## Wednesday, April 22, 2015

### Building a Simple Solar Battery Charger - Polarity Matters!

Definition of Polarity (from the Kids.Net.Au Dictionary):

1. a relation between two opposite attributes or tendencies; "he viewed it as a balanced polarity between good and evil"
2. having an indicated pole (as the distinction between positive and negative electric charges); "he got the polarity of the battery reversed"; "charges of opposite sign"
So, it turns out charging a single AA rechargeable (NiMH) battery with a small (1V) solar cell is both incredibly simple, and incredibly complicated.

As were really just out to power a solar scribble-bot on a cloudy day, we opted to keep it simple.

The main thing to keep in mind is that polarity matters.  POLARITY MATTERS!!!

I know this might sound like a bit of a "duh".  I mean, we've all watched The Magic School Bus "Get's Charged" often enough to know that with DC power (like with the battery) those little electrons move in a single direction through the wires, right?

But, after hooking and unhooking the batteries and solar panel from various hobby motors - it becomes easy to forget.  Polarity only matters with hobby motors if you care which way the axle is spinning.  Since we didn't care, we didn't really pay attention to which wire was positive, and which was negative.  So, when we went to charge our nearly dead battery with the solar cell, we didn't even think to check if the solar panel had positively and negatively marked connections (which you can see from the top of the page - it did).

Needless to say, we had the battery hooked up incorrectly, (-) to (+) and (+) to (-) when we made our first solar charging attempt, yesterday.  This is a good way to damage a battery, and a solar panel.

Today, with a freshly emptied battery (that little fan finally quit turning), we tried again, this time taping one wire to the negative side of the battery, and connecting the other end to the negative post of the solar cell, with the positive side of the battery wired to the positive post.

We left our "charger" out in the sun for a few hours, while we headed off to a homeschool park day.  When we returned, we checked the battery by connecting it to the fan, which started to turn...slowly...and only for a few minutes - but still!  We would have loved to have kept our battery charging for longer, but just as the fan stopped turning...

...the sun went behind a cloud, which was joined by another cloud, and another, and...

...I think it might be a few days before we are attempting any more solar projects.  Cloud study, anyone?

If you have sunnier skies, and would like to build a slightly more "official" solar charger (maybe one with a diode, that would keep the solar panel from sucking the power back out the battery when the sun disappears) for a science fair or teenage science project, I'd suggest checking out the instructions for the DIY \$10 USB Solar Emergency Charger over at the Survival Prepper Joe site (love that name!).

Of course, if you have \$10 to spare, you might want to skip the survival kit (it's a slooooow charge at its best), and put your money where it can do some real (and tax deductible) good - providing full sized solar panels and batteries for college students in Zimbabwe; where, I'm told, the sun is usually shining.  Help "Flip the Switch" on in Bulawayo.

Phyllis said...

You guys are really getting into electricity. Your approach is very interesting!

An Almost Unschooling Mom said...

Phyllis - If by "interesting" you mean "backwards", you're probably right :)

Ticia said...

That sounds like something I would have done.

Angelic Scalliwags said...

Shame about the clouds. That is something which would completely happen in the UK ALL. THE. TIME!

Anonymous said...

Hmm, if that is a NIMH rechargeable then that 1v panel is never going to charge it, no matter how many hours in the sun.

Here is a good visual: http://members.toast.net/joerger/pic2/discharge.gif

Get another .5v panel and connect it in Series and you can try again with it charging at 1.5v.

Visual here: http://www.exo.net/~pauld/activities/physics/solarcellf/solarcell.html