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Ok to answer a few of these questions.Im playing with a lot of different battery-powered reef-pi options during the holidays (other than hacking on model railroad with reef-pi for my son). And wanted to share some insights,
The goal here is to come up with something that can allow reef-pi users to run minimal system (on demand or automatically) during power outage. As usual with reef-pi, the focus here to get something with frugal, affordable components.
My current thought process is to go with mobile power banks , they are cheap and can run multi-hour, easy to source and have different capacity options. Pair them with 5v airstone and submersible pumps (2W or 3Watts at max). Experiement and document the findings then proceed with 12V beefier/expensive power banks if need be.
With that I first used my wife's 4 year old 4600mAh power bank with a vanilla pi zero w. That ran around 13 hours. But i was not sure how the math works or if the powerbank if giving what its rated for after 4 years of use. So i then got a duracell 3300 mAh powerbank (as shown in the photo) and this one gives roughly 16 hours with wifi and reef-pi running . For some reason, i was expecting the pi will experience some kinda of undervoltage situation, and in anticipation i added undervoltage detector in reef-pi :-) . Its a common problem we encounter in some builds. But surprisingly it didnt trip, i.e. the power went out without lowering voltage (or at least thats what im concluding/deciphering).
None the less, i'll proceed testing with temp sensor, float switch, airstone , submersible pumps etc. And keep you all posted. I think this is pretty cool, it solves emergency backup situation as well as portable controller think (i need that for my saltwater mixing use case). If nothing, the airstone and pump can be directly hooked up to the powerbank for maximum run time.
@Bigtrout @Michael Lane @theatrus and other with more electrical/electronics understanding, can you decipher anything from this data. Here are the key summary:
- 3300 mAh powebank with 5v output
- Pi zero w draws 100-140ma
- Runtime was16.3 hours
Simple calcs show that:
Pi drawing 140 milliamps for 16.3 hours
100x16.3=1630mAh
140×16.3= 2282mAh
You may have noticed that it didn't run for the full 3300mAh that the power bank was rated. The reasons why are: Manufacturer ratings are sometimes made in PERFECT conditions with a brand new battery and /or exaggerated...temp of the battery perfect for the battery chemistry and a small amount of load to use the whole battery over a longer time period. You were not in perfect conditions and probably drawing more load than the manufacturers test. Remember there are no official testing "standards" for this rating.
You noticed that you did not get an undervoltage situation. Im assuming that you are using a lithium power bank. As it gets used a lithium acts differently than the old nicad or nimh batteries. Those batteries would power strongly at first and as they were used up voltage would slowly lower until no longer usable. Lithium batteries tend to hold a good battery voltage longer and then just "die" off with no lowering of voltage.
As an example an older NIMH battery powered drill would run strongly at first and then slowly weaken. You could pull the trigger and the drill would run slower with less power on every use. Eventually the drill would turn very slowly with little power but still run.
The new lithium ion powered drills run just as strong on every use and then when the battery is done...the drill just plain dies until the battery is recharged.
