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

33744 Views 24 Replies 12 Participants Last post by floydr, Feb 21, 2022

miernik

Discussion Starter · Apr 7, 2011

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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miernik

Discussion Starter · #4 · Apr 7, 2011

Yabert said:
48 volts electric car will be good idea only for a city car with poor performance(always under 60 km/h, 37 mph). Over that speed, you will need too much power and more power at 48v = too much amps.

That's perfect, I only need to be able to travel max 40 km/h (25 mph) as it will be mainly for city use, and the speed limit in most streets in my city is 30 km/h (18 mph) anyway!

And it is to be a hybrid, so the diesel engine is to stay for eventual highway-use. But the curb weight is 2035 kg (4486 lb) and its a van with which I want to be able to carry some stuff, van+batteries+stuff might go up to 3000 kg (6600 lb) for example.

Do you think a 48V system will be able to pull it at 30 km/h (18 mph)?

I want to put two motors, one for each rear wheel, so the load will be divided in two (two controllers, two motors), so not so much load per motor, controller.

Should I make it a 48V system in such situation?

Especially as I really like hacking, tinkering, modifying all the time and it will be very hard for me to prevent myself touching a wire at some point.

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miernik

Discussion Starter · #9 · Apr 7, 2011

frodus said:
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.

What if I do something like this guy: http://www.electrichubcap.com/

Motor straight on the wheel?

Then the RPM of the motor is the same as RPM of the wheel, which with my 215R14 wheels which are 708.2 mm in diameter, they only roll 300 RPM at 40 km/h (25 mph).

I've put my vehicle data in a spreadsheet, and it calculated that at 40 km/h (25 mph) with 300 RPM the torque demand would be 170 Nm and the power to maintain speed 6736 W (all on flat terrain of course).

I derived the right drag coefficient (0.31) and drive-train efficiency (79%) from the fact that according to specs, the van can move at max 121 km/h with curb weight 2035 kg and the stock max engine power is 58 kW. From that calculation it also turns out that the power consumption at 40 km/h will be 170 W/km.

Does the calculations sound plausible?

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