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

33744 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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There are many factors at work here.

Power is the constant

lets just say you have a small and light conversion at 2000 lbs (and keep in mind your conversion is 2.3 times that weight.)

lets say you want to go 45 mph

It takes 12000 watts for example to hold 45 mph.

Thats 80 amps at 150V but 250 amps at 48v.

Ok not so bad.

Now you want to go 60mph assuming the correct gearing it would take 24000 watts to hold 60mph

160 amps at 150v
500 amps at 48v

Again on a powerlevel its doable (pureley power)
16 x 400ah 3.2V cells = 20KW
48 x 130ah 32.V cells = 20kw

Where you run into problems would be motor and controller ratings.

a warp 9 motor is rated at about 225 amps for 1 hour.

This means at 48v you can go about 40mph for an hour with good cooling before the motor is overheated (there are also efficiency factors at low voltages/rpms that hurt more.)

But at 40mph at 150V its way under spec (long life)

The same problem exists with the controllers. Their 1 hour rating is way lower than their peak advertised rating. For instance mine is 750 amps for 2 minutes, 550 amps for 5 minutes and 275 for one hour.

We are going for long life on the components (and a good top speed.) The compromise is higher voltage and thus lower amps through the motor and controller.

You are right its safer and easier (for you) but not for the motor and controller if you want to go much over about 35mph.
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