But what's significant is what _fraction_ of the power consumption any
portion might be...if the overall is also pretty small as one would
expect/it must be to have a long battery life to begin with, that "very
small amount" may still be a decent fraction of the total and then the
decrease in lifetime is proportional to that fraction.
Again, this is all _purely_ hypothetical w/o any other data...but if the
symptom exists that the battery life doesn't appear to be any longer w/
AC powered unit than non-, it's pretty clear there must not be any real
power-saving circuitry inherent in the device design (or, the overall
device power consumption is quite high and therefore drains the battery
at about the same rate even though there is some extension over what it
would have been w/o the AC).
On Wednesday, October 1, 2014 1:09:40 PM UTC-4, dpb wrote:
Let's do the math. 9V battery. .6V drop across the diode.
8.4V goes to the load. That's 7% of the voltage across the diode
and hence 7% of the power dissipated, because the same current
is flowing through the load and the diode. And that 7% power
dissipation would only come into play when the AC is out. The
rest of the time, it would be zero. You'd have a design where if
the AC is lost, the battery would diminish about 7% faster. But
in return, you'd have a design where the battery lasts the shelf
life, not just a year. I could come up with other designs that
would perform better, but a diode is simple and cheap as an example.
What's apparent is that somehow they are using the battery power as well
as the AC. I can't think of other battery/AC powered devices that do
that. Sump pump backup? Emergency lighting? Battery banks at the phone
(or, the overall
The point is there is no good reason to be drawing down the battery at
all. And why not put in rechargeables? Duh! That's what they do in
those other examples.
The answer as outlined above is more than likely "initial cost".
Again, if these bug you so much for that reason, the obvious alternative
is to do some research and replace them with ones which do have an
extended battery life and/or rechargeables.
Different device/design but the outdoor unit on the local weather
station has started indicating "low battery" at night...it's got a solar
charger and I replaced the initial battery just a month ago but it
showed the missive the other night again already. I suppose the solar
cell is now starting to fail/losing its "oomph" after 2-3 yr in the SW
Not the external station, no. The base in-house display unit does and
it _does_ have pretty good battery life--seems I've changed them out
once, maybe. But, this is a much higher-priced device (Davis
Instruments VantageVUE) so the few pennies kinds of cost-cutting on
mass-produced consumer devices isn't _quite_ so vicious.
Actually, more precisely, I suspect the cut got made at the earlier
"feature selection" level of what were/were not expected capabilities to
meet the target market niche/pricing level for the device rather than
being pared out on an individual component-level culling (altho that
certainly occurs as well as the price/specific component alternative
vendors/etc. evaluations that occur).
On Wednesday, October 1, 2014 3:04:47 PM UTC-4, dpb wrote:
I can show you plenty of rechargeable, cheap, consumer products that
sell for what a smoke detector sells for, or less that have battery
"Backup" or rechargeable batteries, that don't draw the battery down
along with using AC. Your argument about a 10 cent diode is silly.
That the cost/feature analysis comes out at a different place on other
products doesn't negate the general rule. And, in fact, the "feature
set" on your device pretty much confirms it for your particular device.
As said, it isn't/wasn't that the particular one selected component got
taken back out; the feature of which it might have been a key component
wasn't included to begin with. The decision as to whether it was/was
not to be included almost certainly included a consideration of the cost
for the design/manufacture in comparison to the expected margin and
potential increase/decrease in sales and was found lacking...
On Wednesday, October 1, 2014 5:29:59 PM UTC-4, dpb wrote:
Pure conjecture on your part. You have no way of knowing what went into
the design consideration or why anything was or wasn't done. For all you
know it could be just a bad, dumb, design. I've seen battery backup on
cheap consumer products where it doesnt' run the battery down in a year.
The battery is only used if the AC goes down.
Surely no more so than conjecture on your part, either,...I pointed that
out some time ago upthread, as well, that we're both dealing in nothing
but conjecture here (other than the fact that the design did _not_
include the feature I will submit leads to a higher likelihood that was
done on purpose than not).
And, again, that other products have a different feature set and cost
point has no particular bearing on the given one.
I'm bowing out of a futile and pointless subthread at this point...
On Wednesday, October 1, 2014 5:48:22 PM UTC-4, dpb wrote:
I wasn't conjecturing on why they did it. Just pointing it out and
that it's terribly inconvenient and dumb. Even if I buy your argument
that it would make the product cost some tiny amount more, it doesn't
make sense. They could charge $2 more, point out that their design,
unlike competitors doesn't draw the battery down unless the AC actually
goes out and have a marketing advantage. They'd sell more and make more
..I pointed that
It does when they are cheap consumer electronics type stuff.
And when we know how cheap semiconductors, like a diode are today.
That diode could probably be integrated into an IC already in
there, in which case it might not actually cost anything.
You never followed up w/ the particular manufacturer/model so can't tell
anything about who might have been but if it is a foreign knockoff it
may well be that the entire engineering for the specific unit was "copy
the model" in which case the feature set simply mimics that of the one
chosen to ape.
Which if so simply pushes the original design choice of feature set/cost
back to the previous manufacturer...
Kind of like the Muntz TV. Way back when Mad Man Muntz was a car
dealer in California. He branched into TV manufacturing by selling
an extremely cheap TV set. The way he did it was to have his
engineers go through the set and try what would happen if they
left that out. One thing they left out was the IF stage. That
reduced the sensitivity, but since they were mostly sold in the
LA area they were close to the station and would still work, good
enough so that people who couldn't afford a good TV would use
Which, of course, has *never* been illegal. "Except by the consumer"
and all. I remember being 7 or 8, hearing someone "joke" about that
on TV, and being very confused because it was clear to me that the
tags meant that the manufacturer and retailer couldn't remove them,
but we as the final purchaser could.
This was in the late 70s/early 80s; perhaps sometime before then those
words weren't on there? Or it just makes a better joke to pretend
On Wednesday, October 1, 2014 2:02:54 PM UTC-4, Pico Rico wrote:
Wow, really? How very important. Let's see, we have a smoke detector
that's primarily operated by AC. So, it can go forever on AC plus last
about as long as a battery operated one if the power goes out. Power
almost always goes out for hours or a few days, not a year or two. That
voltage drop is important again, why?
I've also cited other common devices where they have battery plus AC and
don't draw the battery down unless it's needed because the AC fails.
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