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Determing how long a battery will last on given project?

Discussion in 'Power Electronics' started by BinaryAssassin, Mar 31, 2016.

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  1. BinaryAssassin


    Mar 17, 2016
    I have two 12v 7AH 350mA lead-acid batteries that I wired in series to power 49 LEDs in parallel (2v FV @ 20mA, each with its own 1.2K ohm resistor) I'm wondering how I can determine the how long the batteries will last? Datasheet of the batteries
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    As described, that's awfully inefficient.

    However, the calculation goes as follows:

    The current per LED is about (24-2)/1200 A. That's 0.183A per LED or about 0.9A in total.

    The run-time = 7Ah/0.9A = about 7:45.

    However the rated Ah is typically for a 20 hour discharge, so the run time will likely be somewhat less.

    In addition, you probably don't want to discharge the battery too far, so about 4 hours would be good.

    As I said earlier, your design is awfully inefficient. If you were to create 7 strings of 7 LEDS with a 490Ω resistor in each string the total current draw will reduce to about 150mA, leading to an approximate run time of 47 hours, with perhaps 24 to 30 hours being safe.
    KJ6EAD likes this.
  3. KJ6EAD


    Aug 13, 2011
    The circuit as designed will consume approximately 22W, 20W wasted as heat in the resistors and only 2W providing light. The resistors would need to be rated for 1W to prevent overheating since each will be dissipating nearly half a Watt!

    Don't let the combined output voltage of the batteries drop below 21V to prevent damage.
    Last edited: Mar 31, 2016
  4. BinaryAssassin


    Mar 17, 2016
    I'm making a firework firing system so each LED is connected to a single cue on the firing system (each LED is connected to an independent cue on the cue module). The LED will only be on to check continuity and then on again once the system is armed. That's why i'm running 24v so as to have enough voltage to run through enough length of cable and fire the e-match.
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