This question is frequently asked by students. I want to provide a detailed explanation of the calculations required to select the right battery specification for an electric vehicle.

In my explanation, i will use miles as the unit for distance. I will also indicate it with “m”, in some cases.

SPEED:

Voltage! Voltage! The faster an EV can travel, the higher the voltage in your Battery System.

Trade off

Similar to ICE’s, speed is a factor in how far you can go. The faster you go, the more fuel is consumed. An EV that goes faster will use more power. This will affect how far you are able to go.

Travel on one charge

Distance:

Speed, Pack KW rating and driving conditions are all important factors.

The distance question.

Distance is determined by the following formula: (KW of pack/w/m) = Distance

*Note: There are some adjustments that must be made to the formula. See usable pack sizes below.

Watt-Hour per mile (Wh/m).

The most important rule for vehicle ownership is:

Small Vehicle 250-300wh/m

Small Pickup 350-400wh/m

This calculation is: Voltsx (Amp Draw/MPH ) = Wh/m

Battery Pack Size (KW):

Pack Voltage x Amp Hour rating of battery = KW

Useful Battery Pack Size:

We can’t use all of the battery packs or our batteries will die very quickly. We can extend the battery pack’s life by using the following:

Do not discharge batteries higher than 80%.

Aside from the fact that an EV discharges the batteries faster then the manufacturer tested and rated it, we also get a phenomenon called “Peukerts”.

We will therefore need to adjust our calculations for this effect. We can ignore the effect on LiFePO4 batteries. They are only marginally affected.

Peukerts effect. Peukerts effect is significant if you use Lead-Acid battery. We only get to use approximately 55%.

The Battery is a source of power.

Size of the usable pack: KW x 0.80x Peukerts = UsableKW

Peukerts:

Lead-Acid = 0.55

LiFePO4 = 1.0

You will get a huge boost in your available power by using Lead-Acid. They are usually cheaper than LiFePO4 battery.

All this together, –

Example:

Batteries: 12 – 12V Lead-Acid, rated 100 Ah

Voltage of the pack: 144V (12 batteries, x 12V each =144V).

Pack Size: 14400 Kilowatt (144V x 100 Ah = 14400 Kilowatt – We can’t use everything!

Useable Pack: 6336Kw (14.400 x 0.8x 0.55 = 6336Kw usable).

We know from experience that an EV powered by a 144V system draws around 90 amps at 50 MPH.

The Wh/m usage is therefore 144V x 90Amps/50MPH = 259Wh/m

This setup will take our EV to 24 miles (at 50 MPH)

A lithium pack with the same voltage and ah would have a range of 44 miles (because Peukerts effect doesn’t play a part).

14400 x 0.0.8 = 11,520 kw useable / 259 =44

Side note: A 144-volt pack of lithium (LifePO4) cells would contain 45 lithium cells (they are nominally 3.2 volts each).

This can be reversed by using LifePO4 cells.

Let’s say you want to travel 44 miles on a single charge at 50 mph. Want to find out what size batteries you will need ……?

We will use the 259 wh/mi average.

Wh per mile / Pack Voltage = ah per Mile

So in our “car” 259/144= 1.8ah per mile

So you’d need to pay ah per mile x miles per charge required x 1.2 (so that you have 20% left after the drive).

Our “car” has 1.8×44 = 79.2×1.2 = 95 Ah batteries at 144 V to cover the 44 mile trip.

Many people would like to travel close to 100 miles, according to our experience. This car can travel 88 miles, which is twice the 44.

The total Kw of the pack must be increased by at least twice. There are many ways to do this, but the most popular is to double your pack’s Kw.

The Ah rating of the battery or the voltage can be doubled by using twice the 100 ah batteries.

In the above example, this would be 45 out of the 200 ah cell or 90 out of the 100.

Remember that the components must be rated to the appropriate voltage and amperage.

You should be careful. Just because you have a higher voltage or a longer range of use, it doesn’t mean that you shouldn’t use batteries lower than necessary.

100ah rating due to the “C” rating. See explanation below.

It is not recommended that you draw more than three times the C rating of today’s Lithium batteries for more than @ 10 second. 1C for 100ah

Battery would draw 100 amps, while 3C draws 300 amps. You could limit your controller to drawing 300 amps maximum from the batteries.

The acceleration would be OK at 144 volts. A 300 amp limit of 288 volts would make it impressive.

It is recommended to use larger ah battery, 160-200 ah, and to adjust your voltage to obtain your KW pack.

3C is between 600 and 480 amps.

Ps. : Do your homework before you ask anyone for a detailed explanation. Lack of homework can hinder one’s ability to think. You can learn more about Electric vehicles by joining forums.