Best course of action for high current DC supply

[Braeden Hamson]

Hi,

I'm on an FSAE team that's making an electric car. We're making a battery pack out of discrete 18650 cells. In the pack every cell must have a fuse between it and its bus bar. So we have to size the fuse and, rate it for the appropriate current. Somewhere from 300A-200A at ~1V. I need to refine my calculations on heat input into nickel fuses. Right now I have to competing ideas as to the design of the power supply.

Option 1:
I take a variable AC power supply and connect it to a modified microwave transformer with the secondary cut out and a few turns of thick wire to get a high current power supply. Then rectify the AC into DC. The trouble is I need some really, really, beefy diodes. I can then vary the voltage of the supply to vary the current output. I will probably make some logic to make it constant current.

Option 2:
I use a car battery, they can do high current. Then build the mother of all buck converters. This idea is less fleshed out, I don't know what the best (good current control, lowish noise and, simplicity would be nice, in that order) method of controlling the power out of the battery would be.

Option 3: (Bonus round?)
Supercaps? An electronics surplus store near me has at least 40 of them and they're about the size of a tall beer can.

As always thank you for the help

 

[BobK]

We're making a battery pack out of discrete 18650 cells. In the pack every cell must have a fuse between it and its bus bar. So we have to size the fuse and, rate it for the appropriate current. Somewhere from 600A-300A at ~1V.

This makes no sense. You are not going to have 300 to 600A going into or coming out of an 18650 cell.


Bob

 

[Braeden Hamson]

This makes no sense. You are not going to have 300 to 600A going into or coming out of an 18650 cell.


Bob

Its a 72s12p pack, ~300V and ~280A. If a cell shorts bad things happen. We're going to rate the fuse to blow at something like 50A over 30 seconds. We don't have that part nailed down yet, input on that is appreciated as well. And like I said 600-300 is just a very crude probably very wrong calculation, I'm in the initial parts of this design. I changed it to 300-200A, after a sanity check. Thanks for that.

 

[davenn]

I changed it to 300-200A, after a sanity check.

and you also said ~ 1V is that wrong as well ? as you say something different in a later post

 

[Bluejets]

Someone on "your team" needs a new calculator.
Appears the expectation of the output possibilities is way over the moon.
Just as an example, 12P @280A = 23A per cell.
10% of that would be pushing a genuine 18650 and even then, perhaps for a fraction of an hour and that is in an ideal world.
Cell properties are never identical.
Supercaps, car batteries....
Check out "batteryuniversity" site and do some reading.

 

[Braeden Hamson]

Someone on "your team" needs a new calculator.
Appears the expectation of the output possibilities is way over the moon.
Just as an example, 12P @280A = 23A per cell.
10% of that would be pushing a genuine 18650 and even then, perhaps for a fraction of an hour and that is in an ideal world.
Cell properties are never identical.
Supercaps, car batteries....
Check out "batteryuniversity" site and do some reading.

This isn't a continuous discharge application. Stab the throttle for 2 seconds then brake. That's racing. Our datasheet says 20A peak discharge. So push them a little over. Again racing. We can only use the car for one year so no point in being nice. And why is "your team" in quotes? Maybe it's a generational thing, but sounds odd.

 

[Bluejets]

This isn't a continuous discharge application. Stab the throttle for 2 seconds then brake. That's racing. Our datasheet says 20A peak discharge. So push them a little over. Again racing. We can only use the car for one year so no point in being nice. And why is "your team" in quotes? Maybe it's a generational thing, but sounds odd.

With the lastest details I forsee a big bang.
How does "your generation" handle them apples.

 

[BobK]

In the pack every cell must have a fuse between it and its bus bar.

Assuming you are correct that your cells can output 20A, then the fuse needs to be for about 25A, not 300 to 600A since the fuse is seeing only the current of one cell.

Bob