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

Tom Parker wrote:
> 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?

No. Consider a simple situation. You have a 300v string of 3v cells.
Some load is connected; a DC/DC converter or voltmeter, for example. If
you put a switch anywhere in the series circuit, what is the voltage
across that open switch?

It is 300v, because the voltage across the load collapses to zero with
no current flowing. It doesn't matter where the switch is, (+ or - leg,
or between any two cells in the string).

Now suppose that the "switch" is really a loose connection. When it goes
open, the voltage goes up to 300v!

If the gap is small, the 300v might cause it to arc over. This
momentarily powers the load, and current flows. The load determines the
current. If it's a DC/DC converter for example, it probaby has some big
capacitors across its input. They can cause a considerable charging
current! But then the current falls. If it falls low enough, the arc
will extinguish itself. But then the voltage builds up, and the process
repeats. Zap, zap, zap! ...

> 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?

The load (be it a DC/DC, motor controller, etc.) is likely to be
"insane" due to the wildly fluctuating voltage. Its response is
unpredictable, and it may even be damaged by the rapidly fluctuating
voltage and current.

> 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.

Yes, that's the basic idea. Bill is suggesting that you look at the
voltage between each pair of points, and make the spacing about
1mm/volt. So you'd have a close spacing between the terminals of a cell,
but a large spacing between the terminals of the two cells at the ends
of the pack.

> What looks much harder is providing this level of isolation between
> cells and the battery box itself.

Bill is talking about 1mm/volt if air is the only insulation. You can
have supplemental insulation at the other points in the system. The
wires are insulated, and may also be in insulated conduit and have other
forms of insulation. The safety regulatory agencies want *two*
independent layers of insulation, each adequate for the voltage in question.

> 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?

Paint and powder coatings are not generally reliable for electrical
insulation. But there are many coatings that are. Look for ones that
have UL, ETL, CSA, etc. ratings as electrical insulators.

It is probably safer to use insulating board materials, like bakelite,
phenolic panels, UL-rated fiberglass-epoxy, etc.

> 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.

I would be very leery of having grounded metal rods with their upper
ends exposed and near the cell terminals. It seems like an accident
waiting to happen.

You could insulate the rods at one (or preferably both) ends. But you
may want to look for materials other than metal to make them out of.
Fiberglass is very strong in tension, for example.

Lee A. Hart | Ring the bells that still can ring
814 8th Ave N | Forget the perfect offering
Sartell MN 56377 | There is a crack in everything
leeahart | That's how the light gets in -- Leonard Cohen

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