EV - Design
Cruising on a flat service does not need much power, but requires energy. The further you go, the more energy is needed and thus a bigger the battery is required. Acceleration takes a lot of power, but not that much energy and so does ascending a hill. The faster you want to accelerate the more power you need. Your motors need to be more powerful and the battery must be able to provide the respective peak current.
Range and acceleration are the two factors driving costs up the most. Think hard about what your driving style is and how far you want to / have to go between charges.
I settled for a max. speed of 35 mph, 20 miles range and moderate acceleration to best fit my city driving and tight budget. I selected a 40 Ah, 72 V battery and two wheel hub motors, 5 kW each.
Peak current of the battery limits acceleration and max. speed -> For a specific power, the higher voltage is just the better way to go!
I developed a simple model for sizing the main components – motors and batteries – and to correlate performance with component specifications. Vehicle mass and resistance greatly determine the power and battery size to move a certain distance at a desired speed.
I want to point out that the energy calculated from basic physics equation is not the same as the energy stored in the battery. You need a battery with higher energy rating based on the fact that batteries cannot be completely discharged and you have other loads, such as your lights, stereo, etc. For LFP batteries, the depth of discharge (DoD) should not be less than 20% (of the initial capacity). For Pb acid batteries the DoD is 50% meaning the battery needs to be sized 2x the needed energy.
Li-Ion batteries offer a much better energy to weight ratio compared to the AGM or Pb-acid batteries used in starter batteries, but typically have lower max. current / cold cranking power. LFP batteries are high power Li-Ion batteries and best suited for my application. I encourage everyone to do their research on batteries before committing to purchase. There is a lot of information out there. I found this website to be a particularly helpful link.
I focus on acceleration only and determine the peak current for 1, 2 and 4 motors in the table above for 72 V when accelerating from 0-60 mph in 10 s. With one motor 330A peak current is required and two motors require 165A max current respectively, which is the spec of the 5 kW motor, delivering 10kW peak power. Based on the worksheet above, it takes 23 kW to accelerate in 10s. 2x 5 kW hub motors meet my desired drive cycle and performance.
I selected the 5 kW motor as the price difference for more power was negligible, but would allow me to upgrade to faster acceleration with bigger batteries later. I selected the 72V motor over the 96V motor as this was almost $1,000 cheaper. The motor is rated up to 120V max., but only warrantied up to 72V. The 96V version reduces the peak current, which drives down the cost for batteries, cables, switches, etc. I also need to take into consideration the diameter of the motor and only the 5 kW motor fits into my targeted rim size of 12”. The 8 kW motor is 13” diameter and the big rims would not fit my vehicle.