Laptop Charging Station for Schools

Re: Laptop Charging Station for Schools

Cariboocoot wrote: »

Welcome to the forum.

I have to ask: what is the purpose of this project? Educational demonstration?
It isn't very practical for a couple of reasons; it's pretty hard to use a laptop outdoors in bright sunlight and they generally have their own batteries in them that will last a couple of hours anyway.

To that end it may be a good idea to look at the DC requirements of the laptops you plan on powering and see if you can't skip the inverter stage altogether; some will run off 12 VDC, albeit perhaps requiring a specific "car adapter".

A laptop running full out tends to be a 35 Watt load. Some more, some less. Very important to know which/what you've got. A dozen such would be 420 Watts +/-. That just misses on being able to be supplied by one of the best small inverters available (which is 300 Watts).

Before you talk about fuses and wire sizes (99% certainty there won't be ay diodes involved) you need to get firm numbers on how much power you need to supply. Otherwise you're just guessing at everything and that is bound to be disappointing.

A couple of other observations: 600 Watts of panel will take up a fair amount of space. Mounting needs to be well thought-out. Boat/scooter batteries may work for demonstration purposes but they won't last long-term.

If you've got the computers in question, spend $30 on a Kill-A-Watt meter and see what kind of power they really draw, both in use and while charging the built-in battery. You might be surprised.

Thanks for the quick reply! In fact, I'm looking at more of a permanent installation. We already have a gazebo structure in an outdoors park, so I think that roof space and glare shouldn't be a problem. I think 600W should take about 5 to 6 m^2 and we've got that kind of space.

Of course you're right that laptops have their own batteries, but it could derail a class if students completely lost power. But since like you say they don't all need to be plugged in all the time, perhaps a smaller setup that can support a half dozen laptops and can use a smaller inverter is preferable. What is that 300W inverter you recommend?

Students will all have different laptops, so I think it's best to guestimate an average power use and leave some wiggle room. The DC to DC adapter solution is interesting but if most computers have different power needs, I don't think I'll risk it.

Why wont boat or scooter batteries last long-term? And what is a better battery solution?

Thanks!
-Trevor

Re: Laptop Charging Station for Schools

Hi again! I like a lot of these suggestions. It turns out that the needs and roofspace I have lend themselves more to a 360W supply system, like you're talking about with a 220Ah deep cycle battery, and then I'll probably use a 300W inverter to limit the draw and keep the the battery from depleting.

I still have a few questions:
1. I'm putting these panels on a gazebo-type structure with one roof facing SE and one facing SW. Since all my panels are angled 45 degrees away from south, the maximum I can get out of them is 0.707 of the maximum rated value, correct? So if I have 510W of panels (255W on each face), the maximum I'll get is 360W at around noon, correct?
2. If I have two types of panels, one pair supplying 5.43A each at 37.7V and the other pair supplying 4.15A each at 18.1V, and I put the smaller panels in series to double the voltage up to 36.2V, what is the effect on the efficiency? Will I get close to 5.43A + 5.43A + 4.15A = 15.01A?
3. Does the situation with the two different angles from the sun have an effect on the Voltage throughout the day? And would MPPT then be better? Or does it only affect current, and can I save on buying a cheaper 30A PWM charge controller?
4. I'm having some trouble finding generic information on charge controllers. Will it be okay to run a 30A charge controller to charge a 12V battery with nearly 360W of input? It seems rather close to the maximum operating amperage.
5. Can I put everything I need (except for the panels, of course) in one box, creatively and safely displayed as an educational tool? I'm talking combiner box, charge controller, battery, DC disconnect, inverter -- the whole kit! And how big would that box need to be? Do I need special ventilation? I was hoping to bury it in the center of the gazebo with a glass roof.

Thanks!

Re: Laptop Charging Station for Schools

I'm a little worried about all this gassing and mist around students. I've read that AGM batteries are safer (if more expensive). If I build a physical divider between the battery and the rest of the system (say, out of wood), could I put them in the same 3ft x 2ft below ground display with a glass ceiling?

Re: Laptop Charging Station for Schools

If I'm using MC4 connectors, which are rated to 20A and 600V, do I need to use a DC disconnect? My system never gets close to those values!

Re: Laptop Charging Station for Schools

Photowhit wrote: »

Actually the amount of wattage will usually be closer to the panels Normal operating Cell Temperature value (NOCT) which tends to be around 75-80% of the panel rating done in standard Test Conditions(STC) so "normal" use I would use this number to start.

I shallow roof with panels mounted SE and SW could both be near max during Summer when the sun is high in the sky and less in the Winter when the sun is lower in the sky. but you do have the general idea.

I think I misinterpreted what you meant by this. Did you mean that rather than the STC 510W of the sytem, I should actually start from the NOCT value, and expect to get about root(2)/2 of that value at an angle of 45 degrees? Or did you mean that even though my panels are facing SE and SW, I can expect a pretty consistent power output close to the NOCT value?

Thanks!