7.2 kWh battery pack, choosing 600 Volts vs. 400 Volts

[elima]

As part of a uni. project, I will be building the above mentioned battery pack. The battery is going to drive two motors. The motor controls can work with both voltages. I am now wondering which voltage to use and therefore seeking your help, thoughts and ideas.
The maximum power dissipation is gonna be 80 kW.
From checking what the big automotive companies use in the battery packs, the voltages that I have seen are max. 415 Volts and I am trying to understand why is that.
When comparing both voltages these are the points that I can think of:
*Higher voltage -> less current -> thinner wires
*Lower voltage -> more cells in parallel -> less resistance -> a bit more balanced pack

One of the reasons I am wondering about the voltage is the need of having dc-dc to have a low voltage system as well. I haven't found any 600 Vdc - 12 Vdc so far.

Any thoughts, comments, ideas will be appreciated.

 

[fellow]

Hello and welcome! Some reasons that passed thru my mind:

Reason nr 1: Cells can't be stucked above certain number. When "bulk" charged, maxiumum voltage on some cells can exceed safe voltage (4,3V/cell on one, and 4,1V/cell next to it for example). This is especially apparent when stucking hundreds of cells due their R_i not being exact the same. BMS must work very hard to keep this imbalace under tight controll. As we know, BMS is only band aid (my first job was designing one and I hate those devices with passion) and there is a practical limit where it would simply became too complicated to properly charge the battery pack. Some cells would tend to overcharge. This can be solved by charging every cell separate, which adds the complexity to the system. Another solution to this is bulk charging smaller groups of cells at lower voltage, and than connecting those sections in series when used. This adds the complexity too, especially in the regen mode...

Reason nr 2: Safety margin. Fairchild, Infineon etc recommend extreme safety margin on their devices because of voltage spikes, and bad performance close to maximum V_DS. 400V systems would need 1200V devices if their recommendation is to be followed (no one on the ES does ). 600V systems would need components that are very expensive. I have 2x75V=150V system on my ebike, but there are no suitable (=cheap) mosfets with low R_ds for use at 150V yet. I would have to use IGBT's at this voltage, and designing that would mean less time to drive my ebike.

Reason nr 3: Every European house (and large part of the world except North America and Japan) has 400V AC as the standard voltage, and a lot of know-how and tested components do exist for this particular voltage (take a simple thing as a wire or circuit breaker for example). 600V is "less used" voltage, 3 x 400V is more logical voltage to take (easier to charge). Not sure why they took 415V, probably because 400V motors already exist and 415V is close to it. It is prefered to improove the existing design than start from the scratch and fight a lot of unknowns (because of the risk of project not being ready in time).

As for 600V DC to 12V DC, there are 1200V rated IGBT's that are suitable for this task. Above 1000V~1200V, and you start to have sparks jumping from one wire to another, if wires are too close. Not very sexy.

 

[Ohbse]

There has been much discussion on these boards regarding voltage VS current when it comes to battery design.

I personally agree with many very clever people that there's no real reason now and certainly not in the future to use high voltage to achieve desired power.

As fellow mentioned, I suspect most of the reason many EV's are designed around ~400v pack voltage is due to historic industrial motors being designed for this voltage. In a battery powered EV none of the reasons that led to high voltage AC being widespread really apply. You do not have to transmit power over long distances, so copper losses and conductor sizing is much less of a concern. Depending on the physical layout of your drivetrain it may be possible to have the phase connections *very* short. The amount of conductor losses can be minimal even at very high 1,000+ amp loads.

The downsides of HV packs are many, fellow is absolutely correct when it comes to BMS complexity and charging concerns. These do not apply nearly so much at lower voltages. There are also obvious safety concerns, 400v DC can kill you, 600v is potentially very dangerous. 84v however is not so scary.

Do you already have a pair of motors/controllers that you need to power? If you don't yet have these components, consider redesigning the whole package to reduce danger, decrease complexity and simplify packaging at a 20s or 72v nominal level.

 

[fellow]

At 80V, 1000A DC fuse is needed. 774 USD each. Picture for fuse fetishists only:

Whatever you do, choose a system of 999V or less, to keep maintenance possible by normal "non electricity pole" people. To work at 1kV+ (1000V+), special licences are needed in many countries. In any case, 80kW has the potential to became a very expensive exercise. By the way, another common voltage in the world is 690V (new de facto industrial standard).

 

[elima]

Thanks for the replies. In the previous year, one guy designed a similar battery pack, and the maximum voltage he used was ~600 Volts. There was a problem in balancing the battery pack due to bad bms/balancing boards and bad cells. I have seen already other guys with 600 Volts and perfectly balanced battery pack.
The motors that are gonna be used are DC motors, so no AC will be used.

If say I have 288 Li-Po cells, 6400 mAh & and 30C continuos discharge for each cell, these are my options for the battery pack:

1. 144S2P - For the 2 parallel cells I increase the capacity -> 2*6400 = 12800 mAh, Maximum voltage: 4.15*144= 597.6 Volts, Nominal voltage: 3.7*144 = 532.8 Volts, Minimum voltage: 3.0*144 = 432 Volts, Maximum continuous discharge current: 6.4*2*30 = 384 A ->Total battery capacity: 12.8*532.8 = 6.82 kWh

2. 96S3P - For the 3 parallel cells I increase the capacity -> 3*6400 = 19200 mAh, Maximum voltage: 4.15*96 = 398.4 Volts, Nominal voltage: 3.7*96 = 355.2 Volts, Minimum voltage: 3.0*96 = 288 Volts, Maximum continuous discharge current: 6.4*3*30 = 576 A -> Total battery capacity: 19.2*355.2 = 6.82 kWh

Now to calculate the maximum current when dissipating 80kW from the battery pack, we use the minimum voltage:
Option 1: 80000/432 = 185.2 A , Option 2: 80000/288 = 277.77 A

What would be the better option?
Thanks!