On Wed, 04 Feb 2004 11:42:25 -0500, Jeff Wisnia wrote:
Bob Engelhardt wrote:

We have a 9.6v Dustbuster that lost it's NiCd batteries rather quickly,
at least compared to the previous, lower voltage, Dustbuster. I have
ordered new cells to replace these, but I'm wondering about the
"charger" that the Dustbuster has. It's just a wall wart and a diode.
I.e., a trickle charger.

I seem to recall that trickle charging is not the best for NiCd's.
Right?

If not, I'm wondering whether I can use the "smart" charger that came
with my 9.6v Makita cordless drill. I'll need an adapter, clearly, but
is the Makita charger somehow specific to the Makita batteries, or will
it smart-ly charge any 9.6v NiCd?

Bob


IMHO continously trickling a nicad at less than 1/10th of it's
milliampere hour rating (i.e. less than 70 ma for a 700 milliamp hour
cell) does them no harm. You might want to measure the charging current
from that wall wart (after the cells are charged up) and if it is higher
than what I just stated, for the cell size in your Dustbuster, reduce
the current by inserting an appropriate sized resistor in series with
the charging circuit.

Overcharging a NiCd is *never* harmless. Overcharging can cause
voltage depression (often mistakenly called "memory"), and shortens
the life of the cell, even if it is at a "trickle" rate. It *may* not shorten
its life enough to matter if the trickle rate is really low (say 1/100 C),
but any overcharge is bad. You want to avoid it if you can.

"Smart chargers" are used to speed up the charging of things like
portable power tool batteries to keep things moving on the job. They
charge at a much higher current initially and have the ability to
measure the voltage rise of the battery pack (and sometimes its
temperature too) and cut the charging current way back when the
circuitry decides the battery is charged.

Smart chargers work in one of two different ways. Older rapid chargers
sensed temperature rise in the battery pack to signal end of charge
(thermistor built into the pack). Cell temperature does start to rise
rapidly when the pack exceeds full charge. This is a workable, but not
ideal, charging method if all the cells in the pack are well matched. It
will result in a battery pack capable of about 300 to 500 charge/discharge
cycles before problems start to show up.

The second, and better, way that smart chargers work is by sensing
battery voltage and looking for a *droop* in battery voltage of about
5 mV per cell just as it reaches full charge. This is a definite and positive
indication of exactly when the cell reaches full charge. If charging is
stopped there, you'll get maximum life out of the NiCd. Again, the cells
in the pack have to be very well matched for this to work properly, if
you're charging a pack rather than individual cells.

The voltage droop method requires pulse charging with battery voltage
sensed across a load between pulses. The chipmakers have implemented
the "brains" of this sort of charger in single chips, so battery charger
manufacturers have a proven off the shelf system to use. Using this
method, and well matched high quality cells, you can expect up to
about 2,000 full charge/discharge cycles out of a pack before you start
to see problems.

Note that can be equivalent to many more *partial* charge/discharge
cycles. The idea that you should fully cycle your NiCds is bad voodoo,
left over from the days of timed chargers which needed a known starting
point for charging to avoid grossly overcharging a partially discharged
battery. With a proper sensing charger, it just needlessly shortens the
useful life of the batteries.

Now the question at hand is which method of charging Makita uses
in the smart charger you have. If it only senses temperature rise, and
many cheaper tool battery chargers do, it won't know when to stop
rapid charging your battery, which doesn't have a built in thermistor.
So it will very quickly cook your battery. OTOH, if it senses voltage
droop, it'll do a good job of charging your battery.

Some chargers do both, with the temperature rise indication a backup
for the voltage droop sensing, and for cases when the battery is too hot
from use when you plug it into the charger to safely allow rapid charging.
(Milwaukee's latest battery chargers are examples of this.) So the presence
of temperature sensing contacts in the charger isn't a foolproof way to
determine whether it uses the voltage droop method or not. It is cause
to investigate further before risking your battery with it.

Gary