Solar Battery Charger V1

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I have a 2006 Suzuki Katana 600 motorcycle that I enjoy riding. However, as I'm pretty much a fair weather rider, it doesn't get ridden much in the winter, even though we have pretty mild winters in Atlanta. This year, the battery went flat due to sitting too long without running it, and I had to replace the battery. Batteries only last 4-5 years anyway, but it doesn't help them to let them run down. I could have bought a Battery Tender, but I hated the idea of having it plugged in all the time. Since I have these two nice solar panels I've never put to use, I figured it was time to build a solar charger to keep the battery topped off.

Googling around found a number of circuits that would charge AA or lithium ion batteries, but nothing really oriented at keeping a car (or motorcycle) battery float charged. I found one very nice circuit from CirKits, but it was $44.95 before shipping. I needed something a little simpler, and I ran across this one by a guy named JAW (which is probably an abbreviation). He lifted the schematic from a company called ETI, and I lifted in from him.


This regulator circuit achieves 3 things: battery cannot discharge into the panel, panel voltage can never go above 14.3V and even when the panel voltage output is as low as 6-7 volts it is still capable of charging a 12V battery.


I had most of the parts on-hand, but had to order the zener diodes from DigiKey. Luckily, I had to order some parts to fix a couple Samsung SyncMaster 204B LCD monitors, so it wasn't a special order. I had planned on using the 20W BP Solar panel I had picked up a few years ago from atop a houseboat, but just after I placed the order I started thinking... "Hmm, those zeners are 5 watts, and with a 20 watt panel, I bet they're going to pop when they can't dissipate the additional 15 watts..." Fortunately I also have a never-been-used 5W panel I picked up when I cleared out some warehouse inventory for a company I worked for.

One really nice feature of this circuit is that because of the capacitor voltage doubler, even when the output of the panel is below the float voltage of the battery, it'll still charge. This helps out since I'll lose a little voltage in the 25 foot cable run from the panel to the charger circuit. And since I'm just maintaining a float charge on the battery, and not trying to do a hard charge, current isn't very important. Although I should be able to get somewhere around 325ma at full output from the 5W panel, I only need 5ma to 10ma to keep it charged. ZD2 acts as a shunt, so any power above what's necessary to maintain the float charge gets shunted to ground.

The solar panel is located just outside the garage, mounted on a piece of conduit driven a few feet into the ground. The cable would be run back into the garage, where it will connect to the charger circuit, located in the ceiling just above where I park the bike. A short drop-cord to plug into the battery, and I'm set.



The circuit was built on through-hole proto-board. I have a stock of Vector #8006 Circbord that I like to use for projects where I don't lay out a PCB. Since this is a one-off project, and fairly trivial, I'd rather save the money and just point-to-point it. By the time a board is laid out and produced, you've spent at least $100, and for that I could have bought the SSC3 from CirKits and been done with it. For some reason, I've seemed to have acquired a largish collection of PacTec enclosures over the years, and a suitable one was found in the stash.


Anderson PowerPole connectors are great for this kind of project. To be able to disconnect the solar panel, I've used a 6" pigtail off the PacTec box. To connect to the battery, a 6 foot drop-cord with PowerPoles connects to the battery. The bike has a small pigtail connected to the battery that brings the connector out where I can get to it without removing the seat. It's not weather-proof, but I almost never ride in the rain, so it's not much of a concern. A small boot might be a good idea if harsher environmental conditions are expected.

Being a total geek, my first thought was to add some analog to digital converters, and log the charge rate and voltage. I decided this was way too much trouble, and built a small box that with a pair of PowerPole connects that I can plug between the drop-cord and the battery connector. This allows my to connect the probes of a volt meter and ammeter inline to check voltage and current. If I find a small small pair of meters that go to 500 milliamps and 20 volts, I may build a box that remains inline full time.


File:Solar Charger Monitoring Box.jpg
Charger monitoring tap box


Now we'll just have to see how the battery holds up through next winter...

Notes, Etc