Recently, after more than a year of research and tiny prototypes, I put together a “mini” 50-watt photovoltaic power system. My intent is to run my computer system off of it. At the heart of the system is a 34 amp-hour AGM lead acid battery. That’s a bit small to power a computer system, but with a slight reduction in use time, and some efficiency modifications, I think it can be done.
I’ve only just put the system together and have been running it for almost 2 full weeks. I haven’t powered my computer using it yet, but I have been running floor lamps and charging my cell phone using it (total overkill).
In researching how to build something like this, I found lots of people online who had built things in a similar size range, but none of their designs could answer one question I had: How do you monitor the battery accurately? Watching the voltage is ok, but it’s not very accurate if the battery is loaded or charging. Deep cycle batteries are tough, but you still need to treat them right. They aren’t cheap either, so protecting the investment is a good idea.
Originally I was going to use something like this “*Turnigy inline watt meter*”:http://www.powerwerx.com/digital-meters/dc-inline-watt-meter-power-analyzer-powerpoles.html, but I hesitated at the thought of running so many amps through a little plastic device. In the end, I took inspiration from how our server battery backup here at Small Dog is monitored.
*The best way to monitor a battery is by counting the amp hours in and out in addition to watching the voltage.* This is like the odometer on your car (if you could somehow put miles back into the engine). To do this without using an inline meter, you use a device called a “*shunt*”:http://ow.ly/uMTqb. A shunt is basically a very low-value resistor which causes a tiny voltage drop. This drop can be measured, and using “*Ohm’s Law*”:http://en.wikipedia.org/wiki/Ohm%27s_law current can be calculated. With some digital logic, you can count how long the current has been flowing, and there you have your amp hours.
In my system I’m using the “*TriMetric 2025*”:http://bogartengineering.com/products/TriMetric. Yes this is a bit overboard for such a small system, but so far I’ve found it to be invaluable, especially since I’m doing a lot of testing. Installation is a bit complicated, but the instructions are very helpful in figuring out what goes where. You also need to program the device for your specific setup. I really found this handy, since I could set the battery charge voltages, charge efficiency, days to recharge reminders, and much more. The TriMetrics also have a J2 serial data out port, so I’m working on a way to pull data from there and upload it to the web in real time. Once online, I could check on the status of the charging from anywhere.
Interested in simpler, shorter term battery backups? Check out “*uninterruptible power supplies (UPS)*”:http://www.smalldog.com/category/Power_Supply/UPS_Backups. These are like surge protector strips except they have batteries in them that will continue to run your equipment if the power at the wall outlet fails. Most are only designed to run a short time (like an hour or less), but that should give you enough time to save and shut down your equipment properly without any data loss or damage.
