Lead Acid Batteries:
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A consolidation of notes from Kitty Rodden's presentation to the Peninsula Chapter
of the Electric Auto Association, by David Coale
What battery should I use in my EV?
This is the age old question; what battery should I put in my EV?
There is no standard answer to this, it depends on what you want
out of your EV. The old rule of "thumb" if you will, is the more
lead the greater the range and the poorer the performance due to
the increase in weight. It is also true that the higher the voltage
the better the performance (acceleration and top end speed). This is
of course also dependent on the type of driving and on the controller.
Since these are fixed for any given EV, these generalizations are not
too far off. Two designs illustrate this:
Long range: 96 to 120 volt system using 6 volt deep cycle batteries.
This will give you lots of amp hours and weigh 976 to 1420 bls.
This type of battery pack should last two to three years depending
on the type of driving.
High performance: 96 to 144 and up using 12 volt batteries. If you
use a starting battery your car will be light (400 to 700 bls.) and quick
and your batteries will last about 3 to 6 months or less. If you use
deep cycle batteries (12V) your batteries can last as long as one and
a half to two years but the car may be heavier depending on the battery.
So what is the range in each case? Isn't that what we want to know;
how far will my EV go? Answering this will help you decide what
battery you will use.
To find out how far a particular battery pack will take us, we need
to know how much energy is in the pack and how much energy the EV
uses per mile.
Range = Energy in pack/(energy used per mile)
The energy in the battery pack (wired in series) is the amp hour rating
times the pack voltage. The amp hour rating is how may amps a battery
can supply over a given time. Most batteries are measured over a 20
hour period. This is a standard that is used to compare batteries with,
and can be found in the specs. on most batteries. The 20 amp hour rating
has to be adjusted for EV use. The faster one draws current from a battery
the less capacity there will be. This is due to the chemistry of the
battery and the internal resistance. Therefore the capacity of a battery
at the 20 hour rate is more then the capacity at the 1 hour (EV) rate. The
following is a table from Trojan Battery Company showing the conversion
factor for finding the X hour rate given the 20 hour rate:
Conversion of 20 hour rate to X hr rate.
X hr Conversion
Note: these values will be a little different for each battery.
One can see from the table above that as you drive faster the range will
decrease due to reduced available capacity. This does not take into account
wind resistance which also increases with speed.
Now we need to know the energy or watt hours per mile that your EV gets.
The table below lists some common EVs and their watt hours per mile rating.
Impact = 130 W-hr/mi (AC drive very good aerodynamics)
Metro = 160 W-hr/mi (PM drive good aerodynamics)
Metro = 200 W-hr/mi (DC drive good aerodynamics)
Truck = 350 W-hr/mi (DC drive poor aerodynamics)
This table was developed using several cars for each category traveling
at highway speeds (60 mph). The numbers reflect the efficiency of the cars
listed. With the following information the range equation for a smaller
type of EV on the freeway would be:
20 amp/hr rating X .57 X pack voltage
Range = -------------------------------------
200 watts hours per mile
From this equation we can make a table using some commonly used batteries.
Bat. 20 AH Wt. pack pack range Cycle miles miles X cycles
type rate Lbs. volts Wt. miles life /Lbs. /Lbs.
T-105 217 61 96 976 59.4 754 .06086 45.89 \
120 1220 74.2 |
T-125 235 66 96 1056 64.3 650 .06089 39.58 > 6 volt
120 1320 80.4 | Batteries
T-145 244 71 96 1136 66.8 625 .05880 36.75 |
120 1420 83.4 /
T-875 165 63 96 756 45.1 540 .05965 32.21 \ 8 volt
120 945 56.4 > Batteries
144 1134 67.7 /
27TM 105 52 96 416 28.7 210 .06906 14.50 \
27TMH 117 60 96 480 32.0 358 .06669 23.88 |
120 600 40 0 |
144 720 48.0 |
30XHS 130 66 96 528 35.6 325 .06742 21.91 |
120 660 44.5 > 12 volt
144 792 53.4 | Batteries
5SHP 165 86 96 688 45.1 560 .06555 36.74 |
120 860 56.4 |
144 1032 67.7 /
As you can see there are no clear cut answers. The miles per pound
are better with the 12 volt batteries but the cycle life is not as
good; and the converse is true. The last column is an attempt to find
a single number to compare the batteries with. This does not show price
however, and this will be a factor in any EV conversion.
The above table is for comparisons only and may not reflect your actual
range. The information shown here is by way of Kitty Rodden formally of Trojan Battery Company. I do not have any affiliation with Trojan Battery Co. This is just the information I have at hand. Please send questions and comments to David Coale.
David Coale ___o\____
(650) 493-4503 =)----/()_____()\