Introduction: Upgrade Desk LED Lamp
The need for upgrade......
This cheap lamp is quite useful in many cases. It has 24 LEDs and gives good light.
However, when you charge it with the included cable, it does not last more than 15-20 min, and it dims when you need it the most.
This needs to be fixed!
Step 1: Let's Have a Look Inside
Of course bolts are under the gushy pads.
Inside we see a familiar setup. There is a battery with no labels, which looks big enough, but does not hold charge. Seems Li-Ion. And there is a capacitive dropper charger with 4x diodes rectifier and a small smoothing capacitor. This was designed to charge from the mains ~220V.
It also has 2 resistors which limit the current for the 2 brightness modes. It uses both of them in series, or only the small one (2,2 Ohms) in high brightness mode.
Step 2: What Shall We Do About It
1. Li-Ion 18650 batteries x2 (unprotected cells)
2. TP4056 charging board - http://www.ebay.co.uk/itm/301905672102?_trksid=p20...
3. 2x 4Ohm resistors
4. Some wires, soldering iron, solder
Cost of all these in my country ~10$
We will replace the battery with TWO 18650 cells connected in parallel. It will give quite long runtime (4400mAh).
To charge these batteries we will use my favorite and familiar from other projects TP4056. Using this we will no longer charge the lamp from the mains, but from the safer USB ports, using standard cable.
To limit the current like in the original setup we need 2.2Ohm resistor. However, I could not find such low value in my drawer, and decided to use 2x 4,3Ohm resistors in parallel.
Lets do some rough calculations:
24LEDs * 20mA = 480mA current (~500mA) (they are all in parallel)
If we have ~4V from the charged Li-Ion stack this means - ~3V on the LED stack, and ~1V over the resistors.
1V / 0.5A = 2Ohms --> so 2Ohms resistance is OK, like the original.
Power anticipation on the resistors will be -> 1*0.5=0.5W -> which means that if you use only one resistor, it might get hot. That is why I recommend using 2x ~4Ohm in paralell OR 2x 1Ohm in series.
Step 3: Make the Battery Pack and Charger
It is very simple - connect the 2 + battery terminals together, and then the 2 - together.
Then solder the wires respectively to the B+ and B- of the charging board. More on that later.
They are almost the same size like the original battery, but I had to break some plastic pieces.
Step 4: Resistors and More Wiring
If your resistors are small like mine, you can twist their ends together and solder them directly to the board OUT+.
Cut the excess terminals if they are too long. The rest of the wiring is clear - I just soldered the blue wire from the LEDs to the OUT- of the board, and 1 wire from the resistors to the switch.
We are using B+ and B- for the battery and OUT+ and OUT- for the load !!! That makes the board work like a protection, if the batteries are too low, it will stop the load !!!
(I did not do this in my previous vacuum cleaner project because the load there is too high, and it could damage the board)
P.S. if you really want efficiency you can use https://www.pololu.com/product/2563 instead of the resistors. It will give you 1-2 hours more runtime, and the brightness will be constant, despite of the batteries getting flat.
Step 5: Test and Glue
When your wiring is complete - Test it ! It should work smoothly !
After that I used again a lot of hot glue under the charging board, and to fix it in place (where the old port was). And then some more on top of it. Don't worry you won't damage it, the hot silicone glue temperature is much lower than that of the solder.
Use some hot glue under the batteries to fix them as well. Finally I put the switch on its old place.
Put the cover back with the screws.
Step 6: All Done
Now I put some insulation tape on the charging hole because it is too big.
Finally we have a USB charging lamp (instead of 220V high voltage).
Charging time: ~4h (red goes to blue when complete)
Always works in bright mode.