Basic Battery knowledge

Battery
A device that converts energy, by chemical reaction or physical reaction, into electric current.
Primary Battery
Energy is exhausted when active materials are consumed (carbon-zinc dry cell, lithium battery, silver oxide battery, alkaline battery)
Secondary Battery
Active materials are regenerated by charging (nickel cadmium (NiCd), nickel metal hydride (NiMH), Lithium Ion, Lithium Polymer, Sealed Lead Acid.
Series Connection
Connection of a group of battery cells by sequentially interconnecting the terminals of opposite polarity thereby increasing the voltage of the battery group but not increasing capacity (i.e. positive to negative connections).
Parallel Connection
Connection of a group of batter cells by interconnecting all terminals of the same polarity, thereby increasing the capacity of the battery group but not increasing the voltage (i.e. positive to positive and negative to negative).
Cadmium
Chemical symbol Cd. This metallic element is the chemically active material of a nickel cadmium battery's negative electrode. When the battery is charged, the negative electrode surface consists of cadmium. As the battery discharges, the cadmium progressively changes into cadmium hydroxide (Cd (OH2)).
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Cadmium Hydroxide
Active material used at the negative electrode of the Nickel-Cadmium Cell.
Metal Hydride
A general name for chemical compounds consisting of metal elements and hydrogen.
Nickel Hydroxide
The active material in the positive electrode of NiMH and NiCd batteries.
Nickel Oxyhydroxide
The chemical name of NiOOH. Indicates that oxidation of Ni (OH)2 has progressed, and that the active material of the positive electrode of an NiCd or NiMH battery is charged.
Capacity
The quantity of electricity that can be obtained from a battery in one cycle from full charge to full discharge when the battery is discharged under conditions of rated current level and ambient temperature within the predetermined range. Generally, capacity is expressed in units of mAh (milliampere-hour).
Nominal Capacity
The standard capacity designated by a battery manufacturer to identify a particular cell model.
Nominal Voltage
The standard voltage used to express the capacity of a particular battery model. It is generally equal to its electromotive force or its approximate voltage during normal operation. Typical Values:
1.2 volts per cell for NiCd and NiMH
3.6 or 3.7 volts per cell for Lithium Ion or Lithium Polymer
3 volts per cell for lithium primary
2 volts per cell for sealed lead acid
1.5 volts per cell for alkaline and carbon zinc
Discharge Rate
The discharge rate is the rate at which current is removed from a battery. When a battery is discharged at a current level "i", for a period until the end discharge voltage is '"h", the discharge is referred to as the h-hour rate discharge, while "i" is known as the h-hour rate discharge current. For practical use, nominal capacity is used as standard.
End-Voltage
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The voltage that indicates the end limit of discharge. This voltage is almost equivalent to limitation of practical use. Typical values:
1.0 volt per cell for NiCd and NiMH
1.75 volts per cell for sealed lead acid
2.75 volts per cell for lithium ion and lithium polymer
2.0 volts per cell for primary lithium
0.9 volts per cell for alkaline and carbon zinc
Open circuit voltage
The voltage between terminals of a battery without any load.
Operating voltage
The voltage between terminals when a battery is subjected to a load. Usually expressed by the voltage of the battery at 50% discharge point.
Polarity Reversal
Reversing of polarity of the terminals of a small-capacity cell in a multi-cell battery due to overdischarge.
Positive Electrode
The electrode which has a positive potential. Electric current from this electrode flows in the external circuit during discharge.
Negative Electrode
The plate which has an electrical potential lower than that of the other plate during normal cell operation. Electric current from the external circuit flows into the cell at the negative electrode during discharge. Also called minus electrode.
Self-Discharge
a decrease in battery capacity which occurs without any current flow to an external circuit. Typical values:
1% per day for NiCd
2% per day for NiMH
~0% per day for Lithium Ion and Lithium Polymer
Short Circui
Directly connecting the positive electrode (terminal) to the negative electrode (terminal) of the battery.
Thermistor
A circuit element with a negative temperature coefficient. It is built into batteries and used to detect ambient temperature or battery temperature. A battery charger may use this device to properly charge a battery.
Is the runtime of a portable device directly related to the size of the battery? The answer should be 'yes' but in reality, the runtime is governed by other attributes than the specified capacity alone.

This paper examines the cause of unexpected downtime and short battery service life. We look at four renegades - declining capacity, increasing internal resistance, elevated self-discharge and premature voltage cut-off on discharge. We evaluate how these regenerative deficiencies affect nickel, lead and lithium-based batteries.

Declining capacity
The amount of charge a battery can hold gradually decreases due to usage and aging. Specified to deliver 100% capacity when new, the battery should be replaced when the capacity drops to below 80% of the nominal rating. Some organizations may use different end-capacities as a minimal acceptable performance threshold.

The energy storage of a battery can be divided into three imaginary sections consisting of: available energy, the empty zone that can be refilled, and the unusable part (rock content) that increases with aging. Figure 1 illustrates these three sections.
Figure 1: Battery charge capacity. Three imaginary sections of a battery consisting of available energy, empty zone and rock content.
Increasing internal Resistance
The capacity of a battery defines the stored energy - the internal resistance governs how much energy can be delivered at any given time. While a good battery is able to provide high current on demand, the voltage of a battery with elevated resistance collapses under a heavy load. Although the battery may hold sufficient capacity, the resulting voltage drop triggers the 'low battery' indicator and the equipment stops functioning. Heating the battery will momentarily increase the output by lowering the resistance.

A battery with high internal resistance may still perform adequately on a low current appliance such as a flashlight, portable CD player or wall clock. Digital equipment, on the other hand, draw heavy current bursts. Figure 2 simulates low and high internal resistance with a free-flowing and restricted tap.
Figure 2: Effects of internal battery resistance .A battery with low internal resistance is able to provide high current on demand. With elevated resistance, the battery voltage collapses and the equipment cuts off.

Nickel-cadmium offers very low internal resistance and delivers high current on demand. In comparison, nickel-metal-hydride starts with a slightly higher resistance and the readings increase rapidly after 300 to 400 cycles.

Lithium-ion has a slightly higher internal resistance than nickel-based batteries. The cobalt system tends to increase the internal resistance as part of aging whereas the manganese (spinel) maintains the resistance throughout its life but loses capacity through chemical reaction. Cobalt and manganese are used for the positive electrodes.

High internal resistance will eventually render the battery useless. The energy may still be present but can no longer be delivered. This condition is permanent and cannot be reversed with cycling. Cool storage at a partial state-of-charged (40%) retards the aging process.

The internal resistance of Lead-acid batteries is very low. The battery responds well to short current bursts but has difficulty providing a high, sustained load. Over time, the internal resistance increases through sulfation and grid corrosion.

Elevated self-discharge

All batteries suffer from self-discharge, of which nickel-based batteries are among the highest. The loss is asymptotical, meaning that the self-discharge is highest right after charge and then levels off. nickel-based batteries lose 10% to 15% of their capacity in the first 24 hours after charge, then 10% to 15% per month afterwards. One of the best batteries in terms of self-discharge is Lead-acid; it only self-discharges 5% per month. Unfortunately, this chemistry has the lowest energy density and is ill suited for portable applications.
lithium-ion self-discharges about 5% in the first 24 hours and 1-2% afterwards. Adding the protection circuit increases the discharge by another 3% per month. The protection circuit assures that the voltage and current on each cell does not exceed a safe limit. Figure 3 illustrates a battery with high self-discharge.
Figure 3: Effects of high load impedance. Self-discharge increases with age, high cycle count and elevated temperature. Discard a battery if the self-discharge reaches 30% in 24 hours.
The self-discharge on all battery chemistries increase at higher temperatures. Typically, the rate doubles with every 10¡ãC (18¡ãF). A noticeable energy loss occurs if a battery is left in a hot vehicle.

Aging and usage also affect self-discharge. nickel-metal-hydride is good for 300-400 cycles, whereas nickel-cadmium may last over 1000 cycles before high self-discharge affects the performance. An older nickel-based battery may lose its energy during the day through self-discharge rather than actual use. Discard a battery if the self-discharge reaches 30% in 24 hours.

Nothing can be done to reverse this deficiency. Factors that accelerate self-discharge are damaged separators induced by crystalline formation, allowing the packs to cook while charging, and high cycle count, which promotes swelling in the cell. Lead and lithium-based batteries do not increase the self-discharge with use in the same manner as their nickel-based cousins do.
Premature voltage cut-off
Not all stored battery power can be fully utilized. Some equipment cuts off before the designated end-of-discharge voltage is reached and precious battery energy remains unused. Applications demanding high current bursts push the battery voltage to an early cut-off. This is especially visible on batteries with elevated internal resistance. The voltage recovers when the load is removed and the battery appears normal. Discharging such a battery on a moderate load with a battery analyzer to the respective end-of-discharge threshold will sometimes produce residual capacity readings of 30% and higher, jet the battery is inoperable in the equipment. Figure 4 illustrates high cut-off voltage.
Figure 4: Illustration of equipment with high cut-off voltage.Some portable devices do not utilize all available battery power and leave precious energy behind.

High internal battery resistance and the equipment itself are not the only cause of premature voltage cut-off - warm temperature also plays a role by lowering the battery voltage. Other reasons are shorted cells in a multi-cell battery pack and memory on nickel-based batteries.
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