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The battery capacity specifies the amount of electric charge a battery can supply before it is fully discharged.The SI unit of battery capacity is the coulomb.A more general unit for battery capacity is ampere-hour（A·h），with 1（A·h）＝3，600 C.For example，a battery of 20（A·h）can supply 1 A current for 20 hours or 2 A for 10 hours，or in theory 20 A for 1 hour.But in general the battery capacity is dependent on discharge rate.

There are two ways of indicating battery discharge rate:C rate is the rate in amperes，while nC rate will discharge a battery in 1/n hours.For example，a rate of C/2 will discharge a battery in 2 hours，and a rate of 5C will discharge a battery in 0.2 hours.For a 2（A·h）battery，the C/5 rate is 400 mA while its 5C rate is 10 A.

Energy Stored（E）

The energy stored in a battery is dependent on battery voltage and the amount of charge stored within.The watt hour or（W·h）is the SI unit for energy stored.Assume a constant voltage（CV）for the battery.Then E（W·h）＝V×C，where V is the voltage and C is the capacity in（A·h）.The capacity of the battery changes with the discharge rate，and the associated discharging current affects the voltage value.The energy scored is thus not a constant quantity and is a function of two variables，namely，the voltage and capacity of the battery.

State of Charge（SOC）

A key parameter in the electric vehicle is the SOC of the battery.The SOC is a measure of the residual capacity of a battery.Typically，the battery SOC is maintained between 20% and 95%.

A common mistake that people may make about a battery's charge is that when a battery“goes dead，”the voltage goes from 12 to 0V（for a 12V battery）.In reality a battery's voltage varies between 12.6 V with a SOC of 100% to approximately 10.5 V with a SOC of near 0.It is advised that the SOC should not fall below 40% which corresponds to a voltage of 11.9 V.All batteries have an SOC percentile vs voltage curve which can be either looked up from the manufacturer's data or determined experimentally.An example of an SOC percentile vs voltage curve of a lead acid battery is shown in Figure 3.4.Note that for a lithium-ion battery，the curve may be much flatten especially for the mid-SOC range of 40%-80%.

Depth of Discharge（DOD）

The depth of discharge（DOD）is the percentage of battery capacity to which the battery is discharged.

Generally，a battery is prevented from having a low DOD.The withdrawal of at least 80% of battery capacity is regarded as a deep discharge.One important precaution is that the charge in a battery should never be discharged down to zero voltage，otherwise the battery may be permanently damaged.So，in this case，a cutoff voltage is defined for the battery voltage so that the voltage at the battery terminals will never drop below this cutoff voltage.This point is referenced as 100% DOD.

Specific energy means how much electrical energy can be stored per unit mass of battery.The SI unit for this quantity is watt hour per kilogram.Knowing the energy stored and specific energy of the battery，the mass of the battery can be easily obtained by dividing the energy by specific energy.Again，the specific energy is not a constant parameter since the energy stored varies with discharge rate.A comparison of the specific energy of various energy sources（typical numbers）is given in Table 3.1.

Energy density means how much electrical energy can be stored per cubic meter of battery volume.It is computed by dividing the energy stored in the battery by the battery volume.The SI unit for energy density is watt hour per cubic meter.

Specific Power and Power Density

Specific power means how much power can be supplied per kilogram of the battery.Note that this quantity is dependent on the load served by the battery and is thus highly variable and anomalous.The SI unit of specific power is watt per kilogram.Specific power is the ability of the battery to supply energy.Higher specific power indicates that it can give and take energy quickly.Volume specific power is also called power density or volume power density，indicating the amount of power（time rate of energy transfer）per unit volume of the battery.If a battery has high specific energy but low specific power，this means that the battery stores a lot of energy，but gives it out slowly.A Ragone plot is used to depict the relationship between specific power and specific energy of a certain battery.

Number of Deep Cycles and Battery Life

EV/HEV batteries can undergo a few hundred deep cycles to as low as 80% DOD of the battery.Different battery types and design result in different numbers of deep cycles.Also，the usage pattern will affect the number of deep cycles a battery can sustain before malfunction.The United States Advanced Battery Consortium（USABC）has a mid-term target of 600 deep cycles for EV batteries.This specification is very important since it affects battery life time in terms of deepcycle number.So，generally，we should reduce the chances of DOD in the control strategy for EVs and HEVs in order to limit the operating cost of the vehicles.

New words and expressions（单词和短语）

1.New words（单词）

2.Expressions（短语）

Notes to the text（难点解析）

★1.One important precaution is that the charge in a battery should never be discharged down to zero voltage，otherwise the battery may be permanently damaged.特别需要注意的是，电池电压不能降到0 V，否则电池将会永久性损坏。

★2.If a battery has high specific energy but low specific power，this means that the battery stores a lot of energy，but gives it out slowly.如果电池有高的比能量和低的比功率，则意味着电池储存了很多能量，但是能量放出的速度很慢。

Exercises（练习）

◆1.Translate the following passages（expressions）into Chinese（英译汉）

（1）Higher specific power indicates that it can give and take energy quickly.

（2）In reality a battery's voltage varies between 12.6 V with a SOC of 100% to approximately 10.5 V with a SOC of near 0.

（4）energy density

◆2.Translate the following passages（expressions）into English（汉译英）

◆3.Directions:Answer the following questions briefly according to the text.

What is SOC？What is DOD？

Reading material（阅读材料）

Ampere-hour efficiency is the ratio between the electric charge given out during discharging a battery and the electric charge needed for the battery to return to the previous charge level.In practice，these two values will never be equal，limiting the efficiency to 100%.In fact，the typical values of charge efficiency range from 65% to 90%.The efficiency depends on various factors such as the battery type，temperature，and rate of charge.