From: Marty Michener <>
>
> At 06:35 PM 3/31/2004 +0000, you wrote:
>
>
>>>I recently bought a Sharp MD-DR7 minidisk and have a question about
>>>battery use. The internal battery is a gum stick AD-N55BT. The
>>>manual instructs to use an Alkaline AA battery in the external
>>>battery case, with a caution against use of a rechargable battery.
>>>I would prefer to use a AA NIMH in the external case. I wonder if
>>>any harm would occur if I did use the NIMH battery? Anyone have a
>>>clue about why the manufacture would not want the use of a
>>>rechargeable battery?
>>>1GDW
>
>
> More probably it is a matter of insufficient voltage - Lithium and
> alkalines typically start their discharge curve when new about 1.6 or 1.7=
> volts. NiMH about 1.3 to 1.4 volts. The old Nickle Cds were even lower=
-
> typically 1.25 volts. The external battery can't charge perhaps if it is=
> below 1.5 - just a guess, though.
This is actually very misleading. Yes, Alkalines are rated at the higher
voltage, and marked as being 1.5 volts. But that's a rating of their
starting voltage. On discharge their voltage drops very quickly reaching
the 1.2 volts of rechargables early in life and continuing downward. The
cutoff point when a alkaline is considered used up is 0.8 volts.
Rechargables are rated by a different method. They start at a lower
voltage, but their rated 1.2 volts is at 80% discharged. So, for nearly
all the life of the NIMH rechargable it's at a voltage above a alkaline
that's supplied the same amount of energy.
The Portadisc's battery indicator is calibrated for alkaline batteries
and based on voltage. For over 80% of the life of a NIMH battery set it
will indicate 99% left. Because the NIMH battery is keeping the voltage
up at the level that a virtually new alkaline would have. A alkaline
pac would have that reading steadily dropping during the same period,
indicating that for most of it's life it's below the NIMH steady voltage.
I would expect the minidisk to run fine off a NIMH rechargable, for a
lot longer than it would off a alkaline. It would certainly not harm it.
The main reason why manufacturers still give that kind of warning is
that nicad rechargables are very low capacity compared to alkalines and
they don't want people complaining about battery life. In contrast,
NIMH, which they tend to ignore, are much higher capacity than
alkalines. If we can ever get everybody off nicads to NIMH, I expect
such instructions will change.
Walt
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