Information on Battery/Solar Powered WAPs

Discussion in 'Off Grid Living' started by melbo, Nov 13, 2007.


  1. melbo

    melbo Hunter Gatherer Administrator Founding Member

    http://www.cuwireless.net/node/104
    From our friends Matt Westervelt in Seattle and Elektra over in Germany:
    Here's the simplest way:

    Take the piece of CAT5 sticking out of the box. Strip the cable down about a foot or two.

    Strip the Blue(+) and Brown(-)s and wrap them around the terminals of a car battery.

    Crimp the rest in your ethernet plug as usual, but without your blues or browns.

    Here's a few pics of one I made a while back with a smaller marine battery.
    http://seattlewireless.net/~mattw/gallery/mobnode

    Since the draw is minimal, a car battery should last you a while, and be easy to find a charger for. You can also run them in parallel for more time, or do a ghetto hot swap. Just make the leads longer and do a one loop around the first one, and then another loop around the second.
    Blue-----(+)--------(+)
    Brown-----(-)--------(-)
    <hr> Let me give some hints for the design.
    • Consider that you may have to protect the system against theft in such a situation
    • Use devices that only draw a minimum of power - every watt you waste makes the system more bulky. You end up with big solarpanels and huge batteries.
    But before I actually start describing the design of a solar system let me share some initial thoughts about such a disaster situation like in New Orleans:

    In a real pinch you dont need a solar system. If you want to provide desperately needed infrastructure during a desaster situation a solar system is luxury - if most cars and trucks are wrecked you dont have to bother about ruining a set of batteries within a month by discharging them a little too deep for a while.

    In a disaster (read: life or death situation) you may not need to think about where to get solar panels/ a full equipped and well designed solar system. If it takes four weeks for the mains power to be restored you could collect a bunch of charged batteries out of wrecked vehicles (in New Orleans there must be plenty of them) and connect them together. All you need is some fuses, cable and a (digital) voltmeter to find batteries in good condition. And a police guy that protects you from being shot by his colleagues for looting while you confiscate some parts from wrecks. That may sound silly but I dont think it is.

    A good and charged truck battery contains 1500 wh at least. Take 4 of them for each node and a system with low power AP will run for a month!

    And last but not least: I truly share the enthusiasm about wifi and what you can do with it. But in a desaster situation where people desperately have to call for help and need means of basic communication while landline phones and GSM fail the easier and feasible way is to resort to VHF/Shortwave Transmitters - many of such devices may be around, but I dont know how popular/widespread Citizen Band Radios nowadays really are. You dont need to set up a complicated infrastructure, just the transmitter, a battery and a simple antenna - and there you go. From a high point you ll have a range of 50 kilometers easily, and 40 to 120 channels.

    Now some thoughts on designing a autonomous solar system:
    I have no clue about the weather situation there. If you want a system with 100 % uptime you have to calculate the system with some safety margin.

    Such a system is consisted by:
    Solarpanel(s)---> Cable----> Charging Regulator (Preferably with MaximumPowerPointTracking)---> Cable ----> Fuse -----> Cable ----> Lead-Acid-Battery (Could be of Sealed-Lead-Acid-Type)

    Consumers (your Accesspoint, Wireless Router or whatever) will be connected to the charging regulator. Most charging regulators come with a protection that protects the battery from being discharged too much - lead-acid batteries degrade dramatically if you discharge them beneath 11 Volts. In fact this should never happen. A system that is designed to last for many years keeps the battery always at least 50 -70 % full. (Depending on battery type, 70% is a good value if you simply take a cheap battery from a car or truck)

    Batteries designed for solar systems can go down to 50% charge without wearing out quickly. But if you design a system for a desaster zone you can live with the fact that a battery may be worn out after a year or even within two months. Thus you may go far below this value. What you dont want is running out of power so the system switches off.

    Lets do a example calculation:
    First we want to know how much power is consumed. Preferrably we want a device that runs on DC directly, with 12 Volts that most batteries provide. (Most autonomous solar systems work at 12 or 24 volts.)

    Something link a Linksys WRT54 or a Meshcube. In fact most APs have a switched mode voltage regulator inside and thus will work in such a voltage range. Just open it and have a look if there are two relatively big capacitors and a inductor near the DC-Input. If they are present you have a switched mode input, maximum input voltage should be somewhat below the voltage printed on the capacitors. Usually 16 or 25 volts. The Linksys is great - it runs at any voltage between 5 and 20 volts. Although the radio could be better... Meshcube works at 8 to 20 volts, and you can plug in really good wifi cards (one to three of them!)

    I assume the WRT54 consumes about 7 watts - that is not measured, just a rough estimation, maybe someone comes up with measurements if this device would be the choice.

    We need that service 24 hours a day - so the device will draw
    24h * 7 watts = 168 watthours.

    Lead acid batteries have approx. 90% discharging efficiency.
    168 wh / 0.9 = 187 wh
    At 12 Volt the current in ampere would be
    187 wh / 12 volts = 15.55 amperehours
    Now lets assume we get a bad weather situation while we harvest *nothing* from the sun because we have shitty weather with cloudy sky for one week.

    15.5 amperehours/day * 7 days = 109 amperehours
    109 ah * 12 Volt = 1308 watthours

    If we allow our battery to get discharged from 100 % to 30% charge thus consuming 70 % of the capacity in such a rare situation (depending on the area where we are going to build the system) we need
    109 ah / 0.7 = 156 ah storage capacity

    A truck battery is available with this size. That would be the cheapest source. (Maybe you find a wrecked truck with working batteries, that the owner wants to donate or lend for a good purpose).

    The amount of energy that you can harvest with a solarsystem depends on the area where you are and the time of the year. A well designed system should be able to fully recharge the battery within a few days in good weather conditions while delivering power to the AP .

    Note that the charging efficiency is 90% - so you lose again 10% of the solar power.

    Usually you'll find information about the energy of the sun radiation from administrative bodies competent for weather. They collect such information over the years and can tell you what to expect for each time of the year. Simulation and calculation programs for solar systems are available, PVSOL being one commercial (and expensive) program, but it is only available for germany and some european countries. There must be similar programs available for the US.

    You may harvest 500 wh during a sunny day with clear sky using a 100 watt panel - that is what I can expect in germany in summer during a sunny day. Note that the amount of energy you get out of a panel is always about 20-30% lower than given in the specs of a panel. Especially if you dont use maximum power point tracking (MPPT). A 100 watt panel may never produce more than 70-80 watts without mpp-tracking.

    So lets do a final calculation.

    500 Wh/day - 168 Wh consumption of AP = 332 Wh charging energy
    332 Wh * 0.9 charging efficiency = 299 Wh charging energy
    1308 Wh (7 days) / 299 Wh = 4.374 days to recharge a 70 % discharged battery while powering the AP.

    I ll assume that the panel(s) are aligned properly and there is no shade (from obstacles) wandering over the panel during sunshine.

    In fact for a system that has to run permanently for many months I would increase the size of the solarpanel - 500 wh energy from the panel at best and 168 wh consumption each day is not very safe. For reliable service I would increase the size of the panel(s) and the battery to twice the values mentioned here.

    Hope I could help.
    cu elektra
     
  2. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    What the sam hill is WAPS?
     
  3. melbo

    melbo Hunter Gatherer Administrator Founding Member

    Wireless Access Points. Like my WiFi router. Not necessarily a SHTF type thing as far as internet is concerned. But, I'm trying to shed my $90 a month Hi speed satellite system and it may mean having a few repeaters set up along the mountain to get me an internet from a neighbor. Or, in a sit without internet, could be used (Maybe) in the field as a command center? Thats a stretch. Sometimes I like to use this site as a bookmark for stuff I find. This is one of those times.
     
  4. <exile>

    <exile> Padawan Learner

  5. zarraza

    zarraza Survivalist in training

    i had to revisit this - as i had a thought.

    you mention repeaters - if you are still considering this - make sure you use a router/bridge combo (if such a thing even exists) as you cannot just forward one access point to another unless they are bridges - then the issue becomes connecting a PC to that - if it is only a bridge, then you may not be able to connect a pc to it - which would really defeat the purpose!
     
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