Calculating power consumption...

Hi,

I am thinking about extending my wireless coverage by adding a WAP
(and a wireless-ethernet bridge) that will be powered by batteries.

The wireless-ethernet bridge will be the client feeding the WAP, so
the WAP can be run as a complete WAP and not a repeater.

The power specs for the bridge and WAP are the same:

INPUT: 100 - 120VAC (0.5A)
OUTPUT: 5VDC (2.5A)

Stupid question: How do you determine how many watts are actually
used? Is this simply by multiplying the voltage with the amperage of
the output specs?

Am I adding this up correctly for a 12V battery?

5V x 2.5A = 12.5W

12W x 2 (devices) = 25W

25W / 12V = ~ 2.10 AaH (Amps per hour)

So, say, a 12V 100A (deep cycle) battery should last ~47 hours if
supplying 5VDC (2.5A) continuously? (Well, somewhat less, because of
inefficiency.)

Then, later on, I could double that amount of time by adding a second
battery and connecting them in parallel.

Am I adding up correctly here? I would, obviously, be using a DC-DC
converters in place of the power adapters, not a DC-AC inverter with
the power adapters.

Thanks!

(E-Mail Removed) wrote in news:6683bc96-12c4-4d8b-b4c0-2e64f67583a1@
34g2000hsh.googlegroups.com:

> Hi,
>
> I am thinking about extending my wireless coverage by adding a WAP
> (and a wireless-ethernet bridge) that will be powered by batteries.
>
> The wireless-ethernet bridge will be the client feeding the WAP, so
> the WAP can be run as a complete WAP and not a repeater.
>
> The power specs for the bridge and WAP are the same:
>
> INPUT: 100 - 120VAC (0.5A)
> OUTPUT: 5VDC (2.5A)

These specs are the for the power adapter, not what the unit actually
uses. The only way to know exactly how much is used, is to use a meter
and measure it....and that will then only be an average power
consumption. The unit uses far less in TX mode than while in RX mode.

And yes...power (W) = voltage (V) * current (W)



>
> Stupid question: How do you determine how many watts are actually
> used? Is this simply by multiplying the voltage with the amperage of
> the output specs?
>
> Am I adding this up correctly for a 12V battery?
>
> 5V x 2.5A = 12.5W
>
> 12W x 2 (devices) = 25W
>
> 25W / 12V = ~ 2.10 AaH (Amps per hour)
>
> So, say, a 12V 100A (deep cycle) battery should last ~47 hours if
> supplying 5VDC (2.5A) continuously? (Well, somewhat less, because of
> inefficiency.)
>
> Then, later on, I could double that amount of time by adding a second
> battery and connecting them in parallel.
>
> Am I adding up correctly here? I would, obviously, be using a DC-DC
> converters in place of the power adapters, not a DC-AC inverter with
> the power adapters.
>
> Thanks!
>
>
>
>
>
>
>
>

In <Xns9B22BFDF155F8thisnthatadelphianet@85.214.90.23 6> DanS <(E-Mail Removed)> writes:

>These specs are the for the power adapter, not what the unit actually
>uses. The only way to know exactly how much is used, is to use a meter
>and measure it....and that will then only be an average power
>consumption. The unit uses far less in TX mode than while in RX mode.

Wouldn't it be the other way around?

(And does a typical access point "power down" when
just sitting there as opposed to actively transferring
big data streams? I really don't know...)


--
__________________________________________________ ___
Knowledge may be power, but communications is the key
(E-Mail Removed)
[to foil spammers, my address has been double rot-13 encoded]

danny burstein <(E-Mail Removed)> wrote in
news:gbbejm$dp0$(E-Mail Removed):

> In <Xns9B22BFDF155F8thisnthatadelphianet@85.214.90.23 6> DanS
> <(E-Mail Removed)> writes:
>
>>These specs are the for the power adapter, not what the unit actually
>>uses. The only way to know exactly how much is used, is to use a meter
>>and measure it....and that will then only be an average power
>>consumption. The unit uses far less in TX mode than while in RX mode.
>
> Wouldn't it be the other way around?

Uh...yeah....it was a long.....long day. Sorry. Boy did I blow that one.
Trying to program rtr's (real rtr's, not consumer gear AP's) that you're
not familiar with really strains the brain.

That is what I meant...TX is much more DC power than RX....DOH !!!!!

You can never go by what the PS says. The PS should always be over-rated
for the device....by 50% of the actual current draw, IMO. We've got a
900mhz DSSS radio that comes with a 1.5A 12v PS and is rated @ 700mA
(which is ~100% over-rated) @ 12V for a full 1W output.

Well I measured it, and in diag keyed mode, as in constant TX, it only
uses ~ 400ma @ 12v.

On Sep 23, 3:14*pm, Jeff Liebermann <je...@cruzio.com> wrote:
> On Tue, 23 Sep 2008 09:52:11 -0700 (PDT), ericm0...@yahoo.com wrote:
> >How do you determine how many watts are actually
> >used?
>
> Measure it. *Apply 5.0VDC from a bench type power supply to your
> unspecified model WAP. *Measure the current. *The power, in watts, is
> the current times the voltage.
>
> >Is this simply by multiplying the voltage with the amperage of
> >the output specs?
>
> No. *What you're quoting are the specs on the wall wart power supply.
> Those are intentionally over-rated and are always more than the actual
> power consumption.
>
> >So, say, a 12V 100A (deep cycle) battery should last ~47 hours if
> >supplying 5VDC (2.5A) continuously? *(Well, somewhat less, because of
> >inefficiency.)
>
> Nope. *You can't run a deep cycle battery below about 75% capacity
> without killing it. *That means you only have about 25% of the battery
> capacity available to run the WAP. *
> * 12 volts * 100 Amp-hrs * 0.25 = 300 watts-hrs
> Assuming typical 80% efficiency for a good 12v to 5v DC to DC
> inverter, that gives you about 240 watts to burn.
>
> If your WAP draws 8 watts (a good typical first guess), you can run it
> for:
> * *240 watt-hrs / 8 watts = 30 hours
> before you need to recharge.
>
> I built a handy spreadsheet for doing the calcs for a solar powered
> ham radio repeater. *See:
> <http://802.11junk.com/jeffl/rf-calc/solar-repeater-206.xls>
> and one for the WRT54G:
> <http://802.11junk.com/jeffl/rf-calc/solar-wrt54g-v101.xls>
> Juggle the numbers to fit your system. *The basic idea is to size the
> solar array so that it can be recharged in less than one day.
>

Thanks for the wonderful information, Jeff. Especially the
spreadsheets. I didn't realize you already crunched the numbers.

Yeah, I'll go ahead and measure it at work. This is starting to look
more involved and costly than what I thought it would be. I'm now
thinking I'll take the bridge out and just run the WAP as a
repeater. It will mainly be for an internet pipe, so that hopefully
won't be too bad.

The WAP that I was thinking about using is a DLink DWL-7100AP since I
have a few of them already, but since this an 802.11a/gb WAP, I'll
probably invest in an 802.11gb only WAP that is compatible to be run
as a repeater with the DWL-7100AP.

Still haven't even gotten to the solar array aspect yet. I need to
crunch the solar with the battery and see where my breakpoint is going
to be.

Thanks again!

On Sep 23, 2:52*pm, DanS <t.h.i.s.n.t.h....@a.d.e.l.p.h.i.a.n.e.t>
wrote:
> ericm0...@yahoo.com wrote in news:6683bc96-12c4-4d8b-b4c0-2e64f67583a1@
> 34g2000hsh.googlegroups.com:
>
> > Hi,
>
> > I am thinking about extending my wireless coverage by adding a WAP
> > (and a wireless-ethernet bridge) that will be powered by batteries.
>
> > The wireless-ethernet bridge will be the client feeding the WAP, so
> > the WAP can be run as a complete WAP and not a repeater.
>
> > The power specs for the bridge and WAP are the same:
>
> > INPUT: 100 - 120VAC *(0.5A)
> > OUTPUT: 5VDC (2.5A)
>
> These specs are the for the power adapter, not what the unit actually
> uses. The only way to know exactly how much is used, is to use a meter
> and measure it....and that will then only be an average power
> consumption. The unit uses far less in TX mode than while in RX mode.
>
> And yes...power (W) = voltage (V) * current (W)

Thanks. That makes sense. I'll go ahead and measure the actual power
being used.

Jeff straightened me out on the battery. I didn't realize that the
battery is only good for the first 25 percent. Ouch!

On Sep 23, 7:21*pm, Jeff Liebermann <je...@cruzio.com> wrote:
> On Tue, 23 Sep 2008 14:45:26 -0700 (PDT), ericm0...@yahoo.com wrote:
> >Thanks for the wonderful information, Jeff. *Especially the
> >spreadsheets. *I didn't realize you already crunched the numbers.
>
> I've built 3 solar powered repeaters and one solar AP. *Two are still
> on the air. *It helps to do the calcs first, and then build.
>
> >Yeah, I'll go ahead and measure it at work. * This is starting to look
> >more involved and costly than what I thought it would be.
>
> Yep. *There are several construction articles on solar powered
> wireless AP's and repeaters. *I'm too lazy to look right now. *If you
> look at the photos, the cost of the box, mounting, panels, controller,
> wiring, and antennas, far exceeds the cost of the access point. You'll
> also run into environmental issues you probably don't want to know
> about. *Condensation will rot out the boards. *Outgassing of the
> battery acid will really make a mess. *Thieves are everywhere.
>
> There are also ready to play outdoor boxes:
> <http://www.hyperlinktech.com/productcenter.aspx?id=14>

Thanks again, Jeff, for all the great info.

I think I am going to step back for a little while before doing this
project. Instead of just jumping into it, I'll continue to edumacate
myself first.

Thanks!