On Dec 22, 2007 3:48 PM, Karl Vacek <kvacek@ameritech.net> wrote:
> Is there any particular reason, other than the cost (which doesn't seem very
> different) that manufacturers send nicads with most rechargeable products
> that don't warrant lithium ion cells ?
>
> I've replaced nicads with nickel metal hydrides everywhere I could and had
> great results. One slight exception is the Motorola HT-1000 radios we use
> for the police auxiliary. NiMH packs work fine, but the radios, originally
> calibrated for nicads, think the battery is getting weak and do their "weak
> battery" chirp for hours before the radio actually dies. I presume NiMH
> cells must have a little lower voltage than NiCds. Other than that little
> problem, they work better, last longer (per charge and per replacement), and
> don't show the memory effects that some people saw when we used nicads.
NiCd cells are cheaper than NiMH ones, which is a big advantage in
itself. A NiCd charger is simpler and cheaper. NiCd batteries havea
lower self-discharge rate than NiMH batteries do, so they work better
for low-drain devices. They have flatter voltage curve than NiMH or
alkaline batteries do, which means it's possible to use them closer to
capacity. They've got lower internal resistance, which allows them to
provide a higher current than NiMH cells, which is useful for things
like power tools.
Where NiMH batteries shine are their greater charge/discharge
lifetime, lower environmental impact, and high-drain use. A NiMH
battery can provide the same energy at a wide range of draw rates; the
number of joules extracted doesn't change much at rates from C/20h to
C/1 h. (20 hour draw to one hour draw.) That's a big advantage for
things like cameras, toys, etc, where you want to use the power in a
hurry.
For things like your radios, I expect the design goal is something
like "lasts a shift, plus a reserve, and recharges before the next
shift". NiCds are good enough for that.
--
David Scheidt
dmscheidt@gmail.com
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