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Lowest Battery voltage

Discussion in 'Electronic Basics' started by John, Dec 7, 2003.

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  1. John

    John Guest

    Hi Guys.
    I have the following problem, could you please help provide a little insight
    into the calculation.

    A piece of refrigeration equipment has a 50hp engine with a 12v control
    system, an alternator for battery recharge and of course a 12v (100Ah)
    battery for starting.
    The control system has a remote temperature and standby/ready control. To
    use the remote standby function the main controller must be on.
    The problem is this.
    When in standby mode the current draw from the battery is about 500mA, if
    the system is left in this state for a few days then the battery does not
    have enough capacity left for the starter to crank the engine.

    What is the lowest voltage/capacity that the battery can be allowed to drop
    to but still be able to start the engine (12.2v seems to be the industry
    standard voltage).
    Finally how does the result equate to time e.g. do we have 20 hours at 500mA
    or 50 hours at 500 mA before the battery capacity is to low to crank.

    Battery standard lead-acid deep cycle type 100Ah.

    With a good battery during crank the voltage drops to 9.5v and rises to

    Same battery, initial engine crank current (cold) is 135A dropping to 36A.

    The engine is a direct injection diesel so starts in around 2 seconds.
  2. Ian Stirling

    Ian Stirling Guest

    (my newsserver won't let me post to alt.electronics)
    I have some doubts that it's a deep cycle, not a starting battery.
    Anyway, for optimum battery life, you don't want to take it much below
    70% charge.
    This is 30AH of usuable charge, or 60 hours.
    To top off the battery then requires the engine to run, charging at
    20A for 1.5 hours. (much faster isn't good for the battery)

    If you don't care much about usable battery life, you may be able to
    take out 60Ah, for around 120 hours between 3 hour charges.
    (IIRC, battery longevity will around double if you only use the smaller

    Can you investigate why the control unit draws 500ma?
    That's a lot of power, my laptop uses around that amount.
  3. default

    default Guest

    Too many variables to answer that question. The actual performance is
    concerned with total power supplied to the cranking motor not the

    You would have to do a plot of voltages under all environmental
    conditions, like temperature of the battery and engine being started,
    state of the battery charge, wear on bearings, brushes, mechanical
    factors etc..

    As batteries age they build up sulfate deposits on the plates which
    insulate the plates from the electrolyte. Your load of 1/2 amp is
    added to the normal self-discharge of the battery. "A few days"
    four days with a 1/2 amp load and self discharge would eat into your
    100 amp hour capacity.

    That type of service (discharging over a period of days with a low
    state of charge) is bound to increase the sulfate problem. Put a
    charger on it if at all possible - something that can replenish the
    half amp and goose the plates enough to prevent or reverse sulfation.

    Failing that solution - rig a means of turning on the engine
    periodically and running it for an hour without regard for the
    thermostat. That will help warm the battery and engine while charging
    the battery.
  4. Divide and conquer. Rig a deep cycle for the control and leave the regular
    battery for starting. Everytime a regula r car battery is fully discharged
    it loses a significant portion of its life and after 10 or 20 of these it's
    worhtlessfor starting but a deep cycle is made to discharge totally hundreds
    of times.
    hank wd5jfr
  5. JeB

    JeB Guest

    I'm wondering if a solar panel could provide the .5 a and fix the
  6. John

    John Guest

    Thanks guys.
    I thought it may be a difficult one, the fridge is on a trailer (semi) so no
    external charging system permitted. Solar had thought of that, and am
    looking at a trial.
    The 500mA is drawn by the controller and management system, the facility to
    be able to have the controller start/stop the fridge from a remote position
    was not part of the original manufactures brief (added at the request of the
    customer) so no provision was made to power down unnecessary systems.
    The battery draws directly from the alternator so no current limitations. To
    further complicate the issue, the engine is also capable of start/stop
    depending on temperature.
    The controller does have a sleep option (restart the unit when battery drops
    to 12.2v or engine temp below ox), this would require changes to software.

    So in an ideal world, with a new 100Ah charged battery how long do we have
    before the battery power is so low that it will not turn the starter and
    power the controller.

  7. default

    default Guest

    Minimum should be ~two days Maximum 5-6 days, given a battery in good
    shape with a 100 AH capacity, but that's only my guess. Your battery
    wouldn't last long in that type of service so the capacity would
    probably drop quickly with time.

    Solar cells and charge controller would be expensive. Not to mention
    that it only takes shade on one cell of an array to limit the entire
    panel's output. With a semi, tracking arrays are probably out so
    your panel(s) would have to be very large, to compensate for the
    amount of sunlight you'd miss. Have to watch where it is parked also.

    I think Henry has the best idea - controller battery and starter

    Small propane, or gas powered genset that runs continuously, or runs
    when necessary to maintain the battery?

    Be nice if there was some chemical means of providing the jolt to
    start the diesel. Compressed air starting would be a natural also.

    Anything you can do to use less power at the controller?
  8. Lane Lewis

    Lane Lewis Guest

    something mechanical that does not need power to initiate the control
    system might work. You could probably go a couple of weeks with this type of

  9. John

    John Guest

    Simplest fix's.
    The simplest fix would be to have the engine restart when the battery drops
    to 12.2v, run for a while then go back into standby.
    Alternate would maybe be a PIC that monitors voltage, if sees 12.2v for x
    period time (overcome spikes etc) start the engine for y period of time,
    providing that the controller is switched on etc.

    Does anyone know of a stand alone voltage/current data logger kit that could
    log the battery for a few weeks (UK).

  10. Luhan Monat

    Luhan Monat Guest

    Even with deep cycle batteries, it is best never to fully discharge
    them. The only difference in these batteries is an extra large gap
    between the bottom of the plates and the casing bottom. This allows for
    more charge cycles before the inevitable 'crud' (I dont know the
    technical term) piles up on the bottom enough to short out the plates.
  11. als

    als Guest

    If your controller has a "sleep" option, then it should have a "cycle"
    option, which would let you run it every day, if necessary. All our
    tower backup systems are set to automatically cycle every week - not
    to keep the battery charged, but to maintain quick start-up times.
    Lots of them. Google is your friend.:)
    Terminal voltage is not a good battery condition indicator (in your case).
    Depends too much on last charge state and idle time, although with
    a .5A constant load, the surface charge should go away fairly quickly.
    Sounds like the best suggestion so far is an isolated starter battery,
    with or without a solar panel. Is the remote wired or wireless?
    Either way, it should be relatively painless to run a trickle charge
    line to the battery.
    100 AH equates to 200 hours at .5A -- but, and this is a big but,
    the AH rating is for 8, 10, or 20 hour discharge (standard) to a
    specific terminal voltage (depends on battery type/vendor/method).
    As someone has previously pointed out, DOD has a major impact on
    battery life, and also determines the type of charge routine needed.
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