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Batteries
can be made out of many different materials. Even batteries made
of the same materials can be built in different ways to make them
have different properties. This means there are lots of different
battery choices and a lot of parameters to be considered. Choosing
the proper battery can make a huge difference in how competitive
your Electric Vehicles is.
One
of the most common battery comparison is weight. For example, lets
put together a reasonable battery pack for the Electric Imp. Lets
choose a 100 amp hour battery. This rating means that if you were
to discharge the battery at the manufacturer's specified rate, it
will deliver a total equal to 100 amps of current for one hour before
the battery is discharged. Now, this doesn't mean you can actually
run the battery at 100 amps for one hour, that would be too simple.
What it usually means is that you can run the battery at 5 amps
for 20 hours (5 amps * 20 hours = 100 amp/hours) Click here
for more information about this.
The
Electric Imp uses a 400 volt pack, so we need enough batteries in
series to add up to 400 volts. This would be over 33 12 volt batteries
(33 * 12 volts = 396). This will give us a battery pack with 40
kilowatts of power. Below is a chart comparing how much each 40
KW pack would weigh.
As
you can see, the difference between the heaviest pack and lightest
is 2500 lbs. If the batteries all performed the same, the Li-Ion
pack offers a huge advantage.
Lets
look at some other comparison such as the Trojans and the Cyclons.
The Trojans are flooded lead acid batteries, similar to what starts
an ICE car. They weight 2/3rds of the advanced
lead acid batteries, but the advanced lead acid batteries are actually
better for racing! The Trojans are optimized for delivering low
current for a long time, such as lighting a light bulb on a sail
boat. When you try and draw a high current, such as 700 amps to
accelerate the Electric Imp, the flooded lead acid will deliver
substantially less amp hours than the advanced lead acid battery.
The flooded battery, since they are not designed for high currents,
will also probably be damaged. It is vital to choose the right battery
for the task.
When
you compare batteries there are more things to compare than just
energy density. Here are some terms used in battery comparisons:
Voltage-
The voltage of a battery.
The voltage will vary depending on state of charge.
Capacity- How much current the battery can deliver over a
set time.
The capacity is given as number of amps delivered for one hour regardless
of the rate of discharge. To properly compare capacity
between two batteries, you need to have the capacity tested at the
same rate of discharge. That rate of discharge should be at rates
close to what your project will draw. Amps/hours.
Specific Energy- How much current at what voltage a battery
can reliably deliver over time per weight. Again this will be different
for different rates of discharge. (Amps * Volts / hours) / pounds.
Energy Density- How much current at what voltage a battery
can reliably deliver over time per volume. Again this will be different
for different rates of discharge. (Amps * Volts / hours) / cubic
inches.
Specific Power- How much current at what voltage a battery
can reliable deliver per pound. This will change with state of charge
of the battery. It is usually important that a battery have a high
specific power at low states of charge. In other words, it can still
deliver the amps when nearly discharged. (Amps* Volts) / pounds.
Power Density- How much current at what voltage a battery
can reliable deliver per volume. It is important that a battery
have a high specific power at low states of charge. (Amps * Volts)
/ cubic inches.
Maximum sustained discharge rate- How many watts that this
specific battery can deliver continuously without damage. This will
change with state of charge of the battery. (Amps * Volts)
Maximum pulse discharge rate- How many watts that this specific
battery can deliver without damage for a specified length of time
such as ten seconds. This will change with state of charge of the
battery. (Amps * Volts for n seconds)
Life Cycle- Number of times a battery can be charged and
discharged before it loses a specified amount of it's capabilities.
This will depend on how you define a battery as being used up; Still
able to deliver 80% of it's capacity? 50% of capacity? 90% of specific
power? This is also different depending on depth of discharge the
battery is tested at. Usually the deeper you discharge a battery,
the lower the number of cycles it will provide.
Self discharge rate- How quickly will a battery lose power
if just sitting.
Operating temperatures- Most battery's performance change
substantially at different temperatures. That means all of the above
ratings are meaningless unless they are done for the operating temperature
you plan to use the battery. The batteries themselves will change
temperature as they are charged and discharged. This means that
the user will sometimes need to design cooling or heating systems
to keep the batteries at a good operating temperature.
Cost- It is difficult to compare batteries due to the wide
variety of voltages and sizes. The chemistry of the battery determines
the individual cells voltage. The manufacture decides how many cells
to combine to make up a battery. This determines the batteries voltage.
The manufacture also decides how big the battery should be. This
determines the capacity. An "Apples to Apples" comparison
need to be for batteries with the same capabilities under the operating
requirements of your application. How many number of cycles a battery
is good for is an important factor when comparing costs.
Lets
look at the requirements of the Electric Imp:
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