When it comes to EV building, you can have the best of the best of everything – aerodynamic body, great motor, excellent controller. However, when it’s all said and done, you’ll sink or swim depending on your batteries.
I’m not going to get into types/sizes/brands here; this is more about the preliminary calculations than anything else. There are some general rules of thumb when it comes to powering an EV:
- Voltage = Speed. It’s not quite that simple, but close enough for basics. The more voltage you can provide to the motor, the faster you’ll be able to go.
- Amperage = Acceleration. Whereas voltage enables the motor to turn faster, amperage is what allows the motor to turn when a load (in our case, a whole car via the driveshaft) is applied.
Also to note is the fact that the lower the amperage you ask the batteries to output, the longer they can output it for. This means you either accelerate VERY slowly, or…increase the voltage!
How does that work, if voltage equals acceleration? It’s a transformation of Ohm’s Law. Ohm’s Law states that the current, I, through a conductor between two points is directly proportional to the potential difference, V, across the two points, and inversely proportional to the resistance, R, between them:
Current is amps, “Potential Difference” is volts, and the resistance is the load/work we want the electricity to be doing – realistically, the wattage. So to simplify, we end up with the following replacements and transformation:
So, for the same amount of power output (in Watts), I can have more amps and less volts, or more volts and less amps. Less amps = good things, so we keep the voltage as high as reasonably feasible.
I’ve got 64x 12V, 7.2Ah batteries. They’re small, both in electrical capacity (amp-hours), and in physical size. However, there are SIXTY-FOUR of them.
That’s the same as 8x 96V 7.2Ah batteries.
8x 7.2Ah = 57.6 Ah total @ 96V
96V * 57.6Ah = 5529.6 Wh; 5.5296 kWh
There are 5.52 kilowatt-hours of electricity in my battery pack. Electric cars use, on average, 250 to 350 Watt-hours of electricity per mile driven.
Let’s use 300 Wh/mi for now, with an 80% ‘Depth of Discharge.’ That’s how much of the total power you use out of the batteries. If you 100% discharge a battery, it’ll basically kill it permanently. 80% is a “safe zone” of discharge for lead-acid batteries, so let’s start there.:
5529.6Wh * 0.80 DoD = 4423.68 Wh @ 80%
4423.68Wh / 300Wh/mi = 14.746 miles
so yeah, we’ll call it 15 miles range at 80%. Most likely, with used batteries like mine, it’d be safer to only go to 50% – which works out to 9.218 miles. The batteries I have are extremely small! Most EV-ers have batteries that are 20 to 60 Amp-hours EACH! This is why we’re starting with the go-kart.