Best way to battery power my Arduino circuit?

Discussion in 'Embedded Systems and Microcontrollers' started by EnjoyIce, Apr 5, 2016.

  1. EnjoyIce

    Thread Starter New Member

    Jan 13, 2014
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    0
    I have an Arduino Uno which is powering an Adafruit TLC5947 24-channel PWM LED driver (Which is in turn powering 12 bi-colour LEDs)

    I have to be able to power this at full power (so all LEDs at maximum brightness) for 12 hours and it is to be constructed as a self contained unit so things can't be too big. (Think a standard size house brick .. very rough estimate but couldn't think of anything else that's easily recognisable!)

    I am looking into ways to battery power this .. and the two options I have come across so far are either:
    1 - use a battery pack (although these tend to be quite large)
    2 - use a load of AA batteries in a series/parallel configuration to give me the required power.

    It would preferably need to be rechargeable so if I went down the route of the series/parallel configuration I would need to include a small charging circuit with it, but the battery pack can just be disconnected and plugged in.

    So, ideally I would like to go for a battery pack - simplicity is key so it just makes sense. However I am not having much luck finding a suitable pack .. I may just be looking in the wrong place, I have never looked for these before.

    I believe I need 8640mA for the 12 hours. Each LED has a maximum forward current of 30mA (and forward voltages of 3.0-3.8V / 1.7-2.3V, bi-colour so two different colours)

    So we have 30mA * 24 + the 25mA for the Arduino itself, all multiplied by 12, giving us 8640mA. This is obviously the extreme maximum and is unlikely to be needed but it *could* be used so I need to assume it will be!

    When looking into this previously it came up that you should look for a battery around 3 times larger than what you require (I assume this is because of voltage/current drops etc) so I need to look for something around 26AH?

    Is this all correct? Also if you know of any good battery packs etc that would be great, thanks!
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,395
    1,607
    Check your math. Just because the max current of a led is such and such doesn't mean you are driving it at that level. Pull every milliamp you can out of this thing.

    Drawing 2/3 of the capacity will hurt your battery life. Less drain is better.
     
  3. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,132
    267
    Measure the current drain while displaying your desired content.

    The current will depend on what you display- the only meaningful number is derived from a test.
    Using the worst-case numbers for an LED sign is a "safe" design practice for mains powered signs, but will yield horrendous overkill for battery sizing.
     
  4. dannyf

    Well-Known Member

    Sep 13, 2015
    1,832
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    Try those battery packs for phones. Typically 10000mah, 5v output.
     
  5. MrSoftware

    Active Member

    Oct 29, 2013
    504
    124
    Lithium Ion batteries are rechargeable and will give you the most energy per unit of space.

    Use PWM to pulse the LED's instead of running them at 100% duty cycle. If you pulse them fast enough they will look like they are on solid to the human eye, but your power use will be reduced due to the reduction in duty cycle.

    Build your circuit and measure its actual current draw. That will tell you what your real consumption rate is.
     
  6. dannyf

    Well-Known Member

    Sep 13, 2015
    1,832
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    He was just using the 100 PCT figure to budget the current consumption and clearly stated that it doesn't always run at that DC.

    The right thing to do.
     
  7. MrSoftware

    Active Member

    Oct 29, 2013
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    Right, I mean if he uses PWM to lower the LED consumption but maintain acceptable brightness, then that level becomes your new 100% on, reducing the peak demand as well as the battery capacity requirement. :)
     
  8. EnjoyIce

    Thread Starter New Member

    Jan 13, 2014
    25
    0
    The project I am working on is not actually an LED sign - its an LED array used for optogenetic stimulation of organic cells. So we need to have the option to drive the LEDs at 100% for potentially 12 hours (experimental length) as the cells will still react to the changes in intensity even if our eyes cannot distinguish the difference.

    If it was a sign or something that people would be looking at then I could definately use PWM to lower the duty cycle to save power (persistence of vision etc) but sadly I cannot do that for this and must plan for 12 hours at 100% duty cycle.

    I will hook up a test circuit and measure the current draw at 100% for a set time and then multiply it up to 12 hours to give me my required current. But once I have that value - is taking aroud 3 times that value the correct path to go down for the battery capacity?

    A phone battery pack would be perfect if it fits the requirements as its easy to recharge and simply plugs in - no need for creating a housing for AA's + a recharge circuit and all that.

    Thanks for the replies!
     
  9. MrSoftware

    Active Member

    Oct 29, 2013
    504
    124
    Cool project! You will want to be sure the current supply to the LEDs is regulated. If you use only passive components then they will get dimmer as the voltage drops in the batteries. I don't know the details, but a co worker found some smd current regulators for LEDs that look like diodes and just go in line with the LEDs. They were on Mouser or Digikey, and might be worth checking out.

    I don't know what industry standard is for battery sizing, but 3x your requirement should be reasonable as you will still have enough capacity to be useful as the battery ages, plus you won't kill it right away by deep discharging is frequently.
     
  10. EnjoyIce

    Thread Starter New Member

    Jan 13, 2014
    25
    0
    Thanks! I am using an LED driver chip ( https://www.adafruit.com/products/1429 ) which is a constant current driver so I should be ok as far as that goes, hopefully!

    I'll have a look around for some battery packs that give me roughly 3x my usage (haven't measured it yet!). Are there any good brands or anything you'd recommend if you know any?
     
  11. MrSoftware

    Active Member

    Oct 29, 2013
    504
    124
    The LiPo batteries I bought were all for flying toys, and even the name brands there seemed to be hit and miss. So unfortunately I'm not sure what the best brand/model would be.
     
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