Build a car for you, not for them ! (the oil
conversions since 1975
QUALITY ELECTRIC VEHICLE CONVERSIONS AND QUALITY
SOME BASIC EV CALCULATIONS
Electric Vehicle Conversions: What’s - What?
As with ICE (Internal Combustion Engine) vehicles, speed, distance, MPG, etc.. will vary between vehicles due to driving habits, road
conditions, etc.. The same will be for an EV (Electric Vehicle). However, here are some basic guidelines to help get you started.
Voltage! The higher the voltage of your Battery System, the faster a given EV can go.
As with ICE's, the faster you go the more fuel is used - the faster an EV goes the more power is used. This will impact how far you can
travel on a single charge.
Speed, Pack KWh rating, driving conditions, aerodynamics, vehicle weight, hills, temperature, driving styles and several other factors
play into the distance question.
The basic formula for determining distance is: ( KWh of pack / wh/m ) = Distance
*note: there are adjustments that have to be made to this formula, see usable pack size below*
Watt-Hour per Mile (Wh/m):
The basic rule of thumb for vehicle is:
Small Vehicle 250-300wh/m
Small Pickup 350-400wh/m
The calculation is: Volts x (Amp Draw / MPH ) = Wh/m
Battery Pack Size (KWH):
Pack Voltage x Amp-Hour rating of battery = KWH
Usable Battery Pack Size:
Unfortunately, we can not use all of our battery Pack or we will kill our batteries extremely fast. To extend the life of the battery pack, we
do not want to discharge the batteries more than 80%.
In addition, because an EV will discharge the batteries faster than the manufacturer tested and rated, we get an effect called “Peukerts”.
Therefore, we will need to correct our calculations for this effect. LiFePO4 Batteries are only marginally effected and we can ignore
Peukerts effect. However if we use Lead-Acid batteries the Peukerts effect if considerable, where we only get to use about 55% of the
power in the Battery.
Usable Pack size: KWh x 0.80 x Peukerts = Usable KWh
Lead-Acid = 0.55
LiFePO4 = 1.0
Yes, you get a BIG hit on your available power when using Lead-Acid. However, they are generally cheaper than LiFePO4 batteries.
Putting all this together - Example:
Batteries: 12 - 12V Lead-Acid, rated at 100 ah
Pack Voltage: 144V (12 batteries x 12V each = 144V)
Pack Size: 14.4 Kwh (144V x 100 ah = 14.4 Kwh - Remember, we can not use all this)
Usable Pack: 6.336Kwh (14.4 x 0.8 x 0.55 = 6.336 Kwh usable)
From experience, we know that a Miata using a 144V system will draw around 90amps at 50MPH.
Therefore, the Wh/m usage = 144V x ( 90Amps / 50MPH ) = 259Wh/m
The distance our Miata will travel on this setup is: 6.336kwh / 259wh/mi = 24 Miles (at 50MPH)
If we had a lithium pack of equal voltage and ah, the range would be 44 miles (because Peukerts effect does not play a role)
14.4 x 0.8 = 11.52 kwh usable / 259 = 44
On a side note, 144 volt pack of lithium ( LifePO4) cells would consist of 45 of the lithium cells (they are nominal 3.2 volts each)
To CALCULATE this in reverse, (using LifePO4 cells)
say you need to go 44 miles per charge at 50 mph and want to know what size batteries you need......
we will use the 259 wh/mi avg.
wh per mile / pack voltage = ah per mile
So in our "car" 259/144= 1.8ah per mile
so you would need ah per mile x miles per charge needed x 1.2 (so you still had 20% charge left after the drive)
in our "car" 1.8 x 44 = 79.2 x 1.2 = 95 ah batteries at 144 volts needed to go the 44 miles.
A lot of people wish to go close to 100 miles in our experience. To make it simple, for this car to go say 88 miles (double the 44 it is
capable of now) the total Kwh of the pack has to be doubled. This can be done in a few different ways, most common would be to double
the ah rating of the batteries used or double the voltage by using double the amount of the same 100 ah batteries.
In our above case, that would be 45 of the 200 ah cells, or 90 of the 100 ah cells.
Keep in mind the components used must be rated for the voltage and amperage
Be careful here, just because you raise the voltage so high or don't need a long range, you should not use batteries much lower than
100ah rating, because of the "C" rating, see explanation below.
With todays Lithium batteries, it is not recommended to draw more than 3 times the C rating for more than @ 10 seconds. 1C for a 100ah
battery would be 100amp draw, and 3C is 300 amps. So if you limited your controller to draw the max of 300 amps from the batteries at
144 volts, the acceleration would be Ok. With a 300 amp limit at 288 volts the acceleration would be impressive.
The usual recommendation is to use larger ah batteries, from 160-200 ah and adjust your voltage to get your needed KWh pack, so that
3C is between 480 - 600 amps.
Some things to remember:
A 5% grade requires twice the power that is needed on level roads.
Poor aerodynamics will use more energy
Poor wheel alignment, low tire pressure, other mechanical drags will use more power
Weight is very important-the lighter the less energy needed to move the vehicle
TEMPERATURE- battery temperature below 50 degrees will diminish the range of the vehicle. Generally, lead acid batteries will lose 30%
of their useful ah at 30* F, and LifePO4 about 15-20%
Driving on hills
Higher voltage comes in handy when going up hills- a long drawn out hill (remember a 5% grade doubles power needed) can easily
demand more than 3C for longer than the recommended time from our 100 ah batteries-putting them in an area that may reduce their
lifespan and create heat in the cell.
If the vehicle is to be used in mountainous areas or for high performance use, larger ah batteries are needed because of this C factor. A
side benefit of course would be longer range- but a costlier pack.
Now we get to the fun part, calculating HP
V x A = watts, and watts/746 = HP so V x A / 746 = HP
If we had a 144 volt pack of 200ah batteries, and a 1000 amp controller, using the above formula we could have 193 HP, (at a 5C draw)
and if we had 288 volt pack of 100ah batteries we could have potentially 386 HP ! (these are calculated without efficiency included,
figure about 85% efficient)
Only one problem, that much electrical power put into the motor could easily destroy it rather quickly ! The common "in the field"
estimate of KW power a 9" motor can handle (for short periods) is 100 KW. So using the above formulas, 144 volt system should be
limited to about 700 amps, and the 288 volt system to 350 amps. Still, 135 hp is pretty good for a small car. NOTE: use high power levels
at your own risk for motor damage.
Using a generator
And for those looking to add a generator as a range extender.....
As you can see from the calculations above, the instantaneous watts needed to drive about 50 mph is about 13,000 watts. So to drive
your car strictly on a generator (at a steady speed), your would need a large one in the neighborhood of 15KW. For any type of
acceleration or resistance (hills) the load could easily be 50 KW, and up to 100KW ! If you used a smaller one, say a 2000 watt
generator, you can see it would add about 10 % to your range. It may be better to use the cost of a generator this size to just add more
cells and skip the generator and its complexity to the system (and exhaust emissions).
Lastly, the subject of heaters. Generally you need about 2500-3000 watts to raise the temp in a car to a comfortable level in a
reasonable amount of time, and about 1500 watts to maintain that temp. So those in a cold climate will lower their range about 15%, or
you will need to add about 15% to the battery KW capacity.
Now the disclaimer: These are all calculation based on theoretical values, averages and assumptions. There are several factors (such as
but not limited to weather, tire pressure, driving habits, battery condition, etc..) which effect all the calculations listed above which are not
used. All calculations should be considered estimates only and can not be relied on as fact.
copyright 2011 EV-propulsion llc Thanks to Dustin O. for editing work
We try to give helpful advice to
all EV builders, and so far have
not charged for it, as many others
This does take a lot of our time but
feel it is worth it for the EV future.
If you feel we gave you some
valuable information and it
was worth something to you,
please make a donation so
we can continue helping
others as we have
Thank you !