Objective

1. To learn the specific charge/discharge characteristics of a Nickel-Cadmium (Ni-Cad) battery through experimental testing of a remote triggered Ni-Cad battery.

2. Each type of battery chemistry, whether it be nickel-cadmium, nickel metal hydride, lead acid, lithium, or others has specific characteristics that define its electrical operation, size, weight and other properties. This experiment introduces the student to some of the electrical characteristics of a NiCad battery. Specifically, we will investigate: 

• Charge and discharge curves - Amongst others, these curves can be used for: 
  • Determing algorithms for safe charging and discharging since over-charging or over-discharging batteries can reduce the lifetime of batteries, damage them, or even lead to fire and explosion,
  • Understanding the float behavior of NiCad batteries, or how the voltage of a battery changes when a charge or discharge process is stopped.
• Energy capacity vs. discharge rate is an important design parameter for NiCad based energy storage systems. NiCad batteries were used extensively in portable power tools and other portable devices. The energy capacity vs. discharge rate affects the weight, size, and cost of a device such as a handheld power tool. Amongst others, the energy capacity vs. discharge rate is useful for:
  • Sizing a battery for a device, by understanding the usable capacity of the battery which changes as a function of the discharge rate,
  • Identifying the duration for which a device or system can operate off battery power by getting the total remaining energy (SOC*Full Energy) by (loaded voltage*current)

Background and Theory

A battery is an electrochemical device in which electrical energy is converted and stored in chemical form for storage. The chemical energy can then be easily reconverted into electrical energy.
Two primary types of chemical batteries exist: Primary and secondary. A primary battery is not normally rechargeable and is designed to only last one discharge cycle, after which it must be replaced. Secondary batteries are rechargeable. They can be discharged and recharged repeatedly.
As we are all aware, a significant number of the modern electronic equipment we take for granted every day, such as mobile phones, laptop computers, music players, cameras and countless others are powered from rechargeable batteries.

 

Basic Battery Operation

Two electrodes (positive and negative, made of two chemically different materials) are separated by an electrolyte - a solution that easily conducts ions (charged particles)

An Electrical Load is applied to the cell, causing the cell to discharge.

• Electrons are pulled from the positive terminal of the battery through a chemical reaction between the positive terminal and the electrolyte

• Electrons flow through the electrical load

• Electrons return to the negative terminal

• Electrons are put back into the negative side of the battery through a chemical reaction between the negative terminal and the electrolyte

• Battery becomes discharged when the chemical reactions are not possible any longer – the chemicals have all been transformed into other chemicals that do not support electron producing chemical reactions

 

Rechargable Batteries

In many batteries, the chemical reactions are reversible when voltage is applied to the battery (Charging). Rechargeable batteries are also called Secondary batteries, as opposed to Primary batteries, which are single use only.

More Battery Basics

The voltage of an individual cell is fixed by battery chemistry.
The current is a function of the rate of chemical reaction in the battery, which is characterized by the Equivalent Series Resistance (ESR). Then from Ohm’s law, we can see that for a fixed voltage, the current is controlled by the resistance.

Current = Voltage / Resistance = V / ESR
The capacity of the battery is defined as
Capacity = (Voltage) * (Amp-hours).

The Amp Hours is the number of Amps that a battery can produce for an hour
OR
the number of hours a battery can produce one Amp.

For example, if the battery has a 10 Ah (Amp hour) rating, it can provide:1 Amp for 10 hoursOR10 Amps for 1 hour.The capacity is usually defined at a standard charge/discharge rate (C-rate), which is the the charge/discharge rate (in Amps) that the battery will provide for the specified number of hours. For example, under discharge, C/10 = 5.2 A implies that the battery will provide 5.2 Amps for 10 hours.
The capacity usually increases for lower charge/discharge currents and decreases for higher charge/discharge currents.

 

Series and Parallel Connection

 

When connected in series, the battery voltages add

– - Positive terminals of one battery connected to the negative of another, and so on

When connected in parallel, the battery currents add

– - Positive terminals of all the batteries connected together, negatives all connected together

Multiple cells are connected in series to obtain higher voltages

 

 

 

 

 

Multiple cells are connected in parallel to obtain higher currents.

 

 

 

 

 

 

 

Mixed – combination of parallel and series connections can be used to obtain the desired

– Voltage (Volts)

– Current capability (Amps)

– Capacity - net energy storage (Amp-hours.

 

Nickel-Cadmium Batteries

Nickel- Cadmium batteries were invented in 1899 in Sweden. However, widespread commercial use did not occur the 1960’s.The chemical reactions governing Nickel-Cadmium batteries during discharge are given below.

 

                       Negative electrode:

                       Positive electrode:  

                       Net reaction:

These reactions are reversible during charging, and the equations will flow from right to left.

 

 

Some of the most important characteristics of Ni-Cad batteries are given in the table below: 

 

 

 

Nickel-Cadmium Battery Characteristics
Nominal Single Cell Voltage	1.2 V
Specific Energy	40-60 Wh/kg
Energy Density	50-150 Wh/L
Specific Power	150 W/kg
Charge/Discharge Efficiency	70-90%
Self-Discharge Rate	10% per month
Number of Cycles	2000

 

 

 

 

 

Ni-Cad batteries have a nominal single cell voltage of 1.2 V, which is fixed by the battery chemistry. In order to obtain higher voltages, cells are put together in series. Ni-Cad batteries typically replace Alkaline batteries in portable electronics. While Alkaline batteries have a higher nominal voltage, at 1.5 V, their voltage also falls quickly with discharge, reaching a minimum of approximately 0.9 V when fully discharged. On the other hand, Ni-Cad batteries have a nearly flat discharge curve, which only drops off after reaching a nearly fully discharged state. Therefore, a Ni-Cad battery has a similar voltage to the average voltage of an Alkaline battery and can thus replace Alkaline batteries in consumer electronics even thought their nominal voltage is substantially lower.

 

The Specific Energy refers to the amount of energy that can be stored per unit weight. This value is very important for portable equipment as heavy batteries will be difficult and energy consuming to move around. The Specific Energy of Ni-Cad batteries is much higher than for lead-acid batteries. It is however lower than for Nickel Metal Hydride (NiMH) batteries and especially for Lithium batteries. As a result, Ni-Cad batteries became very popular for use with portable electronic devices in the 1990’s, but have since been supplanted with NiMH and Lithium batteries as the costs for these two have dropped to become competitive with Ni-Cad.

 

Energy Density describes how much energy can be stored per unit volume. Again, for portable electronic equipment, the space required for a given storage capacity is an important figure. The Energy Density for Ni-Cad batteries is much lower than for Alkaline and Lithium batteries, but is comparable with lead-acid and NiMH batteries.

 

Specific Power refers to the maximum amount of power can that can be delivered. In electrical terms, this is the maximum Discharge Rate of the battery. For high power applications, such as power tools, robotics and remote control toys and other applications in which motors are being driven, the Discharge Rate is a primary consideration that needs to be taken for choosing a battery technology. One of the strongest advantages of Ni-Cad batteries is its durability under high discharge rates. The Specific Power also plays a role in the maximum charge rate, which is essential for quick recharge times.

 

The Charge/Discharge efficiency is also an important factor for the practical use of the battery. Due to their high Specific Power, Ni-Cad batteries typically have a higher Charge/Discharge efficiency, especially at high rates, than lead-acid and other battery types.

 

 

 

 

 

Ni-Cad batteries have a lower Self-Discharge Rate, at 10%, than other similar batteries, such as NiMH, leading to a longer shelf-life. For this reason, Ni-Cad batteries are often chosen in applications where the intended discharge is less than the self-discharge rate. This occurs in devices such as television remote controls and calculators. Previously, Ni-Cad batteries were used extensively in portable, handheld power tool applications such as cordless drills and also in early generation laptop computers. They were later supplanted in these applications by NiMH batteries and then later by Lithium batteries. 

 

The lifetime of Ni-Cad batteries is also long compared with other battery types. Nickel-Cadmium batteries can endure as much as 2000 cycles while still maintaining 50% capacity.

 

One disadvantage of Nickel-Cadmium batteries is their claimed “memory effect”. The memory effect refers to a degradation in battery capacity that occurs when the batteries are repeatedly charged only to partial capacity. The battery will remember this low capacity and think it is full when only partially charged, leading to a reduction in capacity. However, there are many contradictory claims about the memory effect in Ni-Cad batteries, with many people claiming it was more a rumor due to specific and frequent, precisely repeated partial charge/discharge cycles in space satellites. Newer Ni-Cad batteries show very little of this property.

 

A second disadvantage of Nickel-Cadmium batteries is that Cadmium, which is normally 6 to 12 percent of the battery by weight is highly toxic. End of life and recycling concerns cause this to be a large problem. To this end, the European Union has restricted the sale of Ni-Cad batteries to certain market segments, such as medical equipment, in which they have a strong advantage.

 

A battery can be thought of as a voltage source in series with an internal resistance. The cell itself actually has no real resistance because it does not form a complete electrical circuit. Using a multimeter, you cannot measure the resistance of a battery. However, it has an effective or equivalent series resistance (ESR) when placed in a circuit due to the properties and rates of the chemical reactions in the battery. Due to Ohm’s law, in which V = IR, the ESR value serves to put limits on the maximum charge and discharge rates of the battery. Ni-Cad batteries, due to their spiral wound or “Swiss Roll” construction, have a large contact area between the positive and negative electrodes of the battery. This reduces the ESR and allows for the high charge and discharge rates of Ni-Cad batteries.

 

The capacity of a battery can be determined by its average voltage times the Amp-hour rating. This gives capacity in units of Wh or mWh. The capacity of the battery can change depending on the charge and discharge rate. In general, as the charge or discharge rate is increased, more energy is wasted, and the capacity reduces. Ni-Cad batteries exhibit a strong resilience to capacity reductions as a function of charge rate, meaning that the capacity of a Ni-Cad battery changes less due to charge/discharge rate increases than with other batteries.