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Old 07-31-19, 08:46 AM   #1
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Default DIY solar power bank 5V 230Wh

I was talking with MetroMPG the other day about the solar powered alarm clock I made a few years ago. It has really worked great, and I am very happy with it. Whilst talking with him I got the itch to do another similar project with a small scale solar something-or-another.

I thought a 5V power bank for phone or other misc USB charging would be a nice, fun and easy project. I know you can buy those gimmick solar power banks with the tiny solar panels. But, I am thinking an actual functional setup that can charge my phone at 15W (3A) nearly every day of the year. I also want it to be able to charge my wife's phone daily as well. This means it'll have to have a not tiny solar panel, plus decent battery capacity.

I first started looking at our phone's battery capacity which is 3Ah @ 3.7V or 11.1 Wh. My aim was to get at least 8 charges out of this pack. So, it needs to have a capacity of at least around 90Wh. I started looking at battery solutions. Lithium 18650 or 26650 cells seemed like a good solution. However, finding reputable cells with realistic capacities at good prices was challenging. So, I looked at nimh. Since I had such good luck with the alarm clock, and knowing they don't even need a charge controller, this seemed like a good solution. I found a nice deal on D cells that would make a 135Wh pack for $25.

Then, I remembered years ago I had bought four A123 pouch cells. These were lifepo4 cells the manufacturer rejected that didn't achieve their rated 20Ah capacity. I never had a use for them, so they've just been sitting. When I bought them I capacity tested them and most are around 18Ah. So, I have four 3.2V 18Ah batteries good for right around 230Wh. Bingo.

So, I am going to look for a solar panel to charge these up. I think I'll run them all in parallel to keep charging easy (no balancing). However, I am going to need a charge controller as well as a dc-dc boost converter to take the 3.2V up to 5V for the USB output. I'm very open to suggestions on these parts.

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Last edited by Daox; 08-01-19 at 07:25 AM..
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