Batteries

ECE 371Sustainable Energy Systems

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Buzz Words

Energy Density (From the book) = Wh/kg. Most places use Energy Density = Wh/L.

Gravimetric Energy Density = Wh/kg. Specific Energy = Wh/kg. Volumetric Energy Density = Wh/L.

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Comparison of Various Battery Chemestries

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Storage material Energy typeSpecific energy

(MJ/kg)Energy density

(MJ/L)

Hydrogen Chemical 142 9.17

Diesel Chemical 48 35.8

LPG Chemical 46.4 26

Gasoline (petrol) Chemical 46.4 34.2Coal (anthracite or bituminous)

Chemical ~30 ~38

Wood Chemical 16.2[citation needed] 13TNT Chemical 4.6Lithium-ion battery Electrochemical 0.36[5]–0.875[6] 0.9–2.63Flywheel Mechanical .36 – .5Alkaline battery Electrochemical 0.5[7] 1.3[7]

Nickel-metal hydride battery

Electrochemical 0.288 0.504–1.08

Lead-acid battery Electrochemical 0.17 0.56

Supercapacitor (EDLC) Electrical (electrostatic) 0.01-0.036[8][9][10][11][12][13] 0.05-0.06[8][9][10][11][12][13]

Electrostatic capacitor Electrical (electrostatic) 0.00001-0.0002[14] 0.00001-0.001[14][15][16]

Power Density Specific Power = W/kg. Power Density= W/L.

Energy density – Some applications may not need to draw a lot of power but may need a lot of energy – Such as a PV system.

Power Density – Some applications may need a lot of power – Such as an electric vehicle.

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Power and Energy Density

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Power and Energy Density

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Power and Energy Density

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Power and Energy Density

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Battery Energy Storage A battery’s energy capacity can be measured in

terms of : Amp-hour Capacity (Ah) – Note that Amp-hours is

a measure of charge in coulombs. Amp-Hours is how much charge is stored in the

battery.

Since a battery’s voltage is ‘fixed,’ a battery’s energy can also be specified in Watt-hours A battery’s nominal voltage times its Ah capacity is

equal to Watt-hours of energy storage. Example: 1.5 V alkaline battery with a 2000 mAh

capacity has a stored energy of 3 Wh. 10

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Ultra Capacitors / Super Capacitors

Lower Energy Storage than batteries. Higher energy storage than conventional

capacitors. Can be charged and discharged at very high

currents. High cycle life – can last for many charge-

discharge cycles at very high currents

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State of Charge

The State of Charge (SOC) is the percent of charge left in the battery. SOC = 100 % Battery is fully charged. SOC = 50% Half the charge has been removed. SOC = 25% 25% of the charge is still in the

battery.

SOC is kind of like a gas gauge for your battery. Remember that Amp-hours is an amount of

charge.18

Depth of Discharge

The Depth of Discharge (DOD) is the percent of charge removed from the battery. DOD = 100 % Battery is fully discharged. DOD = 50% Half the charge has been removed. DOD = 0% The battery is fully charged.

Note that DOD = 1-SOC.

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More Terminology A secondary battery is a rechargeable

battery.NiCd Lead-AcidNiMH

A primary battery is not rechargeable.AlkalineCoin cells

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C-Rate A C-rate is the is a measure of the rate at which

a battery is discharged relative to its maximum capacity.

The 1C rate means that the discharge current will discharge the battery in one hour. For a 100 Ah battery the 1C discharge rate would be

100 Amps and discharge the battery in 1 hour. For a 100 Ah battery the 2C discharge rate would be

200 Amps and discharge the battery in 1/2 hour. For a 100 Ah battery the 5C discharge rate would be

500 Amps and discharge the battery in 1/5 hour.21From http://web.mit.edu/evt/summary_battery_specifications.pdf

C-Rate For a 100 Ah battery the C/2 discharge rate

would be 50 Amps and discharge the battery in 2 hours.

For a 100 Ah battery the C/5 discharge rate would be 20 Amps and discharge the battery in 5 hours.

For a 100 Ah battery the C/20 discharge rate would be 5 Amps and discharge the battery in 20 hours.

The C/20 rating is fairly standard metric.22From http://web.mit.edu/evt/summary_battery_specifications.pdf

Battery Capacity versus Discharge Rate and Temperature

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Cycle Life Versus DoD

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Cycle Life Versus DoD

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Ah Capacity vs Discharge Rate

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Ah Capacity vs Discharge Rate

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Cell Voltage versus DOD

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Cell Voltage versus Time

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Self Discharge

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Batteries in Series Benefits – higher voltage, lower current, same Ah

rating Longer life More energy storage

Problems One battery fails, you could loose the whole string.

One cell is always the weakest. Charges faster than the others – possible overcharging. Discharges faster than the others – damage and possible

thermal event.

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Batteries in Series Since the current is lower, things are smaller

Wire size, Resistive losses Fuses and switches Connectors

Big wire and the associated components are hard to work with and more expensive.

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Batteries in Parallel

Weakest battery will drag down the bank. Amp hour rating of batteries in parallel add. Typically lower voltage and higher total current.

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Batteries in Series/Parallel

Most efficient arrangement would be all batteries in series. If one battery goes down, you could lose the entire string.

Most reliable would be two parallel strings of batteries in series.

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Most Efficient

Most Reliable

Not Preferred

Battery Charging Typically you charge batteries with a controlled

current source. Typical algorithms:

CCCV Constant Current (CC) – Bulk charge Constant Voltage (CV) – Finishing Charge

Typical Cell Phone Wake Up charge on depleted battery (0% SOC) Constant Current Constant Voltage Pulsed charge

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Battery Charging

Rule of Thumb Build a controlled current source. Charge batteries using the algorithm specified by the

manufacturer. It is safest to monitor each cell’s

Voltage Temperature

Most modern chargers are digitally controlled constant current sources.

General rules apply to big and small batteries.37

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Charging Profiles

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Charging Profiles

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Charging Profiles

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Battery Model

A simple model of a battery is an ideal voltage (VOC) source in series with a resistor (RS).

However: VOC is a function of the battery state of charge and

temperature. RS is a function of the battery state of charge and

temperature. RS is different for charging and discharging.

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Battery VOC

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Battery Charging Resistance

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Battery Discharging Resistance

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Simulink Battery Model

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Battery Charging

For series strings, one battery will charge faster than the others. (Or its voltage will rise faster than the others.)

We need to worry about each battery not having the same amount if charge.

This problem is called charge equalization. Need a charging algorithm to avoid the problem. Can be addressed by the battery monitoring system.

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Battery Monitoring System (BMS)

For large battery banks, you should monitor each battery individually Voltage Temperature Other

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BMS Note that the monitors are floating. (They

are not connected to system ground.) Data must be transmitted wirelessly, or The data communication bus must be

electrically isolated. Not shown on the diagram is a temperature

sensor for each battery. Some BMS’s will have shunt to bypass

batteries for equalization.

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Monitor

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