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Old 08-04-19, 05:36 PM   #11
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I went ahead and ordered two of the ebay dc-dc converters listed above. These are also from China. I searched for a lot of alternatives, but I was unable to find one here in the US that had the power I wanted plus the undervoltage protection that this board has.

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Old 08-04-19, 06:20 PM   #12
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I am looking for one final piece before I continue with the mechanical part of this project. I would like some sort of state of charge meter. I'd like to keep it simple to eliminate phantom load on the power bank. I think something that activates with a push button would be good so it completely disconnects when not in use. It doesn't need to be an active system. However, the only other type of system I'm aware of is a voltmeter. This really isn't the greatest way of measuring state of charge, especially with LiFePo4 batteries. You guys got any suggestions for this?
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Old 08-04-19, 07:43 PM   #13
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Just build a dual comparator circuit with one output to trigger around 3.15V and another around 3.45V. That will give an easy indication of low and high charge. If you want more accuracy, you'll have to use a BMS with that feature which is hard to find for one cell LiFePO4.
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Old 08-05-19, 08:07 AM   #14
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Thats pretty much what I was thinking. Thanks for confirming Mike! You've been quite helpful on this project and I appreciate it.

I was able to find a nice LM3914 board which drives an LED bar graph with a push button right on it. It seems pretty perfect for the job. They have two pots, one to adjust the 'full' voltage, and one to adjust the 'empty' voltage. That is also ordered and now on its way from China.

https://www.ebay.com/itm/LM3914-3-7-...02d0%7Ciid%3A1

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Old 08-06-19, 01:19 PM   #15
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Here is a rough electrical diagram of the power bank. It's pretty simple and we have talked about lost of the parts thus far. The one we haven't yet is the switch. I'm not confident that the dc-dc converters will not suck power while not in use, so I added the switch to power them off when not in use. I imagine the charge controller is made to not drawn down the battery it is made to charge.

I think it would be nice to probably put a led on the dc-dc output as well just to make sure it is actually charging. I'll probably add that.

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Old 08-07-19, 04:38 PM   #16
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With the electrical stuff mostly figured out for now, I am going to switch over to making the enclosure for the power bank. Thankfully, my 3d printer is big enough to handle this task. Here is my initial design. It still needs some work, but I think it looks pretty nice.

Here is an exterior shot. The USB plugs are on the right with the LED meter on the left. Still going to have to stick a switch in there somewhere.





Here is an inside shot. The larger grey boxy thing is the charge controller, and the two other smaller ones are the dc-dc converters.

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Old 08-09-19, 06:49 AM   #17
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To keep things moving, and be able to print items before I get the electronics I had to change the design a little. I added a bunch of mounting holes to the main housing for flexibility. Once I get the electronics, I'll make up some small brackets to hold them to the housing. This will allow me to get the housing printed up. Its going to take approximately 17 hours, so I'd like to get it done sooner than later. This also allows future changes and upgrades or parts replacement with something other than the original parts without having to reprint the large housing.

Here is a picture.

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Old 08-09-19, 07:29 PM   #18
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I also ordered my solar panel for this project the other day. I went with a 20W 12V panel from Newpowa. I got it on Amazon for just over $30. It measures about 18.5" x 14", and puts out 1.16A at 17V.

I am still thinking about how to plug this into the power bank. I'm thinking a barrel connector would probably be nice and work well. Suggestions are welcome.

Newpowa 20w Watts 12v Poly Solar Panel



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Old 08-11-19, 09:00 AM   #19
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I got the second largest piece of this build 3d printed up. It took 10 hours to print, but its on the more dense side. For reference, it is around 7.5 x 3.5 x 1.75 inches.

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Last edited by Daox; 08-11-19 at 05:29 PM..
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Old 08-12-19, 09:12 AM   #20
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The largest print is now done. This one took 23.5 hours to complete. I'm pretty sure that is the longest single piece I've printed.







So, here we have the two halves of the battery enclosure box.


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