The required motor power depends on the speed you want to achieve and the dynamic resistance of the car.
A maximum speed of 100km/hr is required for over half of my journey to and from work. TO be able to keep up with traffic and not contribute to the growing problem of road rage, it is desirable to be able to maintain this speed while climbing a 1:10 incline. There are also a number of small hills to be crossed with a maximum gradient of approximately 1:3 however it is not necessary to maintain full speed on these sections.
I put a spreadsheet together to analyse the required power of the motor based on factors including aerodynamic drag, rolling resistance of tyres, viscous losses in transmission components and road incline. The curves below show the results for the force required to achieve any speed on various inclines. Also depicted is the force available from a typical 9inch DC motor. The diagram indicates that 3rd or 4th gear is suitable for highway use and an option to consider removing the gearbox completely.
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| Force Required and Force Available curves for a typical 9inch DC motor and original Gearbox |
My original calculations did however compare well with the actual results of another electric Hilux I found here. After some discussion with the owner of this vehicle and some friends regarding the method I used to measure the drag coefficient, I decided to proceed with the original design of a 9inch DC motor at 144V and make some allowances in the design to increase the range if I needed too (see range discussion below).
Acceleration
The calculations indicated the 9inch DC motor was not going to be a tyre smoker, with approximately 30 seconds required to reach 100km/hr in 3rd gear with a motor current limited to 500A. 60km/hr is achievable in 2nd gear in 7.5 sec. In order to achieve reasonable performance around town and on the highway I have decided to retain the original gearbox and use a motor controller capable of delivering 500A, preferably 600A.
Range
With this vehicle, range is the most important consideration. As we live on a farm about 20km out of town, the range has to be sufficient to get home comfortably as there are very few places to stop once you leave town. My design calculations were based on many conservative estimates and general figures for things like rolling resistance, drag and drive train efficiency. To undertake the design journey on a 60% depth of discharge (appropriate for lead acid batteries), a 144V 280Ahr battery pack is indicated. If an 80% depth of discharge is used (appropriate for LiPo batteries) a 210Ahr battery pack is indicated. These calculations again compare well with the other electric Hilux which achieved 100km range using 45 x 200Ahr LiPo cells however I still have some concerns about meeting my range target comfortably and I intend to make allowance in the design for increasing the range if need be.
Given that I will have already purchased the battery, the easiest way to increase the battery capacity is to add more cells, increasing the total battery pack voltage. If I need to do this it will involve some modifications to the battery box, additional battery management system components and changing the battery charger voltage - all relatively easy. It also requires the major components, principally the motor controller to be rated for the higher voltage. I will keep this in mind when selecting components, preferably seeking voltage ratings to 196V instrad of the nominal 144V the original design was based upon.
Both 200Ahr and 210Ahr LiPo cells are available with quite different cell dimensions. The choice of cell size and initial battery pack voltage will be determined by the geometry of the area where I intend to install the batteries.
Weight
To be completed.....
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