) and lithium compounds are used in dry cells and storage batteries. In fact the energy of some lithium-based cells can be five times greater than an equivalent-sized lead-acid cell and three times greater than alkaline batteries. Lithium cells often have a starting voltage of 3.0 V. This means that batteries can be lighter in weight, have lower per-use costs, and have higher and more stable voltage profiles.

In PDA batteries, for example, lithium is converted from chemical energy to electrical energy. This process then makes a battery an electrochemical device that stores chemical energy and releases it as electrical energy upon demand.

Chemical reactions are strongly influenced by their environment. The environment of an internal battery includes design parameters, current requirements, capacity and runtime requirements, temperature requirements, and safety requirements.

Critical to battery design is knowing how much voltage is required? Voltage is the electrical measure of energy. To know the voltage requirements we need to know the upper and lower voltage range (nominal range).

The second key component to know about a battery is its current requirements. PDAs, MP3s and other portable devices, for the most part, utilize a constant power discharge to operate. This means that the amount of current will increase as the battery discharges electricity in order to maintain constant power. So we will need to ultimately know the maximum current required. This is important since knowing the max current requirement will influence the necessary protection of chemistry, circuitry, wire, and capacity amongst others. Again we must know the current requirement over the entire nominal voltage range of the battery including start-up currents, surges (intermittent transient pulses).

One other important aspect to know about current requirements is the inert current drain of the device. Devices, even when powered down, require small amounts of current to power memory, switches and component leakage.

The third key requirement to know is the necessary battery capacity and runtime. This will define the overall physical size of the battery. Capacity and runtime is measured in Amperes. Amps – or A – is an abbreviation of Ampere, a 19th century French scientist who was a pioneer in electricity research. Amps measure the volume of electrons passing through a wire in a one second. The electrical current is measured in amperes, where 1 ampere is the flow of 62,000,000,000,000,000,000 electrons per second!

Amp hours – or Ah – measures capacity. Amp hours is what is ultimately important to consumers as it is the capacity or amp hours that tells us how long we can expect a battery to deliver a charge before it runs out. As with all metric measurements, Amps can be divided into smaller (or larger) units by adding a prefix, in this case by adding an "m" to the amp hour we are renaming the amp hour to milli amp hour: mAh; (1Ah = 1000 mAh).

When we consider the design capacity we must determine the chemical needed to insure that the necessary runtime will be met. Lithium is used because of its electrochemical properties. Lithium is part of the alkali family of metals a group of highly reactive metals. Li reacts steadily with water. In addition the per unit volume of lithium packs the greatest energy density and weight available for this grouping of reactive metals.

Ambient operational temperatures are also important because the internal heat of the battery compartment will dramatically affect the life of a battery. Usage and storage patterns are external effect that will also affect battery life and are the responsibility of a user (for example do not leave your device in a hot car with the windows rolled up, or take your device into a sauna).

A safety requirement for a battery that contains lithium requires protection circuitry to prevent the cells in the battery from conditions like over charge, over discharge, high currents, and or short circuits. Protected circuits consists of integrated circuits (programmed digital circuits), several field-effect transistors (FET) that control the current between two points, and resistors (a two-terminal electronic component that resists the flow of current, producing a voltage drop between its terminals). These circuits add cost and space to the battery pack requirements and careful placement is required in physical layouts to preserve system integrity.

Electromagnetic interference (EMI) or protection from electrostatic discharge is another safety concern. EMI, radiated or conducted, can occur throughout the electromagnetic spectrum. The primary problem with EMI is the disruption of performance of electronics. In wireless devices EMI can cause attenuation losses in signal strength and noise during transmission. Battery packs act as radiated sources of EMI and therefore shielding measures must be taken to reduce and or prevent EMI.

Another aspect of lithium battery design is the concept of smart batteries. A smart battery stores, monitors, prevents, and transmits critical battery information stored within the battery.

A smart battery will communicate with the host device through a connector to provide information about remaining capacity, battery voltage, error conditions, cycles completed, internal temperature, current, and several other factors. A smart battery can request a conditioning cycle, which will fully discharge a battery pack and then recharge it to allow the internal remaining capacity value to be accurately calibrated. Smart batteries often have an LED or LCD display that will allow the user to check the state of charge of a battery prior to use.

Lithium based smart batteries typically use coulomb counting to determine capacity, which means the circuit monitors the capacity in and out of the battery by measuring voltage across a sense resistor. For example 1 coulomb is the amount of electric charge carried by a current of 1 ampere flowing for 1 second. Coulomb counting is based on Coulombs law that states that the magnitude of the electrostatic force between two point charges is directly proportional to the magnitudes of each charge and inversely proportional to the square of the distance between the charges.

This review of the internal design of a battery was extensive. By no means thorough. I hope it offers you a basic under the hood understanding of what is inside your battery and how it works.

Until next time – Dan Hagopian,
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