[MLB-WIRELESS] Solar power for access point

Mon Sep 10 14:33:07 EST 2001

is this a remote repeater, or will it be ethernetted into a box some where?
if so, you might consider one of the power-over-ethernet schemes

>Apple Airport Current consumption WITHOUT modem: 180-290mA (2.16-4.08W)
>
>Power on (3 orange lights): 230mA
>Boot phase (1 red light): 180mA
>Final boot (link light on wireless card only): 290mA peak
>Normal Operation: 280mA
>Flash ROM write:        290mA
>ETH flood:      290mA
>Wireless flood: 340mA peak

this is a *lot* lower than i was expecting;  solar powering this unit now
looks quite feasible.

the basic components you will need are as follows:

solar panel (and/or wind generator) into charge controller into battery
then either
into inverter (into plugpack?) into AP, or
into voltage regulator direct into AP

>so basically you need a big battery (or 3) powering the airport that can do
>at least 410mA peak,

call it 500mA = 0.5A , it makes the math easier, and it's always best to be
a little generous.  that way your solar rig will last longer, as it's not
running right up to the rating of each component.

more important than the power RATE of the battery (410mA not being that
great a challenge) is the total power STORAGE in amp-hours.  ie 1 Ah = a
load of one amp running for an hour.

you should get acceptable service if you allow for two days run-time with
no charge - the panels will still charge some on totally overcast days, so
you should only lose service a couple of times a year - allow for more if
you can budget for it, but like a lot of things, it's the last few % of
efficiency that really costs.

anyway, that equals 48h x 0.5A = 24 Ah of storage.  the storage capacity of
a battery is written on the side somewhere.

now the next thing to remember is that you can't really run a lead-acid
battery all the way down - because you'll kill it.  not the first time, but
fairly quickly.  so you'll want to double that, for a "deep-cycle"  battery
designed for this kind of abuse, or 4x it for a normal car/motorbike
battery.  so then we're back up to 48Ah, or 96Ah.

does the airport run from a 240v source directly, or via an external power
adapter/plugpack?  if it's a plugpack, check its label and see whether its
output is AC or DC.  If its DC, and under 12 or 24 volts, then you can
probably run the AP directly from your solar rig.  you'd just need to add a
voltage regulator (cheap) as insurance.

i notice that the power ratings from the perth page given in the previous
reply are all at 12 volts, ie 0.41 A x 12 V = 4.92 W.  myabe this means the
AP runs at 12 volts, this would make life easier...

if on the other hand it uses an AC plugpack, or direct 240v, then you'll
need an inverter to boost up to 240v.  luckily, you can get away with a
very small/cheap one at this power load.  try http://www.jaycar.com.au ,
they should have a suitable one for around $100.

so now you need to allow for the power conversion (in) efficiency of the
inverter - typically 85%.  that means an effective load of
0.410 A / 0.85 = 0.48 A , still within the 0.5 we're using for an estimate,
but cutting a bit fine, so you might want to over-rate some of these
components along the way.

and get a solar panel that puts out enough juice to
>keep up. i think overkill would be the best idea ;)

using the estimate of 4 x the load for charging, you need a 2.0 A @ 12 V
panel.  solar panels are sold by the watt, so this is  24 watts worth of
panel.  average price is $10 per watt.  and this is *peak* watts - for
direct, bright sun - hence the apparent 4x oversupply

so we're looking at:

panel:             3 x 10 watt                = $300.00
               or  6 x  4 watt                = $240.00

battery:           3 x 18 Ah sealed lead acid = $266.85

charge controller: 12 A max 12 V              = $ 87.95

(inverter:         60 watts 12v to 230v       = $ 79.95)

odds n sods, wire, mounting, casing etc       = $100.00
------------------------------------------------------------
rough total                                   = $700 to 800

these prices are all from the jaycar site, just because it's a handy
reference.  their prices are not bad, but feel free to shop around with the
specialised solar places.  solar panels are occasionally available
second-hand, and batteries can be obtained second-hand (with somehwhat
degraded storage life) from places that service wheelchairs or forklifts.
alternatively, a suitable truck/car battery might be cheaper. and obviously
you can scrounge up materials for the mounting and casing.

another thing to consider is whether you need this link up 24/7 - could you
accept it going down between say 1am and dawn some days?  you could then
budget a bit on the batteries.  instead of 48 hours of storage, you might
only need 12 or 24.  i would suggest adding a circuit to switch off the
power when the batteries get down too far.  the more often and the deeper
they are run down, the shorter thier service life

and adding a small wind generator would also extend your battery life - as
well as charging at night, and possibly allowing you to go down to 3x or 2x
load on the panels.  and presumably you're on a hill or some other high
place, so the wind available should be quite good.

the batteries and other components need to be waterproofed, kept out of the
sun and dust - if , and the panels need to be anchored down well so they
won't blow away in a gale.  and consider a lightning rod.

you need to point the panel north, at an angle to the ground.  this angle
depends on your location, as it is optimised to catch the most sun over the
year.  there are lookup tables for your location available in solar power
reference books at your local tech library, or on the net.  and use fairly
chunky wire and short as possible runs on the 12 volt side - it'll cut down
on any losses.

keep us posted.

clae.

>
>the guerrilla.net folks have some more info
>
>http://www.guerrilla.net/gnet_power_systems.html
>
>----- Original Message -----
>From: Robert Farrar
>To: melbwireless at melbwireless.dyndns.org
>Sent: Monday, September 10, 2001 12:04 AM
>Subject: [MLB-WIRELESS] Solar power for access point
>
>
>Hello All,
>
>I would like to put a RG-1000 / apple Airport access point in a location
>that does not have direct access to power, so I would like to get power to
>the unit via solar & battery backup.
>
>Has anyone from the group got any experience in solar powered devices or be
>able to source a how to on making such a unit ?
>
>Any help would be welcomed.
>
>Regards,
>
>
>Robert Farrar
>Mt Gambier, South Australia

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