[Dadrick]

Hello I am another newbie here with some questions; but I’ll start with a little background. Back in June I bought a new Chevy Bolt, which is my first foray into the EV world. Then this month I took the Bolt to a National Drive Electric Week event where I came across a 1976 AMC Pacer Conversion; this got my brain juices flowing. I began researching a DIY conversion which brought me to this forum and I’m hoping that you all can clear up some mysteries of the magic that happens under the hood of an EV.

How is pack voltage related to motor voltage? My though experiment involves a pickup truck, from what I have read it would be wise to go with a 144 VAC motor because of the weight and poor aerodynamics if I want to attain and maintain highway speeds. For now, let’s assume LiFePO4 cells, which are a nominal 3.2v. 45 in series would create a 144V pack. Then depending on the Ah capacity of the individual cells this would determine the capacity: 100 Ah cells would be 14.4 kWh, 200Ah->28.8kWh etc. So far so good?

Now if I double string to 90 cells in series I get a 288V pack (with 100Ah -> 28.8 Kwh). Does the motor controller then regulate that Voltage down to the 144V that the motor is rated for, or does the motor run at 288V? If it runs at 288V does it use less amperage to maintain a given rpm, effectively drawing at .5C? Does it decrease the life span of the motor?

Or is it wiser to take that same 90 cells and pair them in parallel then in a series of 45 pairs of 100Ah cells to maintain the 144v at 200Ah for the same capacity of 28.8 kWh?

Thanks,
Dad

 

[major]

The motor controller can regulate the 288V down to 144V for the motor, if it is rated for the high input voltage and your desired output (motor) current.

Choice of running 90 cells in series or in 2P45S is up to you. Obviously both options yield the same energy and power. Pros and cons to each choice. Less series connected cells reduce BMS complexity. Higher pack voltage can reduce effects of sag should you be running motor hard. Those are the two biggies that come to mind. But there are many other factors from cable size to component costs to consider. You'll just have to do a system comparison.

major

 

[Solarsail]

A motor is designed for a certain voltage and power rating. Which is its optimal operating point. Depending on the motor design, it can take a range of voltages. But there is a limit to what it can take, for example in an AC synchronous motor.

My experience with motors and controllers is quite limited, so please take what I write with a grain of salt.

The controller is also rated for a voltage and power. It will generally want a tight range of voltages, i.e. what the battery will supply, and usually cannot operate at twice the rated voltage. The input and output rating of a controller is normally fixed and not adjustable. So if you double the string, you will have to replace the controller, and most likely also the motor, so that they both operate at the higher voltage and are better matched. Although a 288V controller can produce power for a 144V motor, I would think that for the sake of efficiency, the controller output will be rated close to the input voltage. And a 144V motor would require twice the current as a 288V motor for the same power, and the controller better be prepared for that.

For the same power, a motor running at 288V will require half the current (amperage) as a 144V motor.

Power = volt * A
Range = Energy = volt * Ah

If you want to double the range, it is best to keep the voltage constant, and double the Ah capacity. Doubling the power (to get better acceleration and top speed or hill climbing) while keeping voltage constant will most likely require the replacement of the controller and the motor, so they may handle the doubling of the current.

So doubling range is a lot easier, by putting more batteries in parallel, than doubling the voltage or current. Even there, the central vehicle control system may complain about the doubling of range and get very confused, and shut things down.

So yes, it is a lot wiser to maintain the voltage V and the current A, and double the range Ah. Or a lot of things will have to be redone, including mechanical components that deal with torque or speed, etc.

 

[Dadrick]

Thank you for the replies. That was what I was thinking, I just needed confirmation from others that know much more than me.