On Mon, 23 May 2005 18:12:00 +0100, wig
wrote:
Someone else said NiMh don't like overcharging, I was under the
impression NiMh could be charged at any time irrespective of their
charge status, whereas NiCd should only be charged when empty. And NiMh
could be charged for as long as you like. whilst NiCd don't like
overcharging. I await confirmation from a higher source.
There are basically two ways of charging NiCd and NiMH cells. The
first is to use a charge rate of no more than 1 tenth of the cells
capacity. So for a 1500mA hr cell you could use a charge current of
no more than 150mA. The cell will take about 12 hours (150 x 10 = 10
hours plus some extra for energy losses) to charge from empty to
full. The cell can be left on this charge for well over 12 hours
without harm although for standby use (where they are always on
charge for months on end) a rate of one twentieth of capacity (so
75mA in the example given) would usually be used.
This method of charging is reliable, cheap and safe however it takes
a long time.
The second method is to charge the battery at a much higher rate and
detect when it is fully charged by sensing one or both of the small
voltage change which takes place when it becomes fully charged or
cell temperature.
Voltage change sensing is usually called Delta V (change in voltage)
or DV sensing. It can work either by sensing the small voltage fall
which happens as overcharging starts (-dV) or the rate of change of
charge which happens just before full charge (+dV). With NiCd cells
this voltage change is of greater amplitude than with NiMH so a
charger designed for NiCd will usually fail to sense if a NiMH
battery is fully charged and overcharge it. Most chargers relying
upon dV sensing also use temperature sensing as a backup. The
problem wit h this is that the temperature rises only when the cell
is overcharged so thermal sensing is a bit late to avoid some cell
damage. NiMH batteries also heat up throughout the charging process
so again older NiCd chargers with thermal sensing may fail to detect
the rate of change of temperature which occurs on overcharge.
Putting a NiCd cell in an NiMH charger is always safe for the
battery, putting an NiMH cell in a NiCd charger often isn't. Neither
type of cell will tolerate overcharging at high rates.
THe mythical "memory effect" you will see mentioned everywhere in
connection with NiCd is actually nothing of the sort but something
properly called voltage depression and is simply a reaction of the
battery to _overcharging_. It has nothing to do with
under-discharging (which was the cause of the original "memory
effect" only ever seen on one uncommon type of cell construction in
outer space!).
As an aside the "chargers" supplied with cheap electric drills are
the worst of both worlds. They are unregulated and usually charge at
about one quarter of the batteries capacity. This gives a charge
time of about 5 hours, too long to be useful but with a charge
current far too high to be maintained for more than 5 hours without
causing damage. Leaving one of these batteries on charge overnight
once is usually enough to damage it irreparably.
In general NiCd are better if the application means they are going to
be treated roughly and charged often. They also usually have a
capability to survive more recharge cycles than NiMH. NiMH are
better if the best power/weight ratio is required. So things like
cameras, where a lot of energy is needed in a small package, suit
NiMH whereas electric power tools are usually better suited to NiCd.
--
Peter Parry.
http://www.wpp.ltd.uk/