Background
I was given two broken battery powered LED lights. These lights were powered by three AAA batteries each. A quick test determined the problem to be corroded terminals; quick clean down and they worked as intended.
This is a short write up on how to use LiPo cells from electrical waste (disposable vapes) to convert a simple LED light from battery to rechargeable.
Investigation and Testing
The LiPo cells recovered from the vapes were 350mAh 3.7V cells.
As a typical typical alkaline AAA battery has a capacity of 860mAh, it was decided to combine two LiPo’s in parallel to double the capacity to 700mAh – close enough.
In addition to this, as the three alkaline batteries in series produce 4.5V, a test was carried out to determine if the lower voltage LiPo’s are greater than the forward voltage of the LEDs.
The test was carried out with a sketchy power supply to confirm that the forward voltage of the LEDs was less than 3.3V which is effectively the minimum voltage of the LiPo.
Design
For charging and general BMS, the TP4056 with USB-C module was selected. The two LiPo cells were then individually charged to the same voltage before being added to the same circuit.
Initial tests of this worked a little too well with the LEDs pulling 1.08A and getting rather warm!
A current limiting resistor was added to the circuit. Allowing for a 1V drop across the resistor with the LEDs pulling a nominal current of 400mA gives a value of 2.5Ohm with a 0.4W power. Therefore opted for a 2.7Ohm 0.75W SMT 2010 resistor.
This solution was simple, but short-sighted as it only works during discharge. After realising this mistake the resistor was relocated to the load (rather than the source).
In regards to the chassis and final construction: the existing battery holder was cut out and a small oval cut in the bottom rim for the charging port. The rest was down to guesswork and hot glue.
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Last updated: 19/09/2023