I have a 15 year old UPS that quit working about a decade ago and since then I have been using it as a power strip of sorts. I started reading up on battery packs on instructables.com and I thought about creating a Li-Ion battery pack.

Now, most of the projects I saw on here and elsewhere emphasized the use of a spot-welder to limit damage to the batteries while connecting them. This was a tinkering-around project so I wasn't planning on buying a spot welder. Against all prevailing advise, I planned to solder my batteries.

Supplies

12V UPS : Old UPS with the 12V battery removed

18650 Li-Ion batteries - 24 nos : Power Source

3S 25A BMS : Battery Management System

Copper Wire : To connect the batteries together

Soldering Iron, Solder and Flux : To solder connections

Electrical Tape : To tape the battery pack together

Wire Cutter, Wire Stripper : To cut and strip the copper wire

Multimeter : To check the voltage across batteries

Step 1: Prep Your Batteries

I got my batteries from batteryhookup.com. The ones I got are very similar to these.

Battery Hookup Modem Batteries

The batteries come sealed inside a battery case (the black case in the center of the first picture) and you will need to pry open the cases to get to the battery. The batteries I got were cheap ($0.50 for three) and the website said so that these were hit or miss. Which was fine by me because I was doing this for the first time and I did not want to solder and destroy expensive brand new batteries. I bought twenty of these modem battery packs, which meant 60 batteries.

The second picture shows the battery pack as they look outside the case. They are already stuck together and spot welded in a 3S configuration. Which means the nominal voltage would be 12.6V. While I did not know this beforehand, it meant that all I needed to do to make my battery pack was to snip off the wires where they were soldered so I could solder my own.

I shucked the modem battery packs and was left with twenty sets of batteries. I did not want to remove the spot welding, so I used my multi-meter to measure the voltage across the batteries in a 3S configuration. Of the twenty sets, eight had no voltage, two had voltage between 3-5V and the remaining ten had voltages ranging from 7-11V.

Step 2: Putting Everything Together

I had previously measured the original Pb-Acid battery and the battery compartment in the UPS to determine how many 18650 cells I could fit in. The Pb battery was 4" x 2.5" x 6". The 18650 battery is 18mm wide and 65mm long, which meant I could fit 8 sets of 3S batteries, for a total of 24 batteries, in place of the original battery.

I picked the best eight sets of batteries from the lot and taped them together with electrical tape. I had bought a roll of Kapton tape but I am not sure I even needed it for such a low power pack. Then I set about soldering the ends of the nickel strips. Be sure to use plenty of flux while soldering so you don't spend too much time on the battery terminals. I have a 100W iron but I did not need to use it. My 25W iron did the job just fine.

I previously had no idea how a BMS worked but I found my way around the one I bought from eBay. I have attached a picture of the BMS and it's connections and also a picture of an eBay listing for a BMS similar to the one I used. There are six soldering points on the BMS. One for the battery negative terminal (B-), one for the 3.7V connection (B1), one for the 7.4V connection (B2), one for the battery positive terminal (B+), one for the connection to the UPS +ve terminal (P+) and finally one for the UPS -ve terminal (P-).

Step 3: Done!

And that's pretty much it. After you are done soldering, take the battery pack and insert it into the battery compartment of the UPS, connect the P+ and P- leads and you are good to go.

I checked the voltage after charging the battery pack for twelve hours and got a good 12.8V across the P+ and P- terminals. Charging voltage put out by the UPS is in the neighborhood of 13.5V.

I haven't run any other tests on the UPS other than connecting a 14W CFL lamp to it and it ran for over 75 minutes.

Project Cost:

1. BMS - $6 (There are cheaper ones if you want to get it shipped from China)

2. Battery - $22 (battery + shipping)