Flooded Lead Acid Calculations - Range/Performance

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
rate factor

1 .57 = Most commonly used value
2 .67
3 .74
4 .77
5 .82
6 .84
7 .86
8 .87
9 .89
10 .91
20 1.00

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:


RANGE = (20 amp/hr rating) X (.57) X (pack voltage) / (watt hours per mile of the vehicle)


Note: The above is the range @ 100% DOD, Divide that amount by 2, to get a 50% DOD range on your battery pack, I would try and not go lower than 50% DOD in order to maximize the life of your Flooded Lead Acid batteries. AGM/Gels might be more forgiving.