dlj04(at)josephson.com
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 Posted: Fri Mar 21, 2014 8:59 am Post subject: Earth-X lithium |
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Just as lead-acid electrochemistry is fairly well understood, lithium
cell vendors and types are well known. Every maker of small "aviation"
lithium batteries uses the same LiFePO4 18650 cell type as far as I
know, which is also the same as Tesla uses in their car battery packs,
and the same chemistry used in most smartphones today. There are various
quality levels available, with the leader seeming to be the Panasonic
NCR18650A, which is honestly rated at 2.9 amp-hours at 3.2 volts (more
than 2.9 can be claimed by assuming a lower cutout voltage, which
requires more precise low voltage disconnect to avoid damage.) Packs are
made of strings of 4 cells in series, which is why you see 8-, 12-, 16
etc. cell "batteries."
At last year's CAFE Electric Aircraft Symposium, Eric Darcy from NASA,
who is responsible for designing the lithium batteries for the Space
Station, gave some pointers on testing these ubiquitous cells for
quality (you measure open circuit voltage over a few weeks -- cells that
droop faster than the norm are suspect) and the measures that need to be
taken to assure safe charging and low voltage disconnect. We learned
that quality was primarily related to clean shearing and even winding of
the internal foil materials, with failures mostly due to minute stray
particles of the sheared material causing leakage, or misalignment of
the wound structure allowing the edges to touch.
Ideally each cell is charged separately. Next best is to charge each
pack of 4 cells separately. The trick comes in designing a battery
management system inside the box that manages charge current for
multiple packs and at the very least alerts you if something is wrong. A
battery company that will actually explain how they are doing these
things, and takes the time to explain why a watt-hour of lithium cells
might be more useful than the same watt-hour of lead acid, would be
worth paying attention to.
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