I built a project that I need to power off-grid for at least a few months at a time. It doesn't really do much and it sleeps most of the time (all but a handful of times per day), but does include a XBee-PRO 900HP. I have been able to get the power consumption to approx 25 mA by putting both the arduino and the XBee to sleep using the WDT. I think this is about about as good as I could expect using my Arduino Uno, but please correct me if I am wrong. So, I think the only reasonable way to accomplish my objective is solar. I have found this article, which has been re-posted many times on the internet, including here and on adafruit, but never seems to get very much insightful feedback.
So my concern is foremost whether this is safe. The arduino will be in a primitive setting where I cannot risk a fire. The setup will input up to 12-volt solar (two 6-volt panels in series), in parallel with 7.4-volts lithium ion. I think that is too much for charging Li-Ion safely, but I assume there is a regulator in the battery holder in addition to all the other circuitry it claims. I can't really find any useful specifications on the circuitry of the holder, but maybe my unsophisticated eye is missing it. Temperatures will range at extremes from 10F to 110F. If there is only a very small risk of fire, would it reasonably mitigate the risk to use the fire-retardent bag referenced from the battery holder?
An alternative I am considering would be to use an alternate arduino, such as the Arduino Pro Mini, that requires less Vin than my current Uno and power the project from two 6-volt lantern batteries in parallel. That would yield 52000 mAH and therefore power the arduino for at least a couple months. Is this a better (reader as safer) alternative?
If I were to err on the side of caution, I think I would switch over to the Arduino Pro Mini to take advantage of the lower voltage requirements and therefore the really liberal capacity of the lantern battery - 52000 mAh. The Arduino pro Mini also allows you to configure it to bypass the regulator. I would still need a regulator, but I think I could benefit from a LM7805 switching regulator instead of the linear regulator native to the Arduino Pro Mini. If I did all this, including your suggestion to remove the power LED, is it reasonable that I could get the power consumption to 25 mA - 5mA(LED) - 10 mA(Regulator) = 10 mA? And given the 52,000 mAh from the lantern battery, that I could get a life of 52000 mAh / 10 mA = 5200 hours or 216 days?
You need to look a little deeper into how Regulators work. A standard or switching regulator is not going to work. A 5V regulator as a dropout voltage at around 1.3V. This means that when you get within 1.3V of the target voltage, the regulator will stop working. Since 6V is not greater than 5V + 1.3V, it will not work.
This is one of the reasons that a 9V battery is not a good choice for the Arduino. Once you get to about 7V it no longer works making for some really poor utilization of the battery.
You will need to use a LDO regulator as a worse case option or a boost convertor as the best option.
The Solar Idea is OK, but just use rechargeable NIMH batteries instead of Lithium Ions.
NIMH batteries can withstand overcharging without damage provided that the charge current is C/20 or less.
Using 6 AA 2500 mah batteries will provide between 7.2 and 8.4 V which is just enough for the 5V regulator on the Arduino board.
If you want plenty of battery reserve for cloudy days , then use C size nimh batteries.
Sacman,
Thanks for the response. I think my mistake was that I should use the LD1117V33 switching voltage regulator, which is a 3.3v regulator, rather than the LM7805 that I mentioned in my last post. And, I wasn't specific that this would go with the 8MHz Arduino Pro Min, which only requires 3.35v input. Coupled with mauried's suggestion, I could use five D-size NiMH batteries at 6v and drop the voltage to 3.3v using this regulator to power the 8MHz Arduino pro Mini?
mauried,
I like the idea of NiMH instead of Li-Ion. If my suggestion to Sacman holds water, I would be using five D-size NiMH and this solar panel. That means the battery would produce 6v with 10000 mAh capacity while the solar panels would produce 6v and 330 mA? Would that work and is it sufficient to wire them up directly, in a parallel configuration, to trickle charge?