Hello all,
On Sat, 8 Dec 2001, Sushi wrote:
>
> Then discharge the battery completely. After your Newt automatically
> shuts off, try to turn it back on again. Do this a couple of times.
This is a bad idea!! You should NEVER discharge a battery pack (with
NiCDs,NiMH) below about .6V/cell, .8V/cell is perfectly acceptable.
For a 4 cell pack that's range of ~2.4V to ~3.2V.
What happens in a battery pack(of series cells) is that as you discharge
the pack, since the cells are not perfectly equal (as they can never be)
one cell discharges to empty before the others.
When this happens, this empty cell can go into cell "reversal" and is
being charged "the wrong way". Further discharge of the PACK will
certainly destroy the battery(that is now empty) AND the pack will never
behave the same.
The damage(of the cell) is not repairable by further charge/discharge
cycles, it's permanent!
(Incidentally, you'll notice on any pack of you buy Alkalines, you are
told never to replace just one cell in a device, you should replace all
the cells at the same time. The cell reversal issue is again why you are
told to do this).
Both NiCds and NiMH have a steep discharge curve, so if you actually watch
the voltage as you discharge, the pack/cell voltage will at first slowly
fall then as you near .6V/cell the voltage(cell/pack) will drop off very
steeply.
Again, when discharging, it's important to stop as soon as you hit
.6V/cell, I prefer .8V/cell .
Also keep in mind measuring the cell/pack voltage can be a little tricky.
Pack/cells(of any type, NiMH, LiOn, etc) should measured under some load.
A 1kiliohm resistor across the pack/cell is typically a sufficient
load(don't violate the power rating on the resistor either! Use a 1/4 or
1/2 Watt to be safe).
Even if you are using a good multimeter/scope, if you simply measure the
pack or cell voltage "un-loaded"(without it being connected to anything
that is drawing some power) the voltage reading you get COULD be higher
than what the cell/pack voltage might be in your device (Newton, Walkman,
etc) and would be incorrect.
The "incorrect" results one gets when measuring an unloaded cell/pack
happens because the multimeter/measuring instrument has a high
impedance(so it doesn't draw a lot of power from the device it is
measuring), much higer than the cell's impedance, and the voltage measured
at this voltage divider(by the meter/instruemnt & battery) is incorrect.
When you stick a load on the pack (like a 1k ohm resistor), the resistor
"exposes" the true impedance of the battery pack(which is what we
mathematically model as the cause for the less than 1.2V rating on the
cell) and the meter now sees the voltage drop as a result of the battery's
impedance.
Mathematically, as the battery gets discharged, the impedance/resistance
of the cell goes up, and accounts for the lower voltage reading. Hence, by
measuring the battery voltage, we have a decent idea how empty or full the
battery really is. (It's not an absolute indicator of course).
If you aren't convinced about the loaded/un-loaded issue, try using some
NiCd/NiMHs until the pack is near empty. Un-connect the pack and measure
the pack voltage with a 1kohm resistor across it.
Then come back 2 hrs later and measure the the voltage un-loaded. You
might see that the voltage has actually GONE UP! Does that mean the pack
has more power/is OK to use?
No, try measuring the pack voltage under load(with the resistor) and you
will see that the reading will be close to what you measured the first
time, under load. BTW the unloaded cell voltage goes up because surface
charge builds up on the contacts of the cells, but this charge isn't of
sufficient capacity to do any real work. As you can see, blindly measuring
the voltage might have given an erroneous result.
Also, for those curious about charging, to make sure you never damage
NiMHs OR NiCDs they should be charged in an "intelligent" or "peak
detecting" charger. These chargers carefully watch the cells while they
are being charged and know when to stop to prevent over charging.
Do not use any other kind of charger if you expect to get decent life &
performance from either cell type(the "timer" type chargers have no way to
know when to stop and often result in overcharged/damaged cells).
Furthermore(assuming a peak detecting charger), if a charger says it can
charge NiCds but doesn't say NiMH, then only use it for NiCds, DO NOT use
it for NiMH.
If a charger says it can charge NiMH but doesn't say NiCd, then it is
however OK to charge both NiCds & NiMHs.
Overcharging either cell type results in excess heat building up and the
cells will "vent" -- literally leaking. NiMHs are FAR less tolerant of
overcharging than NiCds so you really have to be careful with them.
All of this advice might sound like a bunch of silly rules, but really
it's all true (and based in sound science).
I didn't mean to get long winded about this stuff -- I could have easily
just said "don't discharge past .8V/cell" and measure the cell voltage
like this, but explantions to rules have always made the rules more
meaningful(at least to me).
Hopefully I've removed some of the mystery surrounding rechargables /
proper care and feeding =).
Finally, if you have bad cells, of any type (Alkaline included) , do not
just throw them away. There are places (both US and in other countries)
that recycle such cells.
Pleas check out:
http://www.rbrc.org/ (US & Canada)
http://www.rebat.com/ (UK)
and/or consult a search engine for other countries.
Thanks!
Speedy2.
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