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.
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.
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