Solar charging using the Arduino with a relay?

I thought this would be easy. It never is. I am building a simple weather station. I have a 10W solar panel attached to my lithium batteries through a relay, controlled by the Arduino.

The hardware has me stuck. I want to use a solid state relay (SSR), but a DC SSR is expensive. A relay will wear out. Where is the middle ground? Is there a cheap SSR that can handle 1A @ 12V? Or is there something else?

Looking to build lots of these stations, so costs add up fast!

Why can't you just use a transistor?

I tried to use a tip120 (Darlington Transistor) but was never able to isolate it enough from the rest of the circuit. Do you have an example/schematic that you think might work? I would love to try it.

Hi DingbatCA,

If you are hoping to simply charge your LiPo battery and power your Arduino you could try one of these: http://www.airsensor.co.uk/shop.html?page=shop.product_details&flypage=flypage.tpl&product_id=2&category_id=4

I also use mine simply to charge LiPo batteries from USB :-)

Would be great to hear move about your weather station when your done - will you be sharing?.... :-)

I have seen the charge shield, and other single chip solutions. They work great. But I am looking for more control.

We have a 16MHz processor sitting there reading from sensors. I figured it would not be that hard to add charging control. This also allows me to have full control over the charge. Such as only charging to 10.1V instead of 10.8V, greatly increasing the cycle life of my cells.

I will be sharing up parts of the station, as i get it done. Currently I need to get the charge circuit off of a relay.

I see where you're coming from - sounds even more interesting - keep us posted :)

I tried to use a tip120

That's an NPN: not your best choice for switching the high side of the circuit.

With a PNP bipolar, or a P-channel MOSFET, you can wire it so it defaults to "off", and only comes on when actively pulled down. Since the voltage on the base or gate will go as high as the maximum panel output, you'd need to use something like a 2N2222 to do the pull-down. Hooking the Arduino directly to the control would exceed the 5V limits, and possibly fry the chip.

Doing a google search for something like "high side p-channel switch schematic" should turn up some example circuits for you.

I would guess something like this? http://www.digital-diy.com/digital-diy.net/Handy%20Tips/Resources/P-Channel%20FET.gif

You would guess right :slight_smile: