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!

Reply to
ericm0009
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snipped-for-privacy@yahoo.com wrote in news:6683bc96-12c4-4d8b-b4c0-2e64f67583a1@

34g2000hsh.googlegroups.com:

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)

Reply to
DanS

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...)

Reply to
danny burstein

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.

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.

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:

and one for the WRT54G:

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.

Reply to
Jeff Liebermann

danny burstein wrote in news:gbbejm$dp0$ snipped-for-privacy@reader1.panix.com:

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.

Reply to
DanS

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!

Reply to
ericm0009

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!

Reply to
ericm0009

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.

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:

I hate single channel repeaters. Details if you want (later). All of the repeaters that I've built, that actually worked use TWO radios, back to back, on different channels. The mesh mob is starting to get the clue as even the most reactionary vendors are now supplying multiple radio mesh nodes (repeaters).

Do a quick test and setup your proposed system inside a small room along with a client radio and a simulated internet server plugged into the router. Try it first with just the server, one router or AP, and a client radio. No repeaters. Use JPerf or IPerf for benchmarking. Then, turn on a repeater or two and watch what happens to the thruput. The moral is that repeaters work if the end points can't hear each other. If they can, the repeater is no better than an interference source.

So some reading on Home Power Magazine about solar arrays, controls, sizing, and prices. I suggest Morningstar controllers.

Reply to
Jeff Liebermann

Well, to be fair, the recommended discharge cuttoff point is all over the map. Conservative solar electric installers, that have to warranty what they sell, will suggest no more than 25% discharge. AT that point, the batteries will last almost forever. Battery vendors, that want to sell lots of replacement batteries will go so far as to suggest a 75% discharge. I won't mention UPS vendors recommendation as my blood pressure is allready too high. For a solar electric system, batteries are probably the 2nd most expensive part of the puzzle if they have to be replaced every 5 years or so.

The idea is to extend the battery lifetime, which means you try to pull down the battery as little as possible. I know of one system that has two battery banks. A small one that gets run down to about

50% and has to be replaced every few years, but is cheap. Behind that, a big pile of Trojan cells, that only gets used in the winter after several consecutive days of darkness. I think the Trojan cells are about 20 years old and show no capacity loss. The junk batteries are now about 2 years old and probably have anywhere between a few days and a year to go before they die.

It's also the same with electric hybrids. Toyota has to warranty their battery pack for 5+ years, so they don't let it discharge more than 60%. Note the LiIon cells can be safely discharged more than lead-acid cells.

Reply to
Jeff Liebermann

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!

Reply to
ericm0009

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