Introduction: $2 Battery Charger/Power Bank for 18650 Battery
Sometimes I buy something that comes in such a nice box it feels a shame just to throw it away. In this Instructable I will be re-using just such a box to make an 18650 battery charger with overcharge protection that will double as an emergency power bank for cell phones, tablets, or any other 5 volt device (just in case - it would be handy in a tight spot, so why not).
I came up with this idea for two reasons:
1.) I bought an expensive wall charger for 18650's that broke 2 months later. Unimpressed was I.
2.) I built a solar panel for my rechargable batteries making the wall charger (even if it still worked) obsolete to me.
** Please note: this circuit will safely charge from any standard USB wall charger. You don't need to use solar panels like me.
So are you as excited as I am? Of course you are! Here we go...
Items That Will Be Used
* a nice small box - I will be reusing a box from an old Protank 2. You can use any kind of box you can get these components to fit into. Ideas include: jewelry boxes, watch boxes, you could even use tupperware or get creative with some cardboard if you'd like.
* some scrap wire - We really don't need much wire for this project. You can use scrap wire from just about anything: old kids toys that don't work, broken headphones, etc. If you need to purchase wire I usually recommend 24 AWG stranded wire. It's thin enough for small electronics but hardy enough to make your projects good quality.
*Micro USB 18650 battery charging module board - Make sure the one you pick has overcharge protection: if it doesn't say "overcharge protection" specifically; look for something in the description that says: "charge cut-off voltage 4.2" or something similar to this, it's the same thing as overcharge protection. I've found these on eBay for $0.75 free shipping for a single board.
*USB DC/DC 5v step-up module board- Make sure the one you order has a type A female USB connection and does not run past 3 volts. Any more than 3 volts and you risk damaging your battery. I've found these on eBay for $1.13 free shipping. The one I purchased for this project was $1.10 at the time of writing this.
Total Project Cost: $2.16 **
** (plus or minus depending on the deals you can find)
Tools of the Trade You'll Need
* soldering iron
* soldering wire
* electrical tape
* whatever tool is your preferred method for wire stripping
* glue - I really recommend E6000 for this project rather than hot glue. E6000 takes about 10 minutes to start to set giving you enough time to work these components exactly where you want them before it begins to harden. (Please note it takes a full 24 hours to fully set.) In addition hot glue tends to un-harden under sunny or other warm type conditions whereas E6000 takes a much high temperature to do so making it great for anything that might be left in the sun on a warm day or as is the case with these components: gets warm when in use. I have had no issues with the E6000 un-hardening using these components once the glue is fully set.
* Something to make holes for the USB ports. This really depends on the durability of the box you're using and what tools you have. Ultimately I found that a razor blade worked perfectly for me.
* OPTIONAL: Helping Hands would be useful if you have them but certainly not necessary if you don't.
* OPTIONAL: Something to decorate the box with. I will be using a very small amount of acrylic paint as well as spray paint (just because it's quick & easy). And finally I will use one of my awesome Instructable stickers! No one wants to see a goofy picture of an outdated Protank 2!
Step 1: Prepare the Inside of Your Box
Mark on your box where you'd like to cut the holes for your USB ports. I very slowly used a razor blade to cut open the holes until each USB port fit through the hole snug.
OPTIONAL: I decided to use some red acrylic paint to color the exposed brown cardboard. My paint job isn't all that great, but I think it looks better than the uncolored cardboard.
Step 2: Connect the Wires & Prevent Short Circuits
While this is a very simple circuit I've included a schematic for those who'd like to see it in the above image.
Solder two wires to the negative wire of your battery holder. Solder one of those wires to the negative terminal of the charge board and the other to the negative terminal of the DC/DC step module.
Repeat this process for the positive wires.
Double check your wiring and then cover all exposed wire with electrical tape.
Don't forget to put a battery in this and double check to make sure everything is working correctly! It would not be fun to glue everything inside only to find out there's a poor connection somewhere!
Please Note: It's been pointed out to me that the pads labeled B on the charge board should be for connecting the battery. In hindsight I feel silly, that seems obvious now! However, I did run some experiments and from what I can tell the B and OUT connections both function exactly the same (same overcharge protection and same current). So if you've already wired to the OUT connections no harm should come to you or your batteries, but for the sake of not feeling silly when your techie friends are checking it out, I'd wire to the B connections unlike me!
Step 3: Glue Your Components in Place
Place a small amount of glue to the underside of your battery holder and glue it in place. If you're using E6000 wait at least 10 minutes for the glue to begin to set.
Now place a small amount of glue to the underside of one board and work it into place. Again, wait at least 10 minutes and then repeat the process with your remaining board.
Remember: E6000 takes a full 24 hours to fully set so I don't recommend putting anything inside of your USB ports until that amount of time has passed.
Step 4: Optional: Decorate Your Box
I spray painted the outside of this box with gloss black spray paint simply because it's quick and easy to work with. I put a final touch on my box once it was dry by using one of my Instructables stickers!
Step 5: Final Thoughts
You'll know when your battery is fully charged when the light on the charge board turns green while a red light means it's still charging.
For the DC/DC step-up board (what you'll use to charge your phone or any 5 volt device) this will have a blue light when charging and no light when the battery is depleted.
Additionally this would make a fantastic companion to have for camping or any other off-grid type of situation to charge devices that are "picky" about a varying current (what happens with solar panels, etc. and true of most modern cell phones and tablets). You'll always be able to keep your 5 volt device charged from a very simple solar (or any other type of renewable energy) setup.
Finally you might be wondering about some more techie details of this circuit and being true to my inner geek I did indeed run some tests. I found the following:
** This charge board charged the battery to 4.14 volts before the overcharge protection kicked in (green light).
** I used the DC/DC step-up board to charge my phone and once complete the battery was depleted to 3.10 volts.
Both of these voltage ratings are within a healthy range for an 18650 battery.
**And for my super geek kin: I used a "well loved" 2500 mAh battery. My phone (a Moto G with a Li-Ion 2070 mAh battery) was at 24% when I started, it charged to 61%, and took 58 minutes to charge.
UPDATE! - My box was big enough that I ultimately decided to add a second set of components giving me the ability to charge two batteries at once. I will be posting an Instructable for making an awesome solar charger for these. Look for that in the next 2 months or so (still waiting on parts to arrive). I can't wait to share it with you!