I need to quickly come up with a way of lighting a trade show booth from a 12v marine battery for 8 hours. My original idea to light the booth was to use a power inverter hooked up to an 80Ah battery and use 120v AC LED bulbs totaling ~45w.

The math on paper looked good but after doing a dry run this evening with a brand new battery, the whole setup lasted a little over 3 hours before the inverter reached the low voltage cutoff and shut down. Obviously I'm losing a lot in the conversion from DC to AC and back to DC or the specs for the bulbs / inverter are way off.

I urgently need to come up with a solution (by tomorrow morning) that will work based on the size battery I have and the amount of time I need it to run. Would I be better off just getting some 12v automotive LED bulbs and keeping everything 12v?

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    \$\begingroup\$ Have you done the math yet? \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 16 '16 at 5:45
  • \$\begingroup\$ Yes. I found some automotive LED bulbs that emit 320LM from 12v and .17A. By my calculations if I use 8 of these bulbs and I run it for 8 hours, I need a battery of 24Ah. \$\endgroup\$ – Mr.SCW Nov 16 '16 at 6:01
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    \$\begingroup\$ Is the battery a starting type or a deep cycle type? \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 16 '16 at 6:21
  • \$\begingroup\$ I have both types. \$\endgroup\$ – Mr.SCW Nov 16 '16 at 6:28
  • \$\begingroup\$ So-called 'strip lights' are continuous reel arrays of 12VDC rated LED lamps. You can cut the strip at marked points, to make a weak 100mm light band, or stronger 500mm light band, or... whatever you like. Just connect to 12V. \$\endgroup\$ – Whit3rd Nov 16 '16 at 9:18

Your problem is using LED bulbs combined with using an inverter. LED light bulbs' internal AC to DC converters are adequate but cheap, and do not tolerate anything unexpected very well. And what they expect is a 50Hz or 60Hz relatively clean sine wave at around their rated voltage.

Your inverter is almost certainly of the 'modified sine wave' variety. This type of inverter works by adjusting the output voltage in 4 large steps. This is, at best, vaguely reminiscent of a sine wave, but its really just a bunch of half-step square waves. For a lot of applications, or even most, this is fine.

Unfortunately, LED light bulbs do not react positively to this abuse. If they are given a modified sine wave like the output of your inverter is giving them, they will literally poop all manner of horrible harmonics directly into the eager mouth of your inverter. This will make the inverter hemorrhage watts in the form of waste heat, while making the bulbs also run hotter and there is even a risk that they will be damaged by these harmonics.

Your math is fine, just remember to add in an extra 200W of harmonic losses, and you get that magic 3-4 hour figure :).

The idea is sound except for this unfortunate 'gotchya'. Possible solutions:

  1. Use a pure sinewave inverter. These are more expensive than modified sine wave inverters, but really not by all that much these days. If time is the most important factor and dropping another $100 or so is acceptable, this is probably your best bet. You can buy them at most retail stores that carry inverters. They may be a bit overpriced at these locations, but I like to think of it as the 'I need it NOW!' tax.

  2. Use an isolation transformer between the output of your inverter and the LEDs. The transformer will 'sine-ify' the squarish output of the inverter and put your LED bulbs at ease. Not sure how easy it would be to find one in a hurry though.

  3. There are other solutions but they all require buying things off mouser and waiting days, so I won't bother mentioning them. I think the best route would be to just get a pure sinewave inverter, one of good quality. Drop that in with your existing setup and you should get 8 hours no problem. Yes, there are conversion losses, but it should be, and this is absolute worst case (it ought to be much better than this), 20-30W tops in addition to the 43W load. Even at 75W, 80Ah is plenty for an 8 hour run time.

  • \$\begingroup\$ Fair enough, but did you catch the third paragraph of the question as well as the comments? \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 16 '16 at 7:19
  • \$\begingroup\$ Yeah. Automotive LEDs are not diffused, have poor light quality/CRI/color temperature., and are not designed to be used as general purpose lighting. Considering that this is intended to light a trade show booth, I would think that actual appearance of the lighting would be very important. Using proper warm-white LED bulbs designed for room-filling light would look much better with less effort. \$\endgroup\$ – metacollin Nov 16 '16 at 7:24
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    \$\begingroup\$ What about LED bulbs made to run off 12v DC? I think they are sold for RV's and solar hookups. Would simply putting the bulbs into the lights and pumping DC into my lamps solve this issue? \$\endgroup\$ – Mr.SCW Nov 16 '16 at 7:26
  • \$\begingroup\$ If you can get those, then they would work great. Depending on how they are regulated inside, they may become slightly dimmer as the battery discharges, but otherwise there shouldn't be any issue. \$\endgroup\$ – metacollin Nov 16 '16 at 7:27

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