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Re: [EVDL] Robotics Institute Video


I thought the video was going to be dry, and it sure was, but the amazing thing,
in my alleged mind, is that I understood almost all of the concepts presented.
That's amazing for me, considering my last attempt at formal education was about
thirty years ago and that didn't work out all that well.

A couple things crop up in the video that folks here may be able to clarify, I

There was reference to starved battery construction. The electrolyte is not a
fluid per se, but is fully incorporated into the separator and the anode and
cathode, except when there's a failure somewhere in the system that causes a
breakdown. Let's ignore the breakdown portion for the purpose of this
discussion, since it won't matter anyway. If there's no liquid in the system
and the venting only happens on a failure mode, why would the batteries have a
"this side up" consideration? I recall reading that it's not a good idea to have
a LiFePO4 cell placed with the terminals on the side, when otherwise they would
be on the top. Is this a misconception, or is it important to have the terminals
on top for cells of that design? I'm looking at it from the point of view of
being able to place a larger capacity cell on its side to make it fit better,
rather than to have a pair of cells in parallel in order to get the same
capacity in the limited space available.

I like Jay Whitacre's attitude about cost effectiveness of a BMS system versus
the return on investment, so to speak. He advocates a bit more work at the
beginning, charting out cell capacity and R-sub-S (?) to get matched cells in a
battery, rather than counting on a large expensive collection of modules to
attempt to keep things in balance. He does qualify his beliefs by suggesting
individual cell voltage monitoring, which I want. Perhaps the money spent on the
monitoring system qualifies not spending money on the balancing system, which is
fine with me.

>From an economical perspective, how would one perform cell capacity and R-sub-S
charting at a home-builder's level? I'm willing to invest in some reasonably
priced data collection devices, especially if such devices would then integrate
into the vehicle after installation, but even if not. I've not seen any
reference to such equipment, but I've also not been looking for anything of that
sort and it would evade my notice therefore.

If I'm going to need 40 100ah capacity cells and then perform cell matching
tests and reject some for not being in the cloud of dots on the whiteboard, how
many will I have to purchase? I get the impression that better companies (A123?)
might provide tighter clustered cells than another less diligent supplier.

Considering that building a battery of say five or six cells that are closely
matched provides a good module, what's the difference between that module and
the next one, which is composed of closely matched cells, but some distance away
on the whiteboard? That is to say, there's two different levels on two different
batteries, but they are eventually connected in the string anyway. This
diagram of two batteries of differently matched characteristics:

|'|'|'|'|'|' (cable) |'|'|'|'|'|'

isn't electronically different from this battery, is it?


Does Jay Whitacre suggest that one should collect forty cells of all the same
dot pattern, so to speak? Based on his presentation, buying from a quality
supplier might allow for that and then the question above is moot.

The temperature considerations presented in the video are fascinating. We've
seen temperatures of more than one hundred degrees Fahrenheit in our vehicles
and I'd feel more comfortable with some form of temperature monitoring as well
as voltage monitoring on all 40 cells. He makes sense about attaching the
thermal sensor to one of the electrodes rather than the case, even though my
Zivan voltage sensor is all plastic and has no means for such attachment. Apples
and Windows, I suppose. If the temperatures got out of hand, I would consider to
attach a CPU heat sink to each terminal for additional thermal shedding, but the
temperature probes would be the final basis for that much work.

I understood the suggestion about charging or discharging the collection of
cells by connecting them in parallel, but there wasn't much detail about that.
He did suggest that using the discharge end was preferable to charging, and I'm
guessing that it's easier to discharge a parallel pack than it is to charge one.
What methodology would be used in such a circumstance? I'm pretty confident
that one would use a low-current load over a long period of time, to keep things
within recommended limits but specifics would be welcomed.

I'm more optimistic than I was about building up a solid, reliable, long lasting
pack, if I can get some answers to the questions above.

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