Objective

To learn the specific charge/discharge characteristics of a Nickel Metal Hydride (NiMH) battery through experimental testing of a remote triggered NiMH Battery.

 

Each type of battery chemistry, whether it be 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 Nickel Metal Hydride 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 NiMH 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 NiMH based energy storage systems. NiMH battery systems were used to power the generation of electric vehicles after lead acid and before lithium based systems. They were also the predominant battery used in hybrid electric vehicles and are used in most versions of the Toyota Prius. The energy capacity vs. discharge rate affects the weight, size, and cost of a battery and device. Amongst others, this information is useful for:
  • Sizing a battery for an application, 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 using the formula: Time = Energy / Power = ((State of Charge of the battery in percentage) * (Total Full Energy of the battery)) / (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 # 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

 

Connecting in Series (Double voltages, Same capacity (Ah) )

Series connection adds the voltage of two batteries, keeps the capacity as same (Ah).

For Example,

              Double Voltage (12V), Same Capacity (10Ah)

Two 6V Batteries joined in series produces 12V, But the total capacity is still 10A.

 

Connecting in Parallel ( Same voltage, double capacity (Ah) )

Parallel connction  increases current rating but the voltage ramains same.

For example,

             Same Voltage (6V), Double Capacity (20Ah)

Two 6V Batteries connected in parallel have the same 6V, But the current increases to 20A

 

Connecting in series/parallel(Double Voltage, Double Capacity(Ah) )

Batteries connected in series/parallel increases both the voltage output and current rating.

For Example,

 

      Double Voltage (12V), Double Capacity (20Ah)

Two sets of batteries already connected in parallel are joined them together to form a series produces 12 V and 20 Ah.

Nickel Metal Hydride

Stanford R. Ovshinsky invented and patented the NiMH battery and founded Ovonic Battery Company in 1982.

Principle of operation:

NiMH works on the principle based on the absorption, release and transport of Hydrogen within the two electrode.

 

Chemical reaction

 

Positive electrode

 

Negative electrode

 

Overall reaction

 

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

Some of the most important characteristics of  NiMH  Battery

 

 

 

Nickel Metal Hydride Battery Characteristics
Nominal Voltage	1.2V
Specific Energy	60-120W.h/kg
Energy Density	140-300W.h/L
Specific Power	250-1000 W/kg
Charge/Discharge Efficiency	66%
Self discharge rate	30% per month (temperature dependent)

 

NiMH 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. NiMH replaces Ni-Cad batteries in portable electronics. 

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 NiMH batteries is much higher than Ni-Cad batteries. It is however lower than Lithium batteries. After 1991, the specific energy of NiMH is doubled. The cost of  NiMH is less than one-third of an equivalent Li-ion Batteries.

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 NiMH is much higher than the Ni-Cad Batteries, Lead-acid batteries. NiMH contains no toxic metals. Applications include mobile phones and laptop computers.

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. Performance is better compared to  alkaline batteries in higher drain devices. Beginning the NiMH  has high energy density with also a high rate of self-discharge. Recent technology  of NiMH has low discharge  rate with approximately 25%  lower energy density. Specific power of NiMH is less compared to Ni-Cad.

NiMH batteries have unique charge and discharge curves (voltage vs. time during charging and discharging). The discharge curve for NiMH is nearly flat during the main portion of it's dischage, whereas most other batteries have a roughly linear, decreasing main discharge curve. This introduces unique challenges into determining the State of Charge of NiMH batteries.

The Charge/Discharge efficiency is also an important factor for the practical use of the battery. The Charge/Discharge efficiency of NiMH is very less compare to Ni-Cad and their efficiency is also less.

NiMH also has high self-discharge of 30 percent per month. Modifying the hydride materials will reduce the self-discharge and reduces corrosion of the alloy, but this leads to decreases in specific energy.

The Life time of Nickel Metal Hydride is less compared to Ni-Cad and Li-ion But more than the lead acid and comparable with lithium polymer.

The discharge curve of a NiMH battery has a large, nearly flat region in the middle of its operation. This means that after the initial discharge, in which the battery rapidly decreases its voltage from the float voltage, the voltage of the battery remains nearly constant, up until the point at which the energy of the cell is nearly depleted and the voltage again rapidly falls of. Unlike lead acid, lithium, or other battery types in which main part of the discharge curve can be closely approximated by a straight line of fixed, decreasing slope, the predominantly flat slope of the NiMH makes it difficult to determine the state of charge (SOC) during the discharge process. As a result, when using NiMH batteries, the energy in and out of the battery must be carefully monitored and summed to determine the SOC. Naturally, this process will accumulate errors over time and will require periodic recalibration of the SOC algorithm. 

 

Advantages:

NiMH has less toxins and it is environment friendly. It can be recycled. Memory effect is lesser and higher capacity than Ni-Cad. Much safer than lithium batteries. It can tolerate overcharge and overdischarge.

 

Disadvantages:

Deep Discharge reduces the life cycle and produces heat when it is fast charged and high load discharge. Self discharge is more compared to other batteries. High maintenance is required, It should be often fully discharged to prevent crystalline formation. Expensive than Ni-Cad battery.