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Lead Acid , Nickel Metal Hydride and Lithium-ion batteries are all used as power packs in electric vehicles. The table below shows key performance characteristics, using Lead Acid as the reference.
W hen reading this information please understand that we are trying to highlight the salient points only. Also note, that some factors of performance are discussed separately. However, in real usage it is not possible to be guided by a single aspect of performance.
For example. We identify Cycle Life and give it a value. However this value is selected with ALL other parameters fixed. So as soon as another parameter changes (eg. temperature) it will impact on cycle life.
Therefore, we recommend you enjoy the information provided and use it to give you a basic education about batteries and a basic means of comparing the technologies presented.
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Items
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Li
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Ni-Mh
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Lead-acid
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Cycle life (cycles)
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300-1000
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500
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300
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Energy efficiency (C discharge/C charge)
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95%
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90%
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75%
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Weight comparison for the same capacity
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1
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1.3
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4
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Size comparison for the same capacity
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1
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1.3
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3.5
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Reliability
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High
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High
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Low |
Lithium cells are the clear front-runner. Unfortunately though, standard Li Ion technology had one MAJOR drawback. Standard Li technology is very volatile, it can catch fire and even explode if over charged or discharged.
A t present Li based cells are still the most expensive, with prices being approximately twice that of comparative NiMh cells. However, as with any new technology, prices will drop over time.
The only disadvantage with ANY Lithium based cells is that if left uncharged for an extended period of time they can 'die' if their voltage drops below the voltage threshold (depending on state of charge this could be as short as 2 weeks). However, this will only occur if the battery is not used for an extended period of time, and is not charged periodically.
There are some major differences among Li Battery chemistries, which are given in the table below.
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Cathode materials
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LiCoO2
Dangerous
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LiMn2O4
Dangerous
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Li(NiCoMn)O2
Dangerous
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LiFePO4
Safe
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Discharge capacity (mAh/g)
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140
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100
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150
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145
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Working voltage (V)
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3.7
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3.8
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3.6
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3.2
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Charged voltage (V)
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4.25
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4.35
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4.3
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4.2
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Overcharge tolerance (V)
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0.1
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0.1
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0.2
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0.7
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Cycle life (no of charges)
@ 0.3C discharge
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<=500
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<=300
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<=500
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<=1000
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55deg Cycle life (cycles)
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300
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100
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300
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800
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Heat Flow by DSC (kJ/g)
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650
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150
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600
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10
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Over charge / Over discharge
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4.9/3C Explosion
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8V/3C
Fire
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8V/3C
Fire
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25V/3C Pass
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Energy density (Wh/kg)
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180
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100
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170
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130
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The primary concern with Li batteries is safety. Overcharging and overheating can cause fire and explosions. The exception to this concern is the LiFePO4 battery.
Lithium Cobalt Oxide (LiCoO2) chemistry has been used for consumer electronics and digital applications, such as mobile phones and digital cameras, since 1993. LiCoO2 battery offers very high energy density. However, this chemistry is not suitable to large format Lithium-ion Battery for EV applications, because that LiCoO2 material is very expensive and unsafe.
The chemistry of Lithium Manganese Oxide (LiMn2O4) is not a good option for EV applications, because of its poor cycle life, especially at elevated temperature. In addition, the energy density of the battery with LiMn2O4 chemistry is the lowest one among all lithium-ion batteries, about 100 Wh/kg, similar to that of NiMH batteries.
A newer material, Lithium Nickel Cobalt Manganese Oxide Li(NiCoMn)O2 is a better candidate for large format batteries. By using only one third of Co metal in the compound the cost is lower than for LiCoO2 with similar safety.
The most recent entry, Lithium Iron Phosphate (LiFePO4), is becoming the "best-choice" for large capacity, high power EV applications. A LiFePO4 battery has the “best of both worlds” characteristics. It is as safe as SLA batteries, as powerful as lithium ion cells and a lower life cycle cost.
The ONLY safe Li technology is Lithium Iron Phosphate. This is the only Lithium based battery that eLation will supply. It is more expensive, but it is safer and has higher number of charge cycles..
If you did not click on the links above you should go to our video page to see the difference in real life.
J ust to help round off our article on batteries, the following allows you to better understand terminology used to specify batteries.
Let’s start with some basics so you can better understand what you are comparing/buying. We recommend you read all of this section to get a complete overview of batteries and how to choose the best for your needs.
Definitions
Batteries use the Ah rating as a means of comparing different batteries and how long they last.
If we have a current flow of 1 amp for 1 hour we have 1 Amp Hour (1 Ah), or 10 amps for 1/10 of an hour, etc. The accepted Ah rating time for batteries is the "20 hour rate". This means that the battery is discharged down to 10.5 volts (12 volt battery) over a 20 hour period while the total amps supplied are measured over this time.
For example, if a battery is a 12Ah battery, it means the battery is capable of supplying current at a flow of 0.6A (12/20) for 20hrs, or theoretically (see chart below) 12A for one hour.
Battery Discharge Rates (or How Long can my Battery last)
When a Lead Acid battery is discharged at a greater rate than its 20-hour rate its capacity is decreased.
Referring to a discharge chart for Lead Acid cells, this means that a 10Ah Lead Acid Battery can only supply 10Amps for 33 minutes. At this point the voltage has dropped to 10VDC and continues to drop rapidly. Note this is a standard discharge chart relevant to ALL Lead Acid batteries.
Nickel Metal Hydride and Lithium Ion batteries do not suffer from this effect. Even at higher discharge rates you obtain the specified Ah rating.
The main advantage of Ah is to allow a simple comparison between different sizes and technology. So in theory it is fair to say that a 24V 10Ahr NiMH battery will last as long as a 24V 10Ah LiFePO4.
The final consideration is Discharge Rate (C). Batteries are able to discharge at rates higher than the specified Ah rating.
For example, if a battery is a 24V 10Ahr 2C battery then it can discharge at a rate of 20A over 30 minutes continuously. When comparing ensure you compare the continuous C rates, not the maximum C rate. The maximum C rate is for short burst only.
Now bring all these factors together, throw in some temperature variations and then sit back and wonder why you read all this. Because the final point to make is that specifications are developed based on a set of criteria, carried out in laboratory conditions, that allow for comparative analysis. An example of this is the LiFePO4 cycle life. Specifications will state +1000 cycles, but that is at 0.3C. this means for a 10Ahr battery pack, if you discharged it at 3.3A all the time to a discharge depth of 20% (ie, use only 2Ahr before recharging), then you will get +1000 cycles.
However, in the real world of electric transport you will draw more than 3.3A (0.3C). Realistically, discharge rates are +1C to depths of discharge (DOD) of 40%. With this type of discharging cycle life drops. Expect figures for the cycle life of around 1C: 800~1000 cycles and 2C: 500~800 cycles. The major point to note here is a comparison.
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