I want to power up for a few minutes a 50W LED (37V at 1.5A) from batteries. I have a bank of 10 NiMH rechargeable batteries (total of 12.9V), each in the 2.1-2.5Ah range. I use a DC-DC step up convertor with XL6009 to increase the voltage (made in China, 5euro, Amazon). enter image description here

I KNOW that I need constant current not constant voltage! This is just an experiment to see if the batteries can keep up for 15 minutes or so.

The convertor is sucking about 2A from the battery bank but it only outputs 0.22A 24V. Why? I was hoping for an output of at least 0.5A. The LED is bright but the radiator is not even getting warm.

The batteries can deliver OVER 4A in shortcut, for a short while.

Summary: the converter draws 10V*2A and outputs 24V*0.22A. Efficiency: 25%!


1 Answer 1


Ah. The usual "I did not consider conservation of energy" confusion:

The power flowing into your converter must be the same as coming out of it. Power, for DC current, is simply voltage · current.

If you want to step up voltage by a factor of \$\frac{37}{10\cdot 1.2}\approx3\$, then you'll inversely reduce the current supplied – so, to produce 0.5 A at 36V, your step up converter would have to draw 1.5 A. And that is probably far beyond its reach.

Also, your batteries only supplying 4A in shortcut is not a good sign. NiMh usually has much higher short circuit current. But in that case, the voltage drop at 1.5 A might already be large enough to inhibit operation.

EDIT: you picked a step-up converter that doesn't even have a maximum current or maximum transferred power specification. It's hence the most likely reason that you're trying to make your step-up converter source more current than it possibly can. Use a step-up converter that is either rated for the power you want to draw or the current at the voltage you want to draw.

  • \$\begingroup\$ The batteries are dropping from 12.9V to 10V at a current of 2A. \$\endgroup\$
    – IceCold
    Jan 23, 2017 at 22:53
  • \$\begingroup\$ Then I doubt your 4A short circuit measurement. \$\endgroup\$ Jan 23, 2017 at 22:54
  • \$\begingroup\$ The converter is already sucking 1.95A. \$\endgroup\$
    – IceCold
    Jan 23, 2017 at 22:55
  • \$\begingroup\$ As stated in my question, the current in shortcut was "over 4A". More exactly was 4A as I started to measure and was increasing as the time lapsed. After 2 seconds was already 5A. I didn't wanted to overload the batteries so I stopped the measurement after 2-3 seconds. I don't know exaclty how much I can safely keep the shortcut. But tomorrow I can try again for 10 seconds. \$\endgroup\$
    – IceCold
    Jan 23, 2017 at 22:59
  • 1
    \$\begingroup\$ Don't! There's folks on this forum that have burnt multiple multimeter fuses by shorting batteries. \$\endgroup\$ Jan 23, 2017 at 23:21

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