My objective was to build a multi-slot Li-Ion charger from mostly scrap and unused stuff.
The only parts that I needed to purchase are the TP4056 circuits which are around $1.5 each. They support 1A charge rate, which is perfect for my needs.
What I already had (what you will need):
– an old Computer PSU with functional 5V rail (you don’t need the 12V rail to be functional, only the 5V one). This provides sufficient current for 12 cells (12A in total). If the PSU does not support enough current, the wires will overheat, the voltage will drop too much from 5.0V and the circuits will start to flash the red LEDs.
– wood (for the case construction – you could pick a better looking piece if you’re into aesthetics)
– wooden screws (shorter than 1cm for contacts and longer than 6cm for the box) and washers that fit
– springs (recycled from old 6V large flashlight batteries – I used washers here too for better contact, but the springs are good enough on their own to make proper contact)
– copper wire – thicker 1mm diameter for common and pairs of recycled-from-CAT5e-cable for the rest of the connections battery-side (should provide 1.5A per pair, which is enough for the purpose)
– plastic parts used for slot delimitation recycled from old notebook batteries
– plastic caps on the top contacts (looks better, keeps battery better in position, helps with contact fitting)
– cordless drill for faster work (low power enough, you don’t need the top tools)
– soldering tools and materials (standard for any electronic works)
– 4h if you know very well what you’re doing and you planned well initially (I wasted a few extra minutes to switch positions for 4 of the circuits because I had 4 with blue LEDs and 8 with green LEDs and put randomly it looked bad, so I set them up as 4G-4B-4G, but that was after the initial completion of the charger)
– 8h if you need to adapt and improvise
Time interval is considered everything from cutting the needed wood to powering up the charger.
Part of the initial planning, given the size of my initial wood piece to be used as main case body, I calculated that there will be space for 12 slots for 18650-type cells. Based on that, I ordered the 12 circuits.
I did some initial measurements and testing to see how things would fit generally. Distance between cells and distance between spring and upper contacts was generally fine, considering it was mostly eye-determined:
I though that the cells would need a little space between them in case there are some with thicker case and to prevent heat transfer between them. So the same plastic parts I used as a trick to properly align them:
Then, so the cells will fit nice into place, I arranged the plastic parts as needed and glued them there. This helps them stay aligned and not touch each other.
On the battery side, both + and – wires are connected separately for each cell. The UTP pair is sufficient for the required current:
After this step I used an ohmmeter to test if all contacts are good to go. Everything was fine, no short-circuits or any bad connections.
Then, I soldered the circuit boards:
Now all I needed were the 2 main power wires and connections between them and the boards. I also partially aligned the circuits so that that the indicator LEDs land on the middle of the cell.
Here are the main rails and the circuits connected to them:
I added a digital volt-meter to make it look more bomb-like (it’s nice, the more cells you add, the more the general voltage drops):
Finally, I connected everything to the hot-wired PSU and did the first the practical test – all worked as planned:
Primary objective achieved; I can now use the charger. What’s good about this charger is that the circuits have no over-discharge protection so I can use it to recover over-discharged cells 12 at a time.
As a bonus later on, I’ll add a capacitor or 2 since there’s some available space near the voltage indicator and I’ll make a nice wooden outer case. I’ll update this later on.