A common arrangement is 3 LEDs in series, then a resistor. One way to control the current is to put a resistor in series. Maybe 2.8 - 2.9 V for an LED like that at 1/10th of its full rating. At low currents, the voltage won't change much. At really low currents you can get odd effects. ![]() Anywhere down to 1/10th or 1/20th will be fine. The exact voltage across will be near 3.2 V, but may be a bit more or a bit less. So if you have a 1W LED that takes 312 mA at 3.2 V, you should supply it with a maximum of 312 mA. LEDs should be supplied with a current, not a voltage. However most of it works, and with lighting you can see if the overall effect is what you want. I agree that a lot of the stuff from Ebay will be poor quality. However for making one rig from off-the-shelf components, and having an easy way to charge the batteries, I would recommend accepting some slight limitations. So using a power bank, a 5 to 12 V converter and those LED strips means that your batteries need to be 1.5 times as big as with the best circuit. Alternatively you could have a converter to convert the battery voltage directly to the LED voltage, with suitable current control, and you might average 90% efficiency, but you would need to have batteries that are not in a power bank, and the loose LEDs. If you had a current limiter and ran 3.2 V LEDs from the battery the efficiency would be between 76% with a full battery and 94% with a nearly empty battery. The overall efficiency is just under 60%, so less than 60% of the battery power will make it to the LEDs and the rest will be lost in heat in electronic components. The LED strips will probably have 3 LEDs in series, maybe 3.2 V each, so 9.6 V and there will be a resistor to limit the current that is also a loss of power, so the efficiency is 9.6/12 = 80%. The converter that I suggested has an efficiency of maybe 85% as well. ![]() The power bank has a boost circuit that increases that to 5 V for any battery voltage but that boost circuit has an efficiency of maybe 85%. You could have a somewhat more efficient set up but it would need electronics to be designed and built, which would be a lot of work.Ī power bank has a Li-Ion battery that runs at between 3.4 V and 4.2 V. Using a power bank like that isn't efficient, but it's an easy thing to put together. Being larger makes it last longer, not produce a higher power. ![]() Even the big power bank you liked to is only rated at 2.4 A (corrected from where I put the wrong units). I suggest you use several small power banks. However the converter won't be 100 % efficient so it will be more than 1 A, maybe 1.2 A, so running two from a power bank pushing the limits. The 12 V strips take 5 W, so at the 5 V output of the USB, that is 1 A. I would suggest you only run one, or at most two, from a power bank. You could use something like this:- to run the light strips at 12 V.
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