I am building a small (~200 mm high) rotating LED light assembly using approx 30 RGB LEDs. I would like to make it as portable as possible so would like to power it with something like a rechargeable 3.7 V Li-po battery, and use a boost converter to step up the voltage to 5 V.

As even a 2000 mAh Li-po battery will be drained within a few hours, I will build the circuit to allow the connection of a 5 V power supply. While powering the circuit, I would also like the power supply to recharge the battery - so everything stays working but the battery also recharges.

I have seen some threads which say to use two diodes on both the power supply and the battery supply voltage when you wish to have power options from both a battery and mains, but I couldn't find anything on recharging the battery when the DC power supply is connected. What other components do I require to achieve this? (e.g. battery charging PCB?)

As well as this, I am interested in anyone's thoughts on the best method of controlling power consumption of the LEDs with something like a 5 V dimmer switch, so the LEDs can be turned on with low brightness. Is a dimmer switch the best option here for controlling the power consumption and lengthening the battery life at times when this is required?


1 Answer 1


You could charge the battery with a wireless tickle charge to extend the operating time or sustain it. A voltage protection to cut off charging current of 3.8V is required when the LEDs are off with a simple float charger.

Wireless can use the stripped conductors as a loop inductor with a reverse diode. Transmitter could be a 5 to 10W amp at 100k to 250kHz with tuning required using metal film caps for resonant coupling. If the secondary LED wiring fails to give the desired results, duplicate the sender wiring and use impedance calculations to choose the right values. Proximity should be less than the smallest loop diameter. The ratio of diameters could be 1:1 or use the same length of wire for many loops in each. LED resistance will be around 15 Ohms each so inductive reactance ought to be less than 15/30 ohms

Use the impedance RLC nomograph in my recent answers.

This is just a paper making design concept.


The simple solution is drop 5Vdc to 3.7 to keep the batteries going using 2 power diodes (10A) or a power LDO like LM317 with a sink.


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