Small portable solar generator questions

[powerpower]

Hello all,

New to the messageboard.  Did a search, but did not find the EXACT answers to my questions, so here it goes...

I would like to charge a portable jumpstart battery to begin experimenting with solar power.  I contacted XPOWER about charging the battery, and they say I need 12 volts and at least 5 watts.

I bought a solar panel off ebay that is 12 volts and 2.5 watts, and I needed another 2.5 watts.

Someone gave me one of the $9.99 solar panels from Harbor Freight.  The panel is used for 12V charging in a car through the cigarette lighter, but the multi-meter voltage shows 24V?!  It does however put out 1.5 watts, so I went ahead and ordered another one off the Harbor Freight website.

So, the question I have is how do I get the 24V panel(s) down to 12V so I can connect them in parallel to get the 5.5 watts?  (2.5 + 1.5 + 1.5)  Is this possible or feasable?  I dissasembled the 24v panel from Harbor Freight hoping it was two 12V panels together, but is actually one 24v panel.    

Suggestions are welcome.

Thanks in advance!

[terry5732]

Your battery probably has a built-in charge regulator for use with an AC adaptor. Bypass this by charging directly to the cables with the switch on. Don't fret the 24V as it will drop to about 12 as soon as connected. You don't really need the 5 Watts but the more Watts the faster the charge. If you are worried about parallelling them, you could put a diode in-line on the 12V one so the the 24s can't put a reverse on it.

[Titantornado]

What's the open circuit voltage from your 2.5 watt panel?  It should be more than 12. (prob. 24v also) You need a higher driving voltage than the battery to charge it, and the 24v can be hooked right to the battery at these low wattages. (for example:  I bought some panels for a 24v system, and their open circuit voltage is 44v)  Both of your panels probably have blocking diodes built in, so they can be hooked up in parallel without issue.  Five watts would still be a very low charging current.  More a maintaining charge than anything.

[powerpower]

terry5732...thanks for the help!

The battery has a 12V cig plug that connects to the battery.  Manual says it is okay to hook up solar panel here...glad to hear that the voltage will not matter when connected.  I am considering the diode.

[Volvo farmer]

If you don't hook the solar panel to a battery and test the voltage, it is supposed to be significantly higher than 12V. 18-21V is the norm for polycrystaline panels. We call this open circuit voltage or Voc. 24V might not be out of the question for a $10 Chinese panel meant to float charge a car battery.

How you determined it is putting out exactly 1.5 watts without an ammeter in the circuit and some mathematical computations is beyond me.

DC Electricity is like water. Your battery is your bucket, your Harbor Freight solar panel is your little tiny hose trickling into the bucket, and your jump start cables are a big huge valve in the bottom of your bucket. All batteries (buckets) leak a little too so if you never put anything in, they will be empty several months later.

Voltage is like the pressure inside the hose filling your bucket. Amperage is the flow of water coming through the hose. Watts is the actual amount of water you collect. A $10 Harbor Freight panel into a jump-start-battery is like a little mouse peeing into your one-gallon bucket. If he pees at 24V into your 12V bucket, the stream comes out really fast but it doesn't really matter because his little urethra is so small, it will not overflow your leaky bucket. Plus your bucket actually slows down his pee because he's not standing above the bucket, he has to push his 24V against the 12V in the bucket from below.

What's really important here is how much water do you want to use out of your bucket? If you want to jump start your car every morning with the thing, five mice would not have enough pee to fill up your bucket every day. If you only want to jump start a car every six months, it's quite possible that one mouse would be able to fill up the bucket, leaks and all, before you need to drain it again.

Now you coud take your one gallon bucket and set it under an elephant's 1000 watt PV array, and it would be full in short order.

So I would argue that the people who gave you the arbitrary 5 watt panel number are feeding you a red herring, unless they asked you how much water you plan on using from your bucket each day.

(Stolen and modified from page eight of the Dan's "Homebrew Wind Power" preliminary draft)

NOT IT!

[powerpower]

Titantornado...I'm uncertain how to measure open circuit voltage, but a multimeter connected to the panel in direct sunlight shows just above 13 volts.  You mention that the battery will be okay at these low wattages...at what wattage do you begin to worry about overcharging?  The impression I'm getting from these posts is that the voltage is not as important as the wattage...correct?  

panel 1 - 12V  2.5 Watts  

panel 2 - 24V  1.5 Watts

panel 3 - 24V  1.5 Watts

This means that the 12v battery will be charging from roughly 25 volts and around 5.5 watts.  This is okay for a 12 Volt battery?  

Thanks again for the help!

[powerpower]

Volvo farmer...thank you for the analogy!  That is great.  All the talk of hoses, buckets, and water made me have to go to the restroom before I replied, but it did get the message across!

quote: "How you determined it is putting out exactly 1.5 watts without an ammeter in the circuit and some mathematical computations is beyond me."  Here I am only going by what is listed on Harbor Freight's website for this panel.  It is listed as a "1.5 Watt Solar Panel"...  The exact wattage probably varies from the manufacturer's specs.

Good call on the red herring!  Now that you mention it, they didn't ask what I would be doing with the battery!  This response is probably 'canned'...not unlike the 'canned laughter' you hear on the cookie cutter prime time programs.    

Thanks for the information.

[Titantornado]

Well, I hate to be the bearer of bad news, but if your 2.5 watt panel is only generating 13 volts when not connected, it will be of no use to charge a 12v battery.  If you can get another one, and hook it in series, you'd have sufficient voltage.  The mouse just won't have enough "push" to get it into the bucket with just one.

(see analogy below for anyone who doesn't know what that last sentance is about)

Yea, in this case, wattage is the most important thing to worry about.

[ghurd]

There is a chance the 'just over 13V' panel has a built in controller. Many of the VW panels have one, keeping the open volts to about 13.7V.

[dinges]

13.7V? That seems like little, to me. That would mean it's only ok for trickle-charging. Batteries could be charged up to 14.4-14.5V, but charging should stop after this voltage is reached (or switch to trickle charge at, say, 13.7-13.8V).

And I wonder whether such a voltage regulator is really necessary with such small panels that put out, say, 100mA max in battery of about 50Ah. No way that such a small panel could overcharge or damage a big battery, I'd say. But then, why DO they incorporate a voltage regulator?

Peter.

[ghurd]

I have seen the VW PVs EBay listed as 2 to 5W, so sounds like it could be one. They are trickle chargers rated at 170ma or 3.2W, however all mine go past 220ma easy, maybe over 250ma. The controller is on/off series type, about 0.1V hysteresis. I overcharged a loaded 7ah AGM under less than favorable solar conditions, in a very short time with a VW PV not having the controller.  No doubt they could ruin an unloaded 24ah quick.

Maybe the controller is provided so if the battery is disconnected the computer will not see excess voltage?

Here, one VW PV catching 2 hours of late afternoon sun, feeding a portable jump starter/compressor/lighter socket does all (not much used here) the air, jump starting and sometimes an inverter for a cordless tool or cell phone. PV is just plugged into the lighter socket. Takes a couple days to recover from a very large SUV tire, nothing else drops the built in battery gauge.  More use in small PVs than most people think.

It is a good way to start playing with solar.

[powerpower]

-- UPDATE --

I clipped the male cigarette plug off the 'volvo' solar panel this afternoon, stripped the wires and tested the voltage ---> 24 volts!  The plug contains a small chip that I assume steps the voltage down to 12 and regulates the 'trickle' charging.

Soooooo....now I have three (3) 24 volt panels that I am assuming (ass: U & ME) will put forth around 5.5 watts of power when paralleled...hopefully that makes things a little easier for the discussin'.

Does this new information change anything?  Do I need a charge controller?  

Or just connect directly to the battery (through the cigarette lighter plug) and just 'set it... and forget it' for weeks on end?  

Comments welcome.

Thanks for the responses...Really enjoying the otherpower.com website.  

[ghurd]

It had a controller for a good reason.  

Maybe search PB137ACV.  PCB137ACV? It'll handle all 6W for $3.

G-

[cyplesma]

http://www.mouser.com/catalog/626/317.pdf