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why do you make EVs with such high voltage? Why not 48V?

33739 Views 24 Replies 12 Participants Last post by  floydr
Why are DIY's electric vehicles made to work on such high voltages, like 96V, 144V and over 200V? Wouldn't it be better to run everything on something around 48V, so its safe to touch?

I know the wires would need to be 3-4 times thicker to get the same resistance (voltage drop) and current handling, but so what? Wire is expensive, but not so extremely expensive not to afford to spend 3-4 times as much on wire (which are not so long in a car anyway) in the name of significant safety.

We are not ever running more then 20kW though the wires, no?
So at 50V that would be 400A. About 200 mm^2 would do, no? Or about ten AWG4 wires in parallel for US guys. That would be about 150 EUR or 200 USD per meter, right? So how many meters of 20kW capable wire do you need in total in an EV?

OK, if you use a few meters, it's quite an amount of money, but still cheaper then getting a new life after touching 200V. And you can design the car to place batteries close to the motor to save on wire. At least I could imagine placing them withing 1 meter.

So are there any other reasons to use anything significantly over 48V, then saving money on copper wire?

If I can afford the wire, should I build a EV out of my 2035 kg van on 48V for safety (only 16 LiFePO4 cells in series, instead of 48 or 64 - what I spend on wires I can save on having a simpler 16S BMS, instead of trying to balance 64 cells...)?
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48V might be a good idea in your application, but realize this. You need to disengage the motors somehow above certain speeds. The issue is, if you gear those 48V motors for lets say 30mph tops, where 48V is 30mph, lets say that the motor rotates at 3000RPM, and you have a 15" radius tire. Lets say you put a 7:1 ratio from shaft to axle. That'd make you right around 30mph.

Now go twice that speed with the diesel engine, if you don't disengage the motor, you're spinning that motor at 6000RPM, which it is not made to do. It will fly apart. I've seen it happen.

Now, for your situation, you don't need a ton of power, but you're talking to EV-only guys. There aren't many (if any) Hybrids here, and many vehicles are built for MUCH more than 20kw. That wouldn't barely move a vehicle past 20 MPH for most of these vehicles. So you're situation is a little different. But for safety, low RPM, low power, 48V isn't so bad. Lots of off the shelf controllers, motors, chargers, batteries, BMS is easier (if used), DC-DC converters are easy to find, but the cable size goes up. Current capacity of the controller may need to go higher as a result.

One thing to remember is that RPM is volts, the less volts, the less RPM. Torque is amps. The more amps, the more torque. So to get more HP at low RPM, current goes way up, and the motor MUST be able to sustain high amps in order to pull the same power levels as a higher voltage lower RPM motor.
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