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

33776 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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dexion

· #2 · Apr 7, 2011

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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Yabert

· #3 · Apr 7, 2011

Hi

48 volts car will be awesome! Like you said, only 16 cells, easy bms wiring, safety, etc.

But do you realise than 48v and 400A is only 16 Kw at motor shaft? It's a poor power. And your peak power with normally availiable controller (1000A) will be only 40 Kw.
And at such high amp, you need a lot of copper to have good efficiency. All wires, motor, battery, controller need to be stronger to pass high amp continously

Many of us need to go at high voltage because we need good performance electric car. My car for example, can give 156v (130v sag) x 1000A to have around 105 Kw at motor shaft.
Also, at high voltage, I will probably need less than 100A to cruise at relatively high speed (80 km/h, 50 mph).

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.

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.

frodus

· #5 · Apr 7, 2011

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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Ziggythewiz

· #6 · Apr 7, 2011

miernik said:
Wouldn't it be better to run everything on something around 48V, so its safe to touch?

Safe to touch isn't as fun to drive!

Safe is a relative term though, 'high voltage' in the 120-144V range isn't really that high. Most of the world uses 220-240V in home sockets, so although accidents happen, it's apparently safe enough for unruly peasants to use.

Defibrilators run in the 300-1000V range, with 1000V most typical, so that's where I'd be worried about safety. While I wouldn't lick a 120V battery (though I grew up licking the 9V ones) an occasional zap is unlikely to cause any real harm. Just a reminder to be more careful.

Yabert

· #7 · Apr 7, 2011

I'm not 100% sure, but personnaly, I think a 72v systems is enought safe.

The main advantage of a 72v systems is you can drop the amps draw by 1/3 for the same power.
200A instead 300A for a motor or a controller is a important difference.

PTCruisin

· #8 · Apr 7, 2011

I've always wondered how 48V is considered safe when anything greater is not. I suspect lawyers were involved. Also, thinking back to Edison's time, he made big public displays of demonstrating the dangers of alternating current to the point of electrocuting horses and elephants on the streets.

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?

steven4601

· #10 · Apr 7, 2011

Think I have posted this before on a different thread regarding efficiency. Most of it boils down on I^2R losses. (That is Current squared times the resistance)

Resistance is everywhere. Batteries, wires, motor windings.
I^2R losses mean if you double the voltage of your system for the same power, you reduce the losses factor 4. (read: only considering ohmic losses)

Something about safety:
High voltages are not dangerous, it is touching them which may have consequences. (no fun intended) If you are uncertain about your system and the (required) voltage's within it, my advice would be to step away and call a friend who can help on site.
(Educated) electronic (design) engineers often pause for a second to re check their decisions before continuing installation/removal/testing high voltage equipment.

Something about ' high ' voltages:
Voltages between 42-60DCVolts (rather large variations exist per country and industry class) are named SELV. Safe/Seperated Extra Low Voltage. Safe to touch, does not cause direct danger.

Between SELV and Low Voltage is the voltage range named ELV. Extra low voltage. This goes up to 120VDC .

Low Voltage lies between 120 to 1.5kVdc
High Voltage is 1500VDC and up....

I doubt any EV car uses High voltage systems

//Steven

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oaiken

· #11 · Feb 10, 2022

How about fresh water electrocution risk ? How bad is it when an EV crashes in a flood ?

cobratom

· #12 · Feb 20, 2022 (Edited)

I think the largest factor at play here outside of obvious amp draw issues on the motor and controller is Charge and Discharge rates of cells/batteries. You want them to last as long as possible, so you should have the lowest amp draw on them to discharge over a longer period of time.
A 100kW motor discharging a 400V battery pack is 250A discharge rate.
A 100kW motor discharging a 48V battery pack is a crazy 2083A discharge rate.

While sure its not a big deal to just have a number of cells in parallel, you would be introduced to a battery management nightmare where a BMS would not be able to regulate a single bad cell in the system or potentially warn you of it, which can lead to a shorted cell of worse. Tesla mitigates this risk on their battery boards by having fused disconnects between the cells. If a cell is shorted or outside of its parameters compared to the rest of the pack, the link heats up and breaks the single cell permanently out of the pack.

brian_

· #13 · Feb 20, 2022

cobratom said:
I think the largest factor at play here outside of obvious amp draw issues on the motor and controller is Charge and Discharge rates of cells/batteries. You want them to last as long as possible, so you should have the lowest amp draw on them to discharge over a longer period of time.
A 100kW motor discharging a 400V battery pack is 250A discharge rate.
A 100kW motor discharging a 48V battery pack is a crazy 2083A discharge rate.

The choice between high overall battery voltage (at low current) and low overall battery voltage (at high current) makes no difference to the current in the individual cells, so that configuration choice makes no difference to cell life.

At 4 V per cell (just for round numbers) with 100 Ah cells and 40 kWh capacity (for example)

100 kW from a 400 V battery would be 100 cells in series, each discharging at 250 A ("C rate" of 2.5), for a total battery current of 2083 A
100 kW from a 48 V battery would be 12 group in series of 8 parallel cells in group, each cell discharging at 250 A ("C rate" of 2.5), for a total battery current of 2083 A

cobratom

· #14 · Feb 20, 2022 (Edited)

brian_ said:
The choice between high overall battery voltage (at low current) and low overall battery voltage (at high current) makes no difference to the current in the individual cells, so that configuration choice makes no difference to cell life.

At 4 V per cell (just for round numbers) with 100 Ah cells and 40 kWh capacity (for example)

100 kW from a 400 V battery would be 100 cells in series, each discharging at 250 A ("C rate" of 2.5), for a total battery current of 2083 A

100 kW from a 48 V battery would be 12 group in series of 8 parallel cells in group, each cell discharging at 250 A ("C rate" of 2.5), for a total battery current of 2083 A

Well of course, but I did not suggest it as its a battery management nightmare. 100 cells in parallel you would be relying on the cells balancing each other, and unless you have redundant connections or fuse links (like the way Tesla packs are designed) where a dead cell or shorted cell can be withdrawn from the system the entire system can be brought down by 1 bad apple as the BMS is not able to regulate the single or small set of cells together.

brian_

· #15 · Feb 20, 2022 (Edited)

cobratom said:
Well of course, but I did not suggest it as its a battery management nightmare.

Not, it's not - in reality, 96 cells in series is by far the most common configuration of EV batteries today... including at Tesla.

cobratom said:
100 cells in a series you would be relying on the cells balancing each other, and unless you have redundant connections or fuse links (like the way Tesla packs are designed) where a dead cell or shorted cell can be withdrawn from the system the entire system can be brought down by 1 bad apple as the BMS is not able to regulate the single or small set of cells together.

Every single production EV has a battery management system which monitors voltages at the cell level and actively balances cell groups. No, Tesla doesn't have redundant connections; there is only one current path for each cell. The fusible links per cell used by Tesla are only used because they insist on using a large number of cells in parallel (other EVs with 2 to 4 large cells in parallel don't need them or use them); your 48 V configuration would require eight times as many cells in parallel as a normal configuration (they're actually 360 V nominal, not 400 V) making any issues with sets of cells in parallel worse.

brian_

· #16 · Feb 20, 2022

cobratom said:
While sure its not a big deal to just have a number of cells in parallel, you would be introduced to a battery management nightmare where a BMS would not be able to regulate a single bad cell in the system or potentially warn you of it, which can lead to a shorted cell of worse. Tesla mitigates this risk on their battery boards by having fused disconnects between the cells. If a cell is shorted or outside of its parameters compared to the rest of the pack, the link heats up and breaks the single cell permanently out of the pack.

You didn't need to add this to your post after my response to make it look like you already thought of this - putting it in your response to my post is more than adequate and much less confusing.

brian_

· #17 · Feb 20, 2022

cobratom said:
Well of course...

Not "of course". You claimed something that was not correct, I provided the correction with an explanation. Appropriate responses could include "thanks for explaining that" or "I didn't realize that"; claiming that something is obvious to you when it completely contradicts what you posted isn't going to impress anyone.

cobratom

· #18 · Feb 20, 2022

brian_ said:
Not, it's not - in reality, 96 cells in series is by far the most common configuration of EV batteries today.

I mistyped, which should be obvious. I meant to type parallel not series.

brian_ said:
Every single production EV has a battery management system which monitors voltages at the cell level and actively balances cell groups. No, Tesla doesn't have redundant connections. The fusible links per cell used by Tesla are only used because they insist on using a large number of cells in parallel (other EVs with 2 to 4 large cells in parallel don't need them or use them); your 48 V configuration would require eight times as many cells in parallel as a normal configuration (they're actually 360 V nominal, not 400 V) making any issues with sets of cells in parallel worse.

I did not say Teslas had redundant connections, I said they have fusible links which ties straight to my point of not even considering a low voltage high amperage pack.

cobratom

· #19 · Feb 20, 2022

brian_ said:
Not "of course". You claimed something that was not correct, I provided the correction with an explanation. Appropriate responses could include "thanks for explaining that" or "I didn't realize that"; claiming that something is obvious to you when it completely contradicts what you posted isn't going to impress anyone.

You are just arguing a point because from a theoretical point of view it's possible, but from a practical point of view it's not. No chance I am thanking anyone for coming in just for the sake of arguing when the practical application doesn't make any sense and is why no manufacturer is doing it.

brian_

· #20 · Feb 20, 2022

cobratom said:
I mistyped, which should be obvious. I meant to type parallel not series.

Well, that completely changes the meaning of your later posts. You could have just said that when you realized the error.

cobratom said:
I did not say Teslas has redundant connections, I said they have fusible links which ties straight to my point of not even considering a low voltage high amperage pack.

You did say

cobratom said:
... unless you have redundant connections or fuse links (like the way Tesla packs are designed) ...

... but perhaps you meant redundancy in cells?

So, we agree that the original claim that battery configuration changes cell current is incorrect?

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