Re: Lithium-ion Batteries
- From: JohnNicolet@xxxxxxxxxxxx
- Date: Sun, 2 Dec 2007 14:49:46 -0800 (PST)
On Dec 2, 1:22 pm, Ned Forrester <n_f_orres...@xxxxxxxx> wrote:
budgie wrote:
On Sat, 1 Dec 2007 15:46:09 -0800 (PST), JohnNico...@xxxxxxxxxxxx wrote:
Hi
I have some lithium-ion batteries from laptops. Is there any reason
why I couldn't repack them and then use them in my cordless drill?
The packs are theoretically plenty to run a drill - especially as I
can pack them in series-parallel etc.
You possibly could, with a number of significant provisos ...
This assumes that the repacked batteries are of a voltage that is
compatble with the drill, and charging is not an issue.
Indeed. The two biggest gotchas are the charging and the reconfiguration.
I pretty much know that I can't. Perhaps my real question is WHY NOT?
In the laptop batteries there will almost undoubtedly be small electronic
protection modules. Their role is to protect the pack against (among other
situations) undervoltage, overvoltage, excessive charge/discharge current,
under/over-temperature operation, and cell voltage imbalance.
These modules are specifically tailored to the number of (series) cells in the
pack and the cell capacity. As soon as you reconfigure the series/parallel
arrangement you invalidate the pack protection module. To have anything like
the original pack's level of protection - which I would strongly recommend - you
would need to source and fit a suitable module, which is not a trivial task at
consumer level and in one-off quantity.
You would also need to arrange a suitable charger. The module may want to
communicate with a charger via a serial/1-wire data bus so this isn't a
straightforward option either. And the "original charger" in most cases is part
of a laptop motherboard.
IF - and it is a big if - you have a pack that provides a voltage, current
capability and Amp-hour capacity to suit your load, AND you have the original
(aka matching) charger for that pack, then a physical rearrangement of the cell
layout would be the limit of what is practical. Everything else is going to
cost you more time/effort/risk than sourcing a replacement pack for your
appliance.
And a final issue. The (probably) Ni-XX standard issue pack on your appliance
is far more tolerant of over-current moments like near-stall operation of the
drill. The overcurrent protection in those Li-XX protection modules we
evaluated/used were snap-action electronic switches, and unless your laptop pack
was rather generously sized I expect the near-stall current load will result in
some frustrating operation on Li-XX packs.
Just my 2c worth.
As a designer of multi-kilowatt-hour Li-ion batteries, I second
everything said above. I would add that the reason Li-ion batteries
need protection circuits, is that some Li-ion chemistries are thermally
unstable, and that instability is increased at an elevated state of
charge; thus, redundant protection against over-charge is very
important. Lithium-cobalt-oxide based cells are commonly used in
laptops because of their high energy density, and these are
particularly susceptible to thermal runaway. This is as opposed to some
high power density (high energy rate) cells, which are used in some new
power tools; some of these high rate cells use other chemistries that
are not as susceptible to thermal runaway, but they have as little as
40-80% the energy of the highest energy cells. As development
progresses, there are always new technologies coming along that may
change this. In any case, the cells from laptops are not really ideal
for the high discharge rate of a power drill; they have a higher
internal impedance than one would want.
Over discharge is not generally hazardous, but but it will damage the
battery so that it no longer holds charge. Hence the need for
protection from over discharge.
When a cobalt based Li-ion cell is stressed into thermal runaway, the
result is quite interesting. You can search for "lithium-ion" on
Google/You-Tube for some video of the outcome. It gets more interesting
when the cells are arranged mechanically so that the heat from one
burning cell propagates to the rest of the pack. One cell vendor we
work with notes that the cells should not be arranged in any pattern
other than in a single plane, because any other arrangement will
guarantee propagation between cells. As seen in the above videos, even
the planar arrangement of laptop batteries does not assure that
propagation will not occur. The key to Li-ion battery design is:
prevention. The triggers to avoid:
Over charge
Short circuit (causes internal heating)
Mechanical abuse (causes internal short circuit or electrolyte leakage)
External sources of heat (external fire)
Thermal propagation (hard to arrange, and not commonly achieved)
and the one that can't be designed around, except by the cell vendor:
Spontaneous internal short circuit.
--
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Ned Forrester n_f_orres...@xxxxxxxxxxxx Hide quoted text -
- Show quoted text -
Thank You very much for your response - much appreciated.
I (sort of) understand how I ruined a perfectly good Laptop Power Pack
when I tried/tested it on a power drill.
But there should be some good uses that discarded batteries can be put
to.
Thanks for the explanation
BobP
.
- References:
- Lithium-ion Batteries
- From: JohnNicolet
- Re: Lithium-ion Batteries
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- Re: Lithium-ion Batteries
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- Lithium-ion Batteries
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