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Re: [EVDL] "One mm per volt" Battery pack layout guidelines

Bill Dube wrote:
> >
> >First, arcing is not a problem below about 30v.
>
> But high-current arcing _is_ a problem below 30 volts in a powerful
> battery pack. Especially a large one. You _might_ get away with 1/2
> mm per volt separation below 30 volts,

Is it safe to assume that the battery will remain intact? If you have 3
3.2V cells stacked together, we can assume that the voltage won't ever
exceed 15V or so and ignore what might happen if they were part of a
300V battery and one of them went open circuit under load? I think this
situation will remain somewhat safe because we expect the current to be
controlled by the motor controller and stop in short order in such a
situation?

If we do assume this, it's reasonably easy to maintain 1mm/V within a
battery pack by laying the cells out correctly and using insulating
plates (which an arc has to go up and over) if we have to make cells
double back on themselves.

What looks much harder is providing this level of isolation between
cells and the battery box itself. At the ends of the battery you would
need 0.5mm/V between the terminals and the frame (as there are two
frame-terminal gaps, one at the +v and one at the -ve end of the battery
and assuming the frame-terminal gap is the same at each end).

I'm using 40Ah thunder sky cells which have very small terminal to edge
distances and constructing a hold down system which doesn't scare me
half to death is proving difficult. Can you trust a generous coat of
paint? What about powder coating?

The best solution for these little cells I've come up with is a rod down
the middle "rib" hole between two cells with a plastic stand-off keeping
the nut away from the terminals. It sounds like the nut should be
recessed into the standoff and a plug inserted on top, there is no
exposed metal in the middle of the cell. In cross section, it looks
like:

|#|
|x|
--ttt--+--ttt--
| | |
| hold |
| down |
| rod |
| | |
|------+------|
========|========
-+- insulting washer
x

Where:

"ttt" are the terminals
"x" are the nuts at the end of the hold down rod
"#" is the plug in the standoff (which is also held down by the same
nut).
"=" is the floor of the battery box with an insulating washer between it
and the bottom nut.

With these hold downs, you can restrain sideways movement with a metal
frame at the bottom and use a plastic or wood or other insulating box to
keep the weather out.

Does this sound sensible?

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