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

33735 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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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

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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.

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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.

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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.

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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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