Battery Powered Contra

July 26th, 2019
battery, contra, power, sound
Update 2019-08-17: see the follow-up.

This Tuesday evening, Kingfisher is going to be playing for an outdoor contra dance in Copley Square. They've asked me to bring sound, which is fine except there's no electricity. I do have a battery and inverter, though, so possibly I already have what we need?

The main question is, does the battery have sufficient capacity to power the sound system for a contra dance. This has two parts: what's the battery's capacity, and how much power will the sound system draw? The battery is rated for 100Ah at 12V which is 1,200Wh, which matched my tests ~10m ago. Since then it's been sitting fully charged, so it probably hasn't lost much capacity. Discharging to 50% would give 600Wh, while 20% would give 960Wh.

Setting up my full sound system for a test is kind of annoying, so I set up some core pieces, and measured the current draw for each as I brought them up. I had the QSC K10 main speaker turned up very loud, loud enough that I needed to wear ear protection while playing the piano.

component meter reading marginal value
inverter 6.4W 6.4W
mixer 26.2W 19.8W
keyboard 32.0W 5.8W
K10 minimal input 47.1W 15W
K10 normal piano 50W 18W
K10 super loud piano with lots of bass 75W 43W

Thinking of everything that draws power in the full system, I get:

  • Mixer
  • 2x Mains
  • 2x Monitors
  • Keyboard
  • Laptop
  • Footboard
  • Phantom power supply
  • Recording laptop (optional)
  • Caller's wireless mic
Let's make some power draw estimates, informed by these measurements:
  • Mixer: 20W
  • 2x Mains: 30W resting, 40W normal, 90W high
  • 2x Monitors: 30W resting, 35W normal, 50W high
  • Keyboard: 5.8W
  • Laptop: 10W?
  • Footboard: 5W?
  • Phantom power supply: 5W?
  • Recording laptop (optional): 10W?
  • Caller's wireless mic: 10W?
This adds to ~125W resting and ~200W on high. While the "high" is an overestimate for how much sound we put out on average when playing, lets still figure 1.5hr of playing on high, and then 2hr of sitting on low, over the course of the night: 550Wh.

That's a bit under half the battery's capacity, so it sounds like we should be ok. The main way this would be likely to be off would be if the speakers end up using a lot more power than I was expecting. I know outdoor events tend to need a lot louder sound than I'm used to, but even if the mains draw 100W each on high, that still only brings the total to 700Wh.

Anyone see any mistakes? Now's the time to catch them, before we risk running out of power during a dance!

Update 2019-08-03: this didn't work. The inverter kept shutting off at around 200W, even though it's rated for 500W (1000W surge). I'm hoping it's that I was using battery cables that weren't up to the current draw, and not that it's an inverter problem, but I'll need to do more testing. We ended up running the dance on a generator.

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