Howdy all, I tried sending this a while ago but had some problems with my mail server. Should be all straight now.
On Tue, 17 Jun 2003, Aaron Markham wrote: <snip!> > This is what I've been reading: > > P = I * E > watts = amps * volts ??? Yes, P)ower (in watts) = current (in amps) * volts > so if the AP's power plug says 5V and 2A... Well, not exactly. The voltage is right, but the current rating on the power supply (wall wart) is what the supply is capable of supplying, not what the access point needs. It is likely that it takes somewhat less. Somewhere in the neighborhood of 200mA to 1A. That info should be printed at the back of the manual or the bottom of the unit. > > 5.0V * 2.0A = 10.0 watts > > I re-read the plug and I see that the above numbers > are the plug's output, so I don't think those are the > right numbers to work with. The input is 0.5 amps and > 100-120 volts. In that case we're looking at 50-60 > watts. I don't like that number cuz now my power > requirement just went up 5 fold. Yikes! Talk about conversion loss!!! Actually since the input is AC the calculations get kinda messy and isn't worth discussing here. Just go by what the AP says it needs. > Aside from the watts calculations, looking at the > solar panels it seems that to get 2 amps (0.5A * 2 > AP's = 1 amp plus the extra required to charge up the > battery to last all night) you'd need at least a 30 > watt panel, but I'm just guessing here. Not bad figurin'! Better budget a little more solar panel as you may need close to two amps to get your batteries charged depending on type. Also you need to look at current consumption over time. In the winter there may be only 8 hours of good charging daylight and 16 hours you rely on battery. You'll need enough solar to run the AP and charge a large enough battery to run it at night. Two 500mA Access points will draw 1A at any given time. For 16 hours of run time we'll need a 16AH battery. (better make it 20) All systems have loss. You will have to put more power in to charge than you expect to get out. To charge a 20AH battery in 8 hours, you need to supply 2.5A for the full 8 hours at a voltage slightly above the battery voltage. Include fudge factor we'll say 3A plus another amp for the APs. Thats 4A assuming a 6V system. You'll need a minimum of a 24W panel setup (better go 30W!). Not too unreasonable. > This page has some good power stats: > http://www.seslogic.com/solarex.html Yikes! Those panels range between $7 and $23/watt!!! Reasonable market prices hover above $3/watt, and deals can be found closer to $2/watt (usually used). Batterys are fairly cheap new. Most UPS use 6V batteries with capacities in the 20AH range. Expect to pay $20-$40 at a computer swap. There is still the matter of a charging circut. A breif search turned up this one: http://www.humboldt1.com/~michael.welch/extras/pvcontrol.pdf Others may be available. Search Google for the term 'charge controller'. -Glen
