I have a small lithium emergency charger for my cell phone. I carry it in a small pouch with a bunch of assorted charging cables...
It does a decent job in an emergency for my phone. I can add 60-70% charge before it gives out.
Perfect to make sure I get picked up from the bus stop after work.
I tried running a raspberry pi but it only lasted a couple of hours. I needed something bigger.
Step 1: Parts
I had an old laptop battery with a bunch of 17500 cells in it. The pack was rated at 4400 maH. Internally it had the cells paralleled in threes. As much as I hate having cells in parallel I decided to put six of them in a box. They were factory matched and were really close in voltage now at 4.09 and 4.10 Volts. That would give me 8800 maH. That should run my pi model A all day long.
I stacked and staggered the cells and took a caliper to it. I used a harbor freight plastic caliper just to be safe. I could have made it a flat pack like the laptop but I wanted to be different.
A quick box modeled in OpenScad and I had the start of a power cell. I made a flat lid and then added a slightly smaller box for the electronics.
I had a bunch of 5V boost converters left from another project....
Make sure you have some kind of low voltage shutdown device. either as part of the boost converters or in the cells. Its best to use a good quality charger/protection board too. Look for one with 6 connections. 2 for External DC (charge, usually USB), 2 for Battery, 2 for DC out. If you decide to use the cells directly through a charge connector you have to be careful not to discharge them too deeply or Charge them too quickly. If you are worried fuse each individual cell too.
You could even add a low voltage alarm made for RC use. It will scream at you when its time to recharge.
Many laptop cells don't have protection circuitry built in. Make sure you get good ones... If you're not sure they have protection built in don't use them. if they weren't paralleled from the factory, don't use them. If you have any qualms about working with dangerous stuff close this window NOW!
I made the box pretty small so there's little cushioning if you drop it...
Failure to use common sense will result in FIRE... Ever see those hoverboard or Lipo with an axe videos?
Step 2: Assembly
I used 2 separate USB boost modules held in with some good super glue. I used a little hot glue to stick the cells together too.
I placed .001uF caps across each of the boards input terminals to reduce noise from the switchers. I added a heavy duty fuse holder and power switch for good measure. I also added a power led but soon realized each module had its own power led built in. You can see the top glowing from them. i could disable them to reduce power consumption but its not a priority, about 15-20ma at idle.....
Step 3: Testing
A quick test with a usb voltmeter shows 5.1V
With the round fuse holder on top It kind of looks like a tank that escaped from an Atari 2600 game.
Step 4: Charging It Up
For charging I added an RC style connector so I could use my RC iMax charger. Make sure you use a Lipo/LiPoly program at 4.2V. Setting the charger to a NiCd Peak program would be devastating.....
I almost used a USB charger module but this way I can tap the power of the cell directly. I really recommend the Adafruit charger protection board. It has USB and barrel plug inputs.
I didn't use one but it's really the way to go. Maybe I'll make a third level to hold one......