I have an outdoor project that I'd like to power with a solar panel during the day and with batteries at night. Will this work?
The Arduino is using approximately 30 mA. The batteries are 2600 mAh NiMH batteries. I think that the 150 mA max. output from the solar panel should be able to keep the batteries charged and power the Arduino.
It isn't clear to me exactly what happens when the solar panel is in low light. Even under low light conditions it shows 12V or more but only a couple of mA. What happens when it is in parallel with the battery which only puts out about 7.5V?
Without something to disconnect the battery, it was always be discharging, even when solar power is present. You need a supervisor circuit to determine when there is enough solar power available, and disconnect the battery.
The general rule of thumb is that continuously charging NiMH batteries at less than 10% of their mAh rating is okay, because they can safely dissipate any energy they don't need as heat. But that rule was formulated back in the days when battery capacities were lower, so I'd go with something closer to 5-7% today.
So the good news is that you don't need a charge controller to protect your batteries from overcharge damage.
But the bad news is that your battery capacity is marginal, at best. If you look at the curves on this page:
you'll see that a 6-cell pack will drop below the 7V needed by a 7805 regulator well before it's fully discharged.
Since you don't have a lot of energy to waste, consider going with an 8-cell battery pack, and a switching regulator, so less of the available energy will get thrown away as heat.
I can give you the details but I am no expert at this. I've only had the system installed outdoors for 3 days. I technically don't even know that it works yet since I estimate that the batteries should be able to run for about 5 days even without the solar panel.
The batteries are just standard rechargable NiMH AA cells. 6 cells in series gives about 8V fully charged. The batteries have a capacity of 2600 mAh. As someone pointed out, it is considered safe to charge NiMH at up to C/10 continuously. Since the solar panel has a maximum output of 150 mA and the project is drawing about 30 mA then there is about 120 mA available for charging the batteries in full sun which is well below the C/10 value. In addition the solar panel probably never charges for more than 10 hours a day anyway.
Because 6 NiMH batteries will only produce about 7.2V once they start to discharge and the 7805 voltage regulator requires a minumum of 7V I did replace it. I am using an LM2936 LDO voltage regulator which is pin compatible with the 7805 but requires no more than about 5.5V to operate. It has a maximum output of 50 mA but that is more than I need for this project.
The solar panel is just a weatherproof panel I found at the electronics store. It was designed to trickle charge car batteries. I just cut off the cigarette lighter plug and soldered it to my project board. Since my project requires 720 mAh per day (30 mA * 24 hours) to operate my estimate is that 6 hours a day of full sun will keep the batteries fully charged. I obviously won't get that every day but it only has to work during the summer growing season so I don't think I'll have any trouble getting that much sun on average.
Here is the panel I found if you are interested in the specific model.
A question for the experts out there. Do you really have to have to regulate the output voltage of the battery with the 7805? I thought that battery output voltage was reasonably steady (especially WRT the max output). I'm asking because if the battery was put inside the 7805, it would use the available battery power a lot more efficiently (if you use 5V batteries of course). Of course with this configuration, it would be trickier to charge it...
Any time you connect a solar panel (or generator) to a battery, the voltage output of the panel stays the same as the battery voltage (which will change as they charge and discharge). Any extra power you get out of the panel comes in the form of additional current (mA). So as long as the batteries are what you need, the solar panel will not over power the Arduino. The batteries will absorb any extra power during sunny times and provide needed power during darker times.
You do need to be aware of overcharging the batteries however. NiMh batteries are more sensitive than NiCd batteries, but no where near as bad as Lithium rechargeables. Here's a very good source of info about batteries: http://www.batteryuniversity.com/ As long as you stay below the trickle charge limits when the batteries are fully charged, you'll be OK.
You should rely on the voltage reg on the Arduino or replace it with a switcher type reg. The voltage of batteries is not stable enough over the drain cycle to be reliable.
