"Paul Giroux" wrote in message
...

The question is, can I use NiMH batteries instead of
the original NiCd? If
I do, will the original AC charger work to charge up
the NiMHs? I'm not
sure how the two battery types differ in terms of
charging needs.



=== Nothing to correct; that's a pretty good writeup!
===
Basically, for the OP, try to stick with the same
battery type as the charger is/was meant for; you'll
get better and more reliable use of the batteries.
Batteries care about their charging rates for their
longest life. "Better" batteries without the proper
charging specs could, not will, result in a short
charging life. Charging voltages (and thus rate of
charge) varies with the type of battery.
Another tip for soldering: lightly sand the battery
solder locations wiht a high grit sandpaper (220 is
good) and wipe down with medicine-cabinet alcohol; do
not touch the solder surfaces with your fingers.
And do solder as reasonably quickly as you can. If
you flub it, wait ofr the battery to completely cool
inside before you apply heat again. Batteries store
heat for several minutes.

Here's what I know so far (and please somebody
correct me if I'm
wrong):

NiCds and NiMH have different charging
characteristics. Smart
chargers for NiCds usually use negative delta-v
detection to sense
when the battery is charged. In other words, when a
NiCd cell reaches
capacity its voltage actually drops a bit, the smart
charger senses
this and stops charging.

When NiMH reach capacity they can go either negative
delta-v or zero
delta-v, so it depends on the cell. I think they
also rise in
temperature, so you have chargers that may detect
this rise in
temperature and stop charging.

NiCds can tolerate bigger current draw and are more
tolerant to
overcharging (if using a 'dumb' trickle-type
charger). I also think
you get more recharge cycles for NiCd than for NiMH.
NiCds also work
better in the cold although this may not be
applicable to you.

You may want to look into the required charging
current for the NiMH
cells you are considering to see if your charger is
compatible. You
have to charge the cells at their rated capacity, in
other words if
the cells are rated 1500mAh you need a 1.5 amp
charging current for a
'quick charge'.

As for soldering cells, you can do so but be very
very quick, don't
let the cell heat up or you'll damage them. Use a
soldering iron with
a wide tip so you have enough surface for a good and
quick heat
transfer; you may need to hold a smaller iron longer
and this may
damage the cells. I use dad's old Fuller 50W with a
quarter inch tip.

Use the copper braid from a old length of coax cable
to connect them,
then use heat shrink tubing to package them up.
Start by shrinking
the ends first then the middle so the pack will be
tighter.

To make a long story short, try to find NiCds with
the same
characteristics as the old cells.

I think you and I are working on a similar project:
I'm looking to
build a battery pack for a bike light, either NiCd or
NiMH (I'm very
much leaning on NiCd), and I'm looking to built it
myself using
7000mAh D-cells. I'll be using a modified DeWalt 9107
(smart) charger
to charge 'em up.