Introduction: Old Torch / Lantern Battery Upgrade

About: I've studied electronics back in the days in my high school. Today it is simply a hobby.

-=The Idea=-

This old Uniross torch uses a single Lead-acid 4V battery.

Why not replace it with a Li-Ion battery, it has a similar voltage.

It is smaller, lighter, and has a bigger capacity.

The torch has 3 modes :

- alternating switch between - 20 LEDs on the side / halogen bulb on the head

- On/Off switch for 9 LEDs around the bulb on the head

Step 1: Disassembling the Torch

Disassembling the torch reveals an old 4V 3Ah battery. I can assure you that it is not 3Ah nowadays, not even close, and it works better with the diodes, but more about it - later.

There is a board with old school electronics, which is not totally OK for the Li-Ion batteries.

Step 2: Necessary Parts

I won't go into much details about this, because I have many other projects where I use TP4056 micro USB battery charger and replace other devices batteries with 18650 cells. I want to show the huge application of this simple setup.

  • TP4056 micro USB battery charger
  • 2x 18650 Li-Ion cells
  • Solder & Soldering iron
  • Hot glue & gun

I won't put ebay/amazon links, because the listings change quite often.

Step 3: Making the New Battery Pack

You can see that 18650 is almost the same length, as the Lead-acid block.

Join the 2 cells together with sticky tape and/or glue, and solder them together in parallel. Solder separate wires for the pack on + and -. Do not connect them directly on the torch terminals, read next step.

It fits perfectly.

Now we have ~4Ah battery, ~same voltage, but significantly lighter. If you want put 3 or 4 cells, or whatever fits.

Step 4: Charging Circuit

Using TP4056.

Solder the new battery pack + and - wires to the bat+ and bat- joints on the board.

Then solder the lantern old wires (extend them if needed) to the out+ and out- joints of the board.

With this set-up we will charge the batteries with any micro USB cable and USB power source, and it is protected, because if the voltage of the batteries drop very low, under 3.2V, the board will cut the load and protect them from over-discharging.

Now remove the old 220V port and enlarge the hole if needed. Use hot glue to fix the TP4056 board there. See the pictures.

Step 5: Plug'n'light

Because the hot glue is transparent you can see the board led, red - when charging, blue - when ready.

The new batteries hold the charge and have a long shelf charge.


Actually the highest voltage the Li-Ion battery reaches is 4.2V. It seems that it is a bit low for the LEDs, because they seem a bit dimmer, but can last like forever (will add test results here). The highest voltage of the Lead-acid is probably ~5V.

However I observe the opposite effect on the halogen light bulb. With the old lead-acid battery, even fully charged, it gets dim quickly, even when the LEDs are OK. This is probably because the halogen bulb has a much higher power consumption, and the lead-acid battery was not able to deliver it and the voltage was dropping. On the other hand the open voltage was higher than the Li-Ion 4.2V and the low consumption LEDs were bright.

Now the halogen bulb gives stable bright light.

You can easily fix this problem, using a 5V booster, however it will slowly drain your battery in time, you will probably need a switch to disconnect the 2 boards, or you can try without.