On Wed, 6 May 1998, John Walter wrote:
> My advise for GPS use is not to use rechargeables . A Mapping and Survey
> firm in Toowoomba who use and sell Magellan told me this, the explanation
> was that alkaline batteries retain their voltage until near their life end
> and with Ni-cad batteries the voltage drops continuously so that you could
> be using your GPS on a damagingly low voltage.
This is reversed.
The voltage of alkaline batteries drop as they discharge. A new alkaline
battery has a voltage near 1.5V. This will drop almost linearly as it
discharges. An almost discharged alkaline battery will have a voltage
near 1.0V.
A charged NiCd battery has a voltage near 1.2V. Its voltage will change
little until it is almost discharged.
As a consequence, the voltage of an alkaline battery gives you a
reasonable estimate of the remaining battery life. The voltage of a
NiCd battery does not. Hence,if you use alkaline batteries in a torch
its brightness should steadily fade over the battery lifetime. If you
use NiCd batteries the brightness should not change appreciably until
just before the batteries are exhausted.
The most importance difference is capacity. The widely sold models of
AA NiCd batteries have 25-50% of the capacity of AA alkaline batteries.
The exact difference depending on the price you pay for each.
NiCds are designed to be recharged upto 1000 times. There is a large
amounts of misinformation about charging NiCds particularly people
talking about a "memory effect". You really only have to worry about
two things. Don't dmagae your NiCds by overcharging them - follow the
time instructions on your charger. Don't damage them by discharging
them below 1.0V. In other words, stop using them when the voltage drops -
e.g. the torch starts to dim.
In theory, NiCds handle low temperature better than alkaline batteries.
At -10C an Alkaline battery has about 50% of its normal (room temperature)
capacity. NiCds should be much less affected but I've never measured
this myself. Canberra birders may find NiCds better value.
I'd be very surprised if use of NiCds could damage a GPS - it would
require a serious design flaw in the GPS.
My apologies for the absence of birds above - I thought some birding-aus
readers who use NiCds might be interested in the comparison.
Andrew Taylor
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