This is kinda a follow up to a previous question I asked here: Parallel 18650 battery pack voltage not stepping-up with Boost converter. Why?

I have now added the correct boost converter to my 20 LED and I am able to achieve higher voltages, however when I measure the current from the output of the booster converter and the wired LEDs, I am not getting the expected current, thus not getting the full power out of it.

My setup is:

So, if I am correct, my LEDs (as it's laid out) should consume 12V at 21A? And my battery pack should supply 3.7V at 15A? Correct me if I am wrong.

I have no intentions of delivering the full 15A or 21A (the latter being impossible) but at 12V at 4A or 10V at 5A. However, my current readings are always giving me 0.7A on my multimeter!

Does anyone know what could be going wrong here?

20 LED setup

6P Battery pack

  • \$\begingroup\$ What are you using for a battery protection circuit? What is it's current limit? Your boost converter is not well documented (they even have mislabeled terminals in one of their images), but note that some boost converters output current at a given voltage is limited by input voltage. \$\endgroup\$
    – K H
    Jan 20, 2019 at 2:09
  • \$\begingroup\$ @KH At the moment I am just directly connecting the terminals, I do have battery protection (as seen in the photo) but I had to disconnect it from the circuit when I discovered i wasn't getting the correct readings. Sorry, I should've posted this link for the boost converter, as this is the one I actually bought from: ebay.co.uk/itm/… \$\endgroup\$
    – Omar Al
    Jan 20, 2019 at 2:16
  • \$\begingroup\$ You're using the wrong kind of power supply for that LED module. You need a constant-current LED driver, not a constant-voltage power supply. 1050mA is typical for commercial LED drivers. In all likelihood you'll need to give it at least 15-18V \$\endgroup\$
    – brhans
    Jan 20, 2019 at 2:17
  • \$\begingroup\$ @brhans Could a constant-current boost converter suffice? \$\endgroup\$
    – Omar Al
    Jan 20, 2019 at 2:21
  • 1
    \$\begingroup\$ High power LEDs (partly due to this, partly for other reasons(variations in current@given Vf) do not share current well in parallel (Although these shouldn't be too bad as they are series strings in themselves) and likely one LED will have more current across it than the rest, causing it to heat up and fail first, causing more energy to be available to the next worst LED, which in turn fails, ETC. The change in voltage required to greatly change the current through an LED is very small as well, so it's easy to make a small adjustment to V, causing a great increase in I and cook your array. \$\endgroup\$
    – K H
    Jan 20, 2019 at 3:00

1 Answer 1


The unit you bought simply does NOT work for the application you have, and never can.

Your math is wrong as well, so lets start there:

I'll ignore heatsinks, conversion efficiency, circuit configuration etc etc.

Your LEDs require 12V DC @ 21A ---> 252W at full power.

With an input voltage of 4.2V (fully charged cells) to supply 252W you would need to be able to supply 60A from your battery pack.

To achieve your goal you'd need a boost convertor that can supply 252W. The unit you linked to shows little real details, but you can see that it is based on the TI LM2587 IC.
Reference to the chip data shows that the 5A current capability turns out to be the maximum peak current that can be drawn on the INPUT side. This can be clearly seen from the block diagram, which shows the current sensing for the output switch:

enter image description here

Ignoring dozens of other elements in this design of a boost convertor, for you this indicates the absolute maximum input power could be 5A * 4.2V --> 21W (you can't actually ever get close to this for some of those other reasons, but I'm doing the shorthand version to show how far from reality you are).

So to summarize:

  1. The boost convertor will not work for you since it CANNOT provide the power you need, even at reduced illumination.

  2. The boost convertor is the wrong type of convertor to drive your LEDs. While the COB modules you seem to have CAN be voltage driven, you have to sense the module temperature accurately to do this. It is always better to use CC drive.

  3. The battery pack is not large enough to power your LEDs for more than a few minutes even if the convertor would work.

  4. The 0.7A (I assume here you are measuring the LED current) you see is about 8.5W, which is exactly what I'd anticipate in a convertor that is current limited in the way this unit is.

Back to the drawing board and work with some real values.

Perhaps as a starting point you could consider:

  1. Build a 4SxP or 5SxP battery pack to suite the runtime you need at the current you need.
  2. Use something like this Buck convertor with one unit for EACH INDIVIDUAL LED. This unit allows the absolute voltage or current to be set (CC/CV) and they work quite adequately for this type of application.

It may also help to read some basic information on the COB LED modules, perhaps start here or here. These modules are a simple series and parallel configuration of LEDs with NO current limiting other than their slope resistance. You need to carefully monitor the current, voltage and temperature of the module to get long term reliability.

  • \$\begingroup\$ Thank you so much for that answer, it was incredibly insightful and helpful! I am still trying to understand everything but I do get the gist of it! If it helps, this is the exact LED I have, the specs are on the page: tinyurl.com/y8vahzdx Gathering from your summary, because the load my LED setup requires mean that I would need a boost converter (or driver) that could at least supply that power, even though I don't intend to use all of it. Am I correct on this matter?? \$\endgroup\$
    – Omar Al
    Jan 21, 2019 at 2:01

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