Magic Staff – Batteries, load, and runtime

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Another few hours of work this weekend saw the Staff cut loose from it’s moorings for the first time.

Staff

Staff running on batteries only for the first time

One of the biggest challenges on this project is power.  The LED strips are 55 tricolor (very bright!) LEDs driven by WS2811 controllers.  The strips are powered at 5v.  Doing some quick load testing on the initial strip (and some online research) showed that each LED has a maximum draw of 60mA, so a 55 LED strip can draw a max of  3.3A @ 5v, and 6 strips can draw something like 20A @ 5v if the entire thing is at full brightness.  That’s a heck of a lot of current.   Compared to that, the Arduino wouldn’t be drawing dink.

I had to balance battery capacity with weight (this thing is meant to be carried in one hand after all).  I considered using D cells, but they’re just too heavy.  I ended up with 8 NiMH C batteries ( http://www.amazon.com/gp/product/B001AX1UQO/ ) from Tenergy.  These are ranked at 5000 mAh @ 1.5v.  I ganged them in groups of 4, connected in parallel.  This gave me 5Ah per group, or 10Ah total.  With a draw of 20A, at full power, it works out to only half an hour of uptime.  Now, there’s a lot of loss in this as well, so realistically, at full power (255,255,255 values on all the LEDs) I expect to get only about 15-20 minutes of use.

In general use, I don’t plan on using this in BLAZING WHITE MODE for more than a few seconds at any given time, but ongoing running will run the batteries down.  The other night I ran the staff using a ‘randomized’ pattern for a good 15 minutes on those batteries without any ill effects, and saw no problems (Check out the video here:  http://youtu.be/LNbIwpk5vdA )

 

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  1. Adam says:

    I’m working on building something with similar power requirements and had planned on using something like a 12V RC car battery and running it through a DC/DC buck converter… but hand’t thought of rechargeable C batteries. Are you just running the strips straight off the 6V from the groups of 4 batteries? Are you regulating it for the Uno?

    • Dave Shevett says:

      Hi Adam – I’m not an EE at heart, so there’s only so much I can do here. Right now I’ve ganged the batteries in 2 groups of 4, hooked in parallel. The power bus there goes through a switch and I’ve tied in the power leads to the strips, and fed a single power line off to the +5V on the Arduino. No regulation other than what the R3 provides on it’s own.

      This has worked fine except when I’m doing really dumb things like trying to drive the entire set of 6 strips with the battery array disconnected. It tries to draw through the Arduino – power levels drop, and the duino reboots.

      You can also see part of the problem in the video. As noted in one of the posts the batteries can get disconnected due to the spring compression – if I try to light the entire strip, the draw gets too much for one set of batteries, voltage gets too low, REBOOT.

      One option would be to drive the arduino from it’s own +5V supply (maybe a set of small penlights or something), but that’s more grandiose than I think I’ll do here.

      (Note that since I’m using NiMH batteries, total voltage from each bank is 5.5v. No need to scale back from 6v).

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