AAs vs rechargeable and tutorials
April 30, 2014Ok, I’m writing up the tutorial. Or I would be if I wasn’t writing this. In the comments for the photo essay, punkiepunkie mentioned putting this together with AAs instead of the rechargeable solution I’ve got going on.
When I started, the Electric Imp drew so much power, AAs seemed crazy to me.
Once I got the battery stuff mostly settled, though, I had trouble killing off even my littlest LiPo. With 150mAh, my littlest Lipo looks like it will last 20+ days.
According to Wikipedia, I can expect 1500 mAh from normal grocery store AAs. I’d need to put two in serial to serial to get 3V. Oh, looking at the power section of the Electric Imp’s breakout board description, I need 3.3V to 17V, so three or four AAs in serial would be best. Lots of power there, probably can go a year between new batteries, at least six months.
I like rechargeable batteries but it definitely adds a lot of cost to my build. If I take out the LiPo charger, the LiPo battery, and the fuel gauge, that makes the build cost $53 instead of $91. (It also makes the build easier.)
There are three different 4-AA holders, all of them $2:
I’m so attached to the fuel gauge, this is difficult. But Electric Imp has a built in voltage meter (hardware.voltage() returns the current voltage in floating point), I was only using the fuel gauge because LiPos are so tricky.
It is hard to get attached to a method of doing things and then try to change. But if I’m going to write a tutorial (I am, I am!), it would be better to focus on the important parts (the Electric Imp parts). I can easily put the LiPo stuff in another file and tell people how to upgrade.
Now I need to redraw my circuit, get a battery case, and take more build pictures. And fix my code. But this makes me happy, it is hard to shift directions but I wouldn’t have known I was making it unnecessarily complicated if punkiepunkie hadn’t mentioned a different path.
I don’t doubt there is goodness in open source (I use Wikipedia daily) but I don’t think it has every applied so directly to one of my projects. It is neat.
Hello Elecia,
I hope my decision to go with 3 x AA alkaline has not diverted you away from your LiPo solution. I like your LiPo idea (and can learn a lot from your fuelgauge code for this and future projects). For me and for low power projects, alkaline are readily available and don’t need a USB connection to a PC or a USB charger to get them going again. And they don’t need me to find the correct USB cable (an minor problem I had when I first received my imp and was impatient to try it out). On my “projects” I can also often swap in 3 x rechargeable AA NiMH instead of the alkaline’s. I also reckon that 3 x AA alkaline will still be enough for a 3.0V circuit when each of the cells are almost exhausted (I think that at about 1.0V terminal voltage they are considered exhausted). There are still the questions about losses in regulators but I’m happy to live with that for the convenience. It took me a while to accept the bulk of 3 x AAs but I know it is a good tradeoff (at least for my projects).
As an aside, I generally use a 3 battery cube type as it does not need a screwdriver to open it. Although I am currently using the flat type for this imp project!
Anyway, just some background to my AA reasoning. Don’t give up the LiPo idea 🙂
P.S. Unless I’m wrong, I notice you don’t have any current limiting resistors on your diffused LED. You might want to get advice on this as the imp pins are specified as being able to source/sink 4mA only. I asked the imp forum if it would damage the imp not to use a resistor but didn’t get a firm yes or no. I’ll be using resistors just in case (and I also appreciate its not a good idea to exceed the specified limits).
Looking forward to the next code iteration 🙂
The benefit of the flat type over the cube of course is the ability to switch the circuit off without removing a battery.