Solar panel sucks power??
July 26, 2010 10:55 PM   Subscribe

I have a mini solar-panel (it's about 2sf) and claims to good for about 75w. In full sunlight it outputs about 19v. Of course when the light fades, the output voltage drops. There doesn't seem to be any 'electronics' built in - it just the silicon panels and output wires (with a female car charger socket soldered on actually). I want to use it to charge a mobile phone and laptop. But ... what seems to happen (with the laptop) is that is actually drains the battery somehow.

If I direct connect the laptop (what risky you say!) - the laptop specifies a 19v volt charger - it's great in full sun. Laptop charges and seems happy. But if I leave it connected and sun is covered by clouds for a few hours (so solar panel output has dropped to 10v or less), it somehow seems to depletes the battery. I come back expecting a fully charged battery and what I have is a dead battery!!

I guess I need some kind regulator ? But web searching only seems to offer me huge and expensive house-power systems.

Help please. And advice on how to get a nice 12v or 5v USB power supply from it too please. I've fiddled around with using to charge a car battery, but this of course is huge and heavy and limits me to 12v - I need more than than to charge the laptop. I want the up side of the panel when it's outputting lots, not the downside when it's outputting less.
posted by Xhris to Technology (10 answers total) 3 users marked this as a favorite
At the very least, put a diode in to prevent the laptop battery from basically losing power to the solar panel whenever the battery voltage is higher than the solar panels voltage. That being said, connecting a completely unregulated power source to your laptop battery is a pretty bad idea. Shouldn't be too hard to solder together a suitable regulator using off the shelf components.
posted by Diplodocus at 11:09 PM on July 26, 2010

Best answer: You need a blocking diode (1 2 3) or a charge controller.
posted by drdanger at 11:14 PM on July 26, 2010

You've made sure you have the right polarity, right?

I'm sure it's obvious & that no one else fouled this up before.
posted by Pronoiac at 11:33 PM on July 26, 2010

Best answer: Most of the charge controllers sold for that purpose will have output voltages suitable for charging a 12V lead-acid battery, and this won't suit your 19V laptop.

I'd be looking for a car power adapter to suit your model of laptop.

Car electrical systems are nominally 12V. However, a fully charged car battery with no load is good for 13.8V, and the standard system voltage under charge is generally 14.4V, and there are assorted high-inductance components in circuit that can generate spikes of over 40V. Anything designed to run off the car's nominal 12V supply should easily tolerate the higher output voltage of your fully illuminated panel (which, by the way, will drop significantly lower than 19V as soon as you start drawing current from it).

Any car power adapter with a 19V output is pretty much forced to include a switching DC-DC converter to step the voltage up, and any such converter should cope just fine with the fact that your panel will occasionally droop below 12V.

The same can't be said, unfortunately, for cheap in-car USB power adapters. These are quite likely to contain a horribly lossy linear regulator that just turns the voltage difference between input and output into waste heat.

I've seen multi-output-voltage universal laptop power supplies designed to run from mains electricity, and these often include a power-only USB socket. If you can find something similar designed for in-car use, it will probably have the DC-DC converter you need while still being switchable for 12V output.
posted by flabdablet at 12:48 AM on July 27, 2010

Yep, you need a charge controller. You'll find circuit diagrams online and you can buy all the parts you need from if you're comfortable soldering. If not, there are controllers available for sale that have adjustable voltage settings.
posted by drewgillson at 12:49 AM on July 27, 2010

This is a standard issue with solar panels, since everyone hooks them up to batteries pretty much universally. The easy answer, stated above, is a diode. That will keep it from draining when the voltage isn't there to actually charge the battery.

Also, 2 square feet is not 75W unless it's a lot more efficient as anything I've seen for sale. Most 75 watt panels are 20" x 48", which is about 6-½ square feet.
posted by aubilenon at 1:51 AM on July 27, 2010

I'm with aubilenon. There is no way you can get 75 W from a 2 ft2 panel.

If you're receiving the maximum possible insolation, nearest to the equator (the Sahara desert for example), then you'll be getting something like 300 W/m2. Two square feet is 0.186 m2 which gives you 56 W and that's assuming that the solar panel is perfectly efficient. Factor in an efficiency of around 20% (and that's being very optimistic) and that gives you 11.2 W.

Could you give us the make/model of the panel? I think you've been had.
posted by alby at 4:50 AM on July 27, 2010

I wouldn't be at all surprised to find that there's been a decimal point lost somewhere in the shuffle and that the panel is actually rated 7.5W.
posted by flabdablet at 5:25 AM on July 27, 2010

Modern Outpost sells equipment that will help with this, as well as helpful articles about charging laptops from solar.
posted by Emanuel at 6:13 AM on July 27, 2010

How to Make a Solar Power Generator for less than $300

Since you already have the $100 solar panel, this can guide you through the rest.

posted by I am the Walrus at 6:14 AM on July 27, 2010

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