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Daox 07-31-19 09:46 AM

DIY solar power bank 5V 230Wh
3 Attachment(s)
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.

NiHaoMike 08-01-19 01:03 AM

From a quick search, it looks like 1S LiFePO4 BMS boards are hard to come by, but 4S LiFePO4 BMS boards are really common. If the 4 cells are closely matched, you can just go for a 4S pack, would also make the rest of the setup easier to design. Otherwise, you can use a standard 1S BMS for overdischarge protection but the trip point will be too high for overcharge protection.

For a boost converter, just find one that can supply well above what you need (I suggest 4A or more) with a low idle current. I'm not sure of any ready made modules to recommend since I generally build that kind of circuit from individual components. You can try a board for a portable USB battery, but the built in charger will not be usable without modification to limit the charging voltage.

For a charger, you'll need a MPPT step down if you're planning on using a standard 12V solar panel. (Or look into building your own with about 3.6-4V MPP voltage - that's surprisingly economical for your use case since the cost per watt of small panels is way high.) A shortcut that helps us is the fact that LiFePO4 has a pretty constant voltage over a good part of its cycle, so a 555 timer driving a buck stage with a fixed duty cycle combined with another circuit to switch it off if the battery voltage reaches 3.6V will do the trick.

Daox 08-01-19 08:57 AM

Thanks for the info Mike! I looked around a bit. I still like the idea of a 1s4p design for simplicity and forgoing any balancing issues. The batteries vary from 17.6ah to 18.6 ah.

Here is what I found on single cell charge controller chips:

I really like the idea of using a 12v panel and mppt charger because it will charge at least somewhat even during not ideal conditions. If I am going to use this year round I'll want to charge whenever possible. The LT3652 chip seems to do this and it looks like I can order a board from China for $20 or a bit less. However, finding info on the board is proving difficult.

Daox 08-01-19 01:56 PM

After a bit more looking, I am thinking more about using the CN3722 chip. It can handle a max of 5A charge current vs the LT3562's 2a. I was hoping to use a 20w panel, and this gets me close. However, I have never heard of the manufacturer Consonance. This makes me question that 5A rating. Another thing is that the controller doesn't start charging until the panel voltage gets to the set MPPT voltage. It seems to me that his would leave some power on the table, especially in my case where my battery voltage is going to be so much lower than my panel voltage.

The only other alternative is the BQ24650 which seems more complicated and all the boards are more expensive because they have more external components.

NiHaoMike 08-01-19 09:14 PM

From my experience, that sort of simplistic MPPT controller is not very good and a 555 will be way better.

At 20W and low voltage, it would probably make sense to build your own solar panel. Go for 10 or 11 cells in series (or 12 if you want the option of using conventional lithium cells later on), and then a LDO regulator with reverse current blocking set to about 3.6V would be all you need for a charge controller.

Go higher power, then just buying a premade panel would make more sense.

Daox 08-02-19 08:42 AM

Could you link me to a 555 control circuit? I'm not exactly sure how that would work. I know the 555 is a timer chip, but I have never used one.

I did order a CN3722 board for $9 off ebay. It will be here in 2-5 weeks, haha. I am a little concerned that I will burn it up working at it's max output. We will see. I will explore other options in the mean time.

I was able to find some inexpensive ($5) 6A dc-dc step up converters for the output. They even have undervoltage protection which is a great thing. I was thinking I would use two of them.

jjackstone 08-02-19 09:48 AM

Lots of ckts.

Daox 08-02-19 08:02 PM

6 Attachment(s)
I worked on the battery cells and got the pack assembled. As mentioned, these are A123 Amp20 pouch cells. They measure about 6.25" x 9" plus the connector tabs on top which bring them to about 10" tall. They are roughly a 1/4" thick.

To start, I got the connection tabs all cleaned up and oriented correctly. Then, I used shipping shrink wrap to hold them together.

As you can see, the tabs now need to be connected to create a 4p (4 cells in parallel) battery pack.

To fill the gaps, I chose to use 1/4" thick aluminum bar stock.

Two of the pieces got tapped for an M3 bolt so I can connect a wire to them easily. There are also two end plates made up with holes so it can all be clamped together with threaded rod.

And here is the pack fully assembled. Its quite the solid sturdy thing. For reference, the pack weighs just under 5 lbs.

NiHaoMike 08-02-19 08:33 PM


Originally Posted by Daox (Post 61206)
Could you link me to a 555 control circuit? I'm not exactly sure how that would work. I know the 555 is a timer chip, but I have never used one.

It's a 555 in an adjustable duty cycle configuration driving a buck converter.

Daox 08-02-19 08:42 PM

I'll have to read up on that. Like I said, I haven't used one before. I've never built a dc-dc converter either. Thats a bit over my head at this point.

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