On Nov 21, 9:08*pm, "DoN. Nichols" wrote:
On 2011-11-21, wrote:

On Nov 20, 9:55*pm, "Martin Riddle" wrote:
wrote in message

....

I just measured a Harbor Freight (Chinese) 8-incher.

Drain: 13.5uA (off), 14.5uA (on)
Battery low threshold (blinking display): 1.37V
Lowest operating voltage: 1.01V

* * * * [ ... ]

Why bother including an on off switch?

It's really not worth it for 1uA.

If I designed these, I'd shoot for 2uA active draw, like the
Mitutoyos, and set the battery low threshold at 1.1V (for alkalines).
The battery consumption is the biggest fault with these. *Apart from
that, they're impressive.

I sometimes think about wiring up a "AAA" or solar cell and just
forgetting it, but for $0.50 a year it's not worth the trouble.

* * * * Actually -- there *are* some "solar powered" ones -- by Mitutoyo
IIRC. *They would probably be excellent used daily in a well-illuminated
shop.

I saw a solar-powered Mitutoyo at a flea market and was intrigued, but
resisted. Something about having a glass window on a machine tool
close to all that hard steel just didn't sit well--it brought up
memories of scratched and broken watch crystals.

OTOH, I paid $10 for most of my HF calipers. Those you can take
chances with. Using the 8x25mm solar panel from a $1 calculator, a
super capacitor for storage, and an LED as a regulator diode was my
notion. $2 in parts, $500 labor ;-).

I fitted one to the lathe carriage--best thing I ever did.
Removable. I fitted another to the tailstock ram. With it you can
bore to 0.002" depth every time without even trying. Magic.

*In my shop, often dark for days at a time, until a project lures
me there, the replaceable batteries are a better choice.

* * * * I did recently get an auto-darkening welding hood from Harbor
Freight which is solar powered, and based on what I have read about them
dying if not used regularly, I've put mine on a folding workstool facing
out the window so it sees daylight to maintain the charge. *I'll proably
eventually have to perform surgery and replace the rechargeable cells in
there. *There is a temptation to provide a connection for an AC-powered
trickle charger so I can store it more conveniently. *What would be
particularly nice would be an induction coupled charger like those for
electric toothbrushes. *Just put it on a stand and expect it to be fully
charged when I come back.

* * * * If the charge is good enough to work on the first strike, it
should work fine for the rest of the day, because it will be getting a
charge boost from the arc -- close enough to vigorous sunlight. :-)

Sounds like a LiIon cell. If so, those can't be allowed to go dead,
as you've surmised.

--
Cheers,
James Arthur