On Fri, 08 Jul 2005 17:08:34 GMT, Bill Thomas
wrote:

Andy wrote:
I was just looking through the Mar/Apr issue of Wood magazine, where
they did an article on myths of rechargable batteries. One thing they
said really confused me - they said that rechargable batteries should
NOT be discharged completely, and that you should recharge them as soon
as you feel your tool slowing down, because if a cell is totally
discharged it can switch polarity and ruin the whole battery (or
something like that). I'd always heard that with NiCad batteries, you
DO want to discharge them completely, so they don't develop a 'memory'
and accept less of a charge each cycle. Does anyone have a good
explanation for why one or the other is true, or firsthand experience
trying it both ways?
Also, they said that NiMH batteries are really not superior, because
even though they can have larger amp-hour ratings, they don't last for
as many charge/discharge cycles. I'd also read elsewhere that NiMH
batteries don't develop a memory, which seems to me like it would make
them last longer. Has anyone used both types side-by-side through the
whole life of a battery?
TIA,
Andy

Greetings,

NiCad battery memory problem can only occur under special circumstances,
which never occur in how any one uses NiCads, so don't worry about it.

You do want to discharge a cell, but not reverse the polarity.
A battery has many cells. Take the battery voltage and divide by
1.3 to get the approximate number of cells. If you have a 14V
battery pack, you do not want the voltage to drop more then 1.3 Volts.
If more than that, you have a good chance of reversing the polarity
on one of the approximately 10 cells.

Nicads have a fairly constance Voltage until they reach the discharge
level, then the Voltage drops quickly. If you start the recharge
just as the Voltage drops, i.e. when the tool starts to slow, then
you recharge when the battery has mostly discharged.

People have done side-by-side testing. Let's hope they reply to
your posting with the results or references to the results.

Sincerely,
Bill Thomas


FYI: Cell reversal is caused when a cell in a pack has a lower
capacity than the other cells and that cell reaches 0 volts before the
others. The other cells in the pack then pump current backwards
through the depleted cell causing the reversed voltage.

The NICAD memory effect, according to the history/legend that I heard,
dates to the late 50s and the Vangard space program. the US lauched
the Vanguard satelites into orbits that maintained precisely the same
amount of time time (to the second) in the sun, charging the
batteries, and in the dark, discharging the batteries. Thus, the
batteries were charged/discharged to exactly, and I mean exactly, the
same point for their entire life time. They developed a memory.

So there are 2 conditions for memory.
1. You must be using ancient NICADs (I think someone else pointed this
out)
2. You must discharge precisely to the same point each time.

Using todays hand tools in normal everyday use, you are unlikely to
achieve these conditions.

Now, there is an effect called charge depression, or capacity
depression where the discharge curve is lowered, not shortened. The
end effect is to make it look like there is a memory. However, it is
caused by overcharging and not by repeated discharging to the same
point. This effect is what most people refer to when they say they
fixed a capacity problem by deep cycling a battery.

Also, some cells develop large crystals on their plates that can grow
large enough to puncture the insulating separator between the two
plates of the cell. This shorts out the cell and unless you can blow
out the short the battery pack will exhibit reduced voltage and will
appear to have reduced capacity. Pulse chargers, where high current
for brief periods of time (pulses) are used to charge the battery, are
sometimes good at blowing out the short but you have to realize, once
that insulator is punctured, the cell is essentially destroyed. you
may be able to bring it back to life by zapping it but the cell
essentially self discharges and the crystals grow back. You might as
well buy a new pack rather than fool around with zapping cells. Or at
least start saving your pennies because you're going to have to buy a
new pack sooner rather than later.

BTW, in order to effectively zap cells, you need a really high current
source. the best way that I found is to connect another fully charged
nicad cell across the shorted cell. positive to positive, negative to
negative. I'm talking cells here, not the entire battery. The charged
cell will discharge into the bad cell and each cell should end up half
charged. You should monitor the voltage of the pair to see that the
short really is cleared or else you're going to be dumping an extreme
amount of current into a low resistance short which can only lead to
bad things happening.

Obligatory warning time: zapping cells can result in dangerous buildup
of hydrogen gas in the depleted cell. Be careful and monitor cell
voltage to ensure the short is cleared.

On the subject of these crystals, they really only short out a cell if
the cell is discharged. On a fully charged cell, they tend to blow
themselves out with a current surge when thy puncture the separator.
Thus you should always try to keep your packs charged, especially over
the winter when they're sitting in drawers, and the incidence of
shorted cells should be reduced. They will still tend to self
discharge, but they will be at full voltage.

dickm