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Battery sizing and recharging.

Discussion in 'Power Electronics' started by MadMechanic, Mar 8, 2013.

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  1. MadMechanic

    MadMechanic

    48
    0
    Aug 28, 2012
    Hello all,

    I am needing advice on what battery I should choose. My system in question has a 35A load at 12V. I need a battery that can supply this. The battery can be fed continuously during load by a power source of 14V, 60A or 70A via alternator.

    I would like to stay as small and as light of a 12V battery as possible. I have looked at lawn-mower batteries and motorcycle batteries in wet and dry cell.

    The problem is, most of them do not list Amp Hours, but only Cold cranking amps. My question is, how do I size a battery to be put under a 40A max load without picking the wrong one and destroying it? This will be a constant continuous load at times.

    If these are listed by Cold cranking amps, how high is high enough to be safe for a 40A load? I will be feeding this battery with an alternator @14V much like a car, so it wont be discharging super quick. Any advice greatly appreciated.
     
  2. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    If you are constantly charging the battery in excess of the drain there is no significant impact on the battery, the charging circuit will be supplying the current and the battery in essence just becomes a big current smoothing device or backup under excess loads...

    What are you size limits? If size is not an issue you can use a 'marine deep cell' battery instead of a car battery as they are designed for longer drains vs initial cranking amps...

    In your case as long as the charging circuit is working properly even a small motorcycle battery will suffice, or even no battery at all...
     
  3. MadMechanic

    MadMechanic

    48
    0
    Aug 28, 2012
    Thanks for the info Coca! I was thinking this might be the case. My application is actually to drive a small boat using a trolling motor which will be powered by an alternator generation unit.

    As for the charging circuit, I am doing some research. For something this simple, would I need anything between the 14V alternator, the battery and the motor other than fuses? I was reading about IC regulators which increase and decrease the strength of the rotor's magnetic field.

    For this design, should I incorporate a regulator like this? What types of components are normally in the circuit of alternator - battery - load, given there is no starter? Thanks again,
     
  4. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    Get a 'one wire' alternator they have all the regulation built in and you simply wire it to the battery positive and ground it common to the battery, a fuse won't hurt but it's not generally necessary...

    This is the 'dumb' way to hook this up, if you want 'feedback/control' you can look into three wire alternators, FYI you can jump a three wire to function similar to a one wire if you want...

    Hit up some 'hot rod' forums for the difference between one wire and three wire to get more details... Three wire has advantages but is not a necessity...
     
  5. MadMechanic

    MadMechanic

    48
    0
    Aug 28, 2012
    I think the one I was looking at is a 1-wire, so it should be the simple way.

    When looking at batteries...If I were to measure the resistance of the battery in question, and apply it to ohms law, Could I assume the amperage value I get from the calculation at 12V would be the maximum amperage the battery should ever see across its terminals before failing?

    Could I treat a battery this way as a resistor in parallel with the load when im looking at sizes?
     
  6. john monks

    john monks

    693
    1
    Mar 9, 2012
    Seeing as how you do not want to start an engine with the battery why not just pick up any old car battery from a gas station? Even if the battery is slightly sulfated it should still work. A wet cell seems most practical.
    From my experience gel-cells are a disaster. If left discharged too long they're toast.
     
  7. MadMechanic

    MadMechanic

    48
    0
    Aug 28, 2012
    If I were to measure the resistance of a battery, Could I treat it as a resistor in parallel with my load?

    Could I assume that if it reads less than the load resistance, It will be of sufficient size to handle the amperage drawn across the circuit?
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    No, the resistance of a battery is effectively placed in series with the load.

    What it will tell you is the voltage drop at the current drawn.

    So, if the ESR is 0.05 ohms, the battery voltage will fall by 0.5V if you draw 10A.

    DO NOT try to connect a meter across the battery in a resistance mode -- it won't work. You determine the ESR by placing a load across the battery and measuring the change in terminal voltage.

    You would probably want the ESR of the battery to be under 10% of that of the load.
     
  9. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    I think at that end of the day you are really over complicating this, if as you said the charging circuit is supplying in excess of the drain the battery is not even technically needed... At this point the battery is simply acting to smooth out the current, and possibly provide a temporary surge backup... As John said just grab a used car battery and run with it (any one will work) that is unless you are a point where you are going to want the battery to act as a backup if the charging circuit fails... In that case look at marine deep cells, the manufactures will provide drain rates and run times for those batteries...

    A 35A drain in nothing out of the ordinary for a car battery, many car horns draw upwards of 20A to 25A on their own, factor in headlights, radio, and blower motor and you can see where drain is easily in excess of 35A while people sit with the car off in a driveway honking their horn for someone to come out... This is why almost all new cars have alternators well in excess of 35A nowadays...
     
    Last edited: Mar 10, 2013
  10. MadMechanic

    MadMechanic

    48
    0
    Aug 28, 2012
    Makes sense. Thanks again guys I have gathered most of the parts I will need for this, now I just have to combine and test. If I have enough output left over I might add a 12V water pump to circulate water into the exhaust manifold of the engine for water cooling. I will post up a video once I achieve success :D
     
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