Battery Design Feedback for 20s10p

[chuyskywalker]

I'm shooting for a 72v system using 18650 cells and have an odd space to fill. (See thread)

In this design attempt, I setup each parallel group as 5 cells vertical stacked with button tops pressed against another 5 stack. The flow would look something like this:

Is this a sane design? Or is that P grouping more like a "mini series" and a bad idea?

 

[serious_sam]

That's 40S5P (and it's missing the parallel connection and balance point between every second set of series connections).

 

[chuyskywalker]

That's 40S5P (and it's missing the parallel connection and balance point between every second set of series connections).

You are 100% right and I was pretty dumb about this. Back to my drawing board >.<

 

[john61ct]

10P first, then serial

 

[chuyskywalker]

Reduced down to 20s5p with the intention to make two of them and run the output from the BMS's in parallel. Is that ok to do? What problems might that lead to?

 

[john61ct]

Just two strings, should be fine, double the BMS cost of course but round here people don't usually spend hundreds on those.

But have you got 5P20S, parallel first?

 

[serious_sam]

If you're going to run 2 packs/BMSs in parallel, you may want to consider the failure mode of one pack suddenly disconnecting .

ie. If the BMSs have LVC capability, and one disconnects its pack during a peak current draw condition, the second pack suddenly experiences double the peak current. Is this condition acceptable for the hardware you are going to use.

 

[john61ct]

Compared to all the power getting cut.

3 parallel strings is the highest I'd go, without balancing gear between the packs

 

[chuyskywalker]

If you're going to run 2 packs/BMSs in parallel, you may want to consider the failure mode of one pack suddenly disconnecting .

That's the only major downside I could think of as well. Using 30Q cells (15amp) could handle about 75amp potential draw. They _can_ do up to 20, but it's (clearly) not a good idea. With an 80amp controller, it's cutting things _very_ close should one side fail. However, the controller can also be put into different speed modes, which I'm pretty sure means it lowers the total amp threshold.