Project feasability - Solar charger and monitor

Hey guys,

I am looking for a little bit of help, i currently own a 12v 180W solar panel array on my house,
I'm wanting to build a 12 volt solar charger and energy monitor using the arduino uno,
I want it to be able to charge a battery upto a certain voltage, then when full, it dumps the excess charge into a dump load (obviously using relays to do the conversion from 5v logic levels to 12v circuit).
Also possibly a way to monitor load output from the battery, when they battery gets low, it shuts off the load.

On top of this fairly simple operation, i'd like for my project to use my arduino ethernet shield and post stats on battery voltage (volts), solar output (watts and volts), and maybe load output (watts) , and maybe total energy collected on average that day/week?

I'm sure this project is possible, i have a fair knowledge of electronics, but i'm fairly lost on the coding that would be needed to get the arduino to fetch/record this data for me.

Thanks for any help in advance.
Liam.

For the first part (I want it to be able to charge a battery up to a certain voltage, then when full, it dumps the excess charge into a dump load), use a SPDT relay or a SPCO / SPTT (stable off position for between dummy load and charge). That relay will move the 12v output from the solar panel to the charging circuit and the dump off. So youll also need to make a charge circuit for the solar panel -> battery. The relay can easily be controlled using an arduino. to log data about how much it is charging / dumping, youll need to make voltage and current sensors. here is in example that i made for switching dc to change between rev and fwd for some dc motors: http://1.bp.blogspot.com/-q_O0LU735H4/UYstU-VV1YI/AAAAAAAACfA/_q_CKT7TdWI/s640/motors.png

for the data logging you mentioned later, you just need some of the voltage / current measuring things i mentioned earlier. i've never used a Ethernet shield, but with a few tutorials, you should be ok!

for a voltage sensor, you just need a voltage divider. it essentially "divides" the voltage to a lower range. so instead of 10 to 15v or something measured on teh battery, a 0 to 5v signal is sent to an arduino analog in. go here: http://www.raltron.com/cust/tools/voltage_divider.asp set r1 as 22, and r2 as 10 (ohms or kohms doesnt matter, just the ratio). put 15v as the in voltage and click calculate, you should see the max voltage the arduino will read. now delete the in voltage and replace it with 11v and delete the out voltage number. click calculate and you''ll see the minimum voltage the arduino will see. now, connect the battery(fully charged) to the divider and check the output (to the arduino) with a voltage meter. then find the analog value it corresponds to with the arduino. do the same for a dead battery. then change the analog value back into a voltage number and store it in a byte, int whatever and write it to the ethernet.

this explains the current sensor: Measuring Current with the Arduino

surely possible!!!

Unlike a wind or water turbine there is no need to dump excess power form a solar panel, just don't allow the current through to the batteries, which of course is what your regulator will do.

Also look at the open-circuit voltage for your panel, you may be surprised how high it will go with little or no load. You measuring circuit has to handle that voltage at least plus a fudge factor.

a way to monitor load output from the battery, when they battery gets low, it shuts off the load.

There's no easy way to measure a battery's state of charge. The easiest thing to do is just look at the voltage and shut it down if too low, but that's a very bad indication of the battery's state, especially if there was no load when you looked.

You can "coulomb count" which is counting the amps in and subtracting amps out x about 1.4 (Phukets (sp?) factor) and make a decision based on the result.

Another thing to consider is the charging rate, as a general rule of thumb you should not charge at more that 1/10th the battery's Ah rating, so a 120Ah battery should not be charged at a rate above 12A. With a single panel that's unlikely to happen unless you have a very small battery.

And one more thing, most chargers have a three-stage algorithm, bulk, absorption and float. In the bulk phase you dump as much into the battery as makes sense, then when charged you sit the battery at a high voltage (about 15v) for some time, even a couple of hours, and finally to revert to float where you try to keep the battery at about 13.4v.

I may be out a bit with some of the numbers but that's the guts of it, all this is well documented on the web I'm sure.


Rob