What is the best method to measure output of a solar cell ? uCurrent ??

[tdconsul]

I am currently experimenting with a small (20 watt i believe) solar cell. Its just a small battery tender type used to plug into cig lighter and keep batt up. My question is what would be the best way to measure the out put of the cell. The only test equip I have is a fluke 72 dmm (well a couple other dmms also but nothin fancy) and I am also playing with an arduino to log the data so I can track my out put for different sites and tracking devices/programs so I can optimize for max out.

At present I have tested the voltage of the cell in direct sun and have seen as high as 38vdc. I connected the cell to a 9.2 ah 12v sla from a scooter (plain ol sla) which was at 10.17 vdc when I started. The first time I connected the AMP side (+ lead into the 10A hole)  up I did see 0.03 adc in direct sun and about the same if I connected the cell directly to the dmm with no load. I was able to gain approx .87 volts the first day ( I thought it was not a bad day for free lol) so the batt tested 10.94 when the sun went down. At that time I did not see any amperage, but the sun was down so I went to bed. I went out the next day to a higher location and set up but when I tested the amps in series with the cell and the batt I did not see any ( i did get a little flicker in the +-) and when I tested the voltage with the cell hooked up it barely flickered from the batt voltage which was now at approx 10.89.

So it was very difficult for me to tell if I was seeing how much if any charge I was getting from the cell. My question would then be ,,,

What type of measurement would be most accurate (volts or amps) when monitoring the effective output of a solar cell.

What would be the best (cough cough - cheapest) way of measuring that output

Is is possible to build a simple breadboard type circuit and connect to an arduino that would measure in at least micro volts or amps so I can see the minute changes in the charge of the battery.

Just off the top of my head I am thinkin that since my real target is measuring the "Charge" of the battery then I would want to be measuring Volts. But I am also thinking that I might be missing some important electrons sneaking out to have a smoke if I only capture the end result and dont keep and eye on those pesky little buggers as they are running down the wire.

Oh well I am digressing into the depths and think I better stop b4 it gets out of hand.

One of the reasons why I am here is I believe the uCurrent might b very helpful with my quest !! ;~)
~Tim

[darrenb]

Hi,

A dmm should be all you need to get a good idea of what is going on.  With two you could measure volts and current at the same time.

uCurrents from all I've heard are great but they are for measuring down in the uAmp (millions of an amp) range whereas a normal meter in the ten amp range would be suitable for measuring a 20 watt solar panel.

A battery down at 10.17 Volts may be dead.  Anything under 11 Volts for a 12 Volt battery is a worry.  They should be left full not empty.  I'd suggest seeing if the battery is any good before going too much further with the solar panel.

When you connect a 20 watt panel to a battery this size in sunlight you should see a voltage rise of half a volt or more pretty much straight away.

Measuring amps you should get one amp or more from a solar panel this size to this battery.

Keep an eye on the battery voltage when the panel is connected without a regulator.  You probably don't ever want the battery voltage to go over 15 Volts.  14.2 Volts is ok but only while bulk charging.  For more permanent connection 13.6 Volts is probably the highest you want for a SLA battery.

Connecting an Arduino is another story.  Some things to consider is that it measures the voltage relative to a reference voltage so you have to do a bit of work to get a stable reference voltage.  Also measuring a large range of current across a shunt will probably require a voltage amplifier or a dedicated monitor IC.

[poot36]

If you are really getting 38V unloaded voltage out of the panel it is a 24V panel.  If you drag the output down with a lower voltage device you will not get much power out of the panel at all.  Try a 24V battery and see what happens.  You can test the short circuit current of the panel with your meter on the 10A range it will not hurt the panel.  I am going to guess that it will be around 1A or so.

[tdconsul]

Ahhh yea now things are starting to make sense to me,,, yea now that you mention it I was having problems with one of the batts ( hence why it is in the garage) ok kewl it is all kinda makin sense to me,, soo I guess my question would then be ,,, would just measuring and recording the voltage (seems to be less expensive overheadwise) with an arduino and just forgetting about the amperage unless there is a prob or somethin going to be a sufficient judge of the system performance,,, I am most likely going to be watchin the voltage with the arduino to fine tune the 2 axis tracker to max out so I can just use that ref for efficiency reports (holy smokes did I just spell that rite )

tks poot36 also reminding me its a 24v system and need to run em in par 2 get anything. I have a couple of 50ah slas from another chair that I was planning on using 4 an elec tractor,, so might as well charge em up see what I can get out of em.. I am thinking off the top of my head if I am seeing one amp and I usually charge at 10 or so on my charger then I should get a pretty good couple of ahs after a couple of days,, gimme sometin 2 play with at the least,, and 100ah is a pretty good bit of time to be runnin just some water pumps and what not to move hot water around and heat things up.

Thanks again 4 the replies super appreciated (I am sure Dave does 2 so I won't bug him with this simple stuff lol)
~Tim ;~)

[tdconsul]

ohh yea i was just thinking also that as far as I can tell ,, if I hook my batts up in series, then I have to b concerned about "balancing" and as far as I can see it would most likely b in my best interest to keep and eye on the voltages of each individual cell ( especially if I have the pins on the ardy) and stop the charge if any of the cells reach the high voltage spec ( and possibly set up some special circuits later on that would "direct" the current to the cells that need it) I was just rethinking that and prob b best to set up a simple "comparator" type circuit that I can set up in an "or" state so if and of the cells are over a certain ref voltage then I would just eat one io pin for over volt on a batt, I was also thinkin about doin a temp sens on each batt and I think I can use the same type of circuit for the temp sensors so I will only eat one pin 4 temp and one 4 volts,, this way it would b scalable if I add more batts later on ,, hmmm sounds possible in theory in my mind at least  ~Tim ;~)

[Seekonk]

Solar panels are basically a current source.  The current will be about the same  whether it is shorted connected to a 12 or 24V battery.  For that reason they work best (most power) if they are operated at the power point.  It is relatively simple to take a $2 LM2596 type regulator power board and modify it to work at power point.  Get a HV version or limit the input to 35V with a zener.   An Arduino with a 490hz PWM and a power transformer/FET can make a good power point charger.

[zapta]

Since you mentioned Arduino, you wan to look at this sensor http://www.adafruit.com/products/904  (they also have a 60A version).

Another option is to buy or build a programable electronic load. It will allow you to do more controlled tests.  Search google for arduino programmable electronic load.

[Seekonk]

I'v been thinking about an electronic load project for the uno as a RE example project but since I already have a couple EL-300 and a Swinger I it has been low priority.  With a UNO you could switch in a couple resistors and PWM a third for fine control.  No current sensor is needed because you know the resistor values, voltage and % PWM, that will give you current.  Then you can just clock down with the monitoring screen in TOOLS and find the power point Interesting board, but partial to the ACS712 chip because of isolation in high voltage.  I pair these 5A versions up with a good  100A shunt.