Help with a little math for power vs battery.

Discussion in 'General Electronics Chat' started by maxpower097, Nov 18, 2010.

  1. maxpower097

    Thread Starter Well-Known Member

    Feb 20, 2009
    Sorry but I'm out of practice with my power and battery calculation formula's. What I'm wanting to know without going and research if anyone knows off the top of their head. How long would a large boat or truck battery last powering 10w a day for 16 hours a day?
  2. debjit625

    Well-Known Member

    Apr 17, 2010
    That depends on the AmpHour(Ah) ratting, i.e.. for one hour how much current (in Amp)the battery can supply.For example a 6V 4Ah battery can supply 24Watt for 1 hour now the same battery will supply 12Watt for 2 hours.

    Good Luck
  3. SgtWookie


    Jul 17, 2007
    Well, 10W would be roughly a 16.13 Ohm load, 787.4mA current.

    You would not want to use a truck battery, as they are not deep-cycle. If you discharge auto batteries below 80% of full without recharging them immediately, they will have a rather short life.

    Boat batteries are usually a trade-off between automotive-type batteries and true deep-cycle batteries. It's best to not discharge them below 70% of full. If you discharge them to 50%, you'll cut their expected service life by 2/3.

    There are lots of variables to go in the equation. I posted a spreadsheet in the Tips & Tricks thread, reply #36 if I remember offhand. You could plug in your AH numbers for a boat battery to see charge remaining vs voltage over various temps.

    If your boat battery is rated for 150AH, then discharging it to 70% would be roughly 45AH. That would be roughly 57 hours of operation for that 787.4mA load, or ~3.6 days of operation before the battery would need recharging.
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    20W is roughly 850 mA, round to 900mAH/Hr

    roughly 19.5 Amp Hrs per 16 hours, rounding up for losses and worse case battery terminal voltage drop.

    Starter batteries shouldn't have a huge problem with that load, but there are huge swings in battery ratings, hard to get an "Amp Hours" rating instead of the Cold Cranking Amps and Reserve Capacity.

    That said, I have left dome lights on overnight in my car, which is about 20AH with no problems, but that is with an Optima battery, designed for deep cycle/high capacity.
  5. maxpower097

    Thread Starter Well-Known Member

    Feb 20, 2009
    Ok got yah! I'm basically trying to run between 5w-10w 14-16 hours a day for 2-4 weeks. Seems like if I shop for the right pack, and right circuit I'll be good. I don't mind spending a lil extra for a pack with extra capacity. I could even go li-poly because safety is not an issue.
  6. timrobbins

    Active Member

    Aug 29, 2009

    It sounds like you're not quite sure of what your load is ?

    Are you talking default 12V operation?

    Is there no form of charging available over the 2-4 week time span?

    Your energy capacity requirement spans from 980Wh to 4,480Wh, and your discharge rate is very low relative to your stored capacity.

    Ciao, Tim
  7. maxpower097

    Thread Starter Well-Known Member

    Feb 20, 2009
    Correct. It will be a very minor load for 16 hours a day. Not sure exactly what the load will be but it will be powering LED's to the tune of about 5-10w at 2.5v - 3.5v