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  1. #31
    Join Date
    Nov 2014
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    68

    Default Re: 12 volt Electrical Battery Problems. Where is all the power going?

    Quote Originally Posted by pvb View Post
    I don't quite believe this! You try reading again what you wrote, which was "If your 2 fridges are constantly drawing 8A each then your batt monitor will read 16A and your 110Ah battery will be drained in 3.4 hours (55/16 = 3.4)." Are you capable of understanding what you wrote?
    1) Approximate real world capacity = Nominal capacity/2 = 55Ah
    2) Current drain from 2 8A fridges = 16A
    3) Hours until drained = [Approx real world capacity]/[Current drain] = 55Ah/16A = 3.4Hrs.

    Which statement don't you like? Whichever it is, see if you can disagree with it without making it personal...
    Last edited by roaringgirl; 23-08-19 at 08:31.

  2. #32
    Join Date
    Jun 2016
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    172

    Default Re: 12 volt Electrical Battery Problems. Where is all the power going?

    Would you be willing to operate without your fridges for a day?

    Why not simulate the same event by removing yourself from shore power, but switch off everything drawing power. If the batteries drop off at all, then you've confirmed they are dead or that the solar panels are delivering nothing. Then turn on everything else that you use except the fridges, and see if you get anything like your usual plummet.

    Then turn on each of the fridges separately, and see what happens. Then do both. You need to test the various scenarios to make sure you know what is doing the damage. It would be a shame to put in new batteries and kill them because you had a faulty fridge. or faulty something else not yet identified.
    Last edited by Polly's Kettle; 23-08-19 at 09:00.

  3. #33
    Join Date
    May 2001
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    UK East Coast
    Posts
    36,280

    Default Re: 12 volt Electrical Battery Problems. Where is all the power going?

    Quote Originally Posted by roaringgirl View Post
    1) Approximate real world capacity = Nominal capacity/2 = 55Ah
    2) Current drain from 2 8A fridges = 16A
    3) Hours until drained = [Approx real world capacity]/[Current drain] = 55Ah/16A = 3.4Hrs.

    Which statement don't you like? Whichever it is, see if you can disagree with it without making it personal...
    Statement 3 is completely incorrect. I explained why in post 23, and said that the battery would be drained in about 1.5 hours, not 3.4 hours.

    As you seem not to understand battery theory, it would be better if you didn't post incorrect advice.

  4. #34
    Join Date
    Jul 2005
    Posts
    3,313

    Default Re: 12 volt Electrical Battery Problems. Where is all the power going?

    Quote Originally Posted by roaringgirl View Post
    1) Approximate real world capacity = Nominal capacity/2 = 55Ah
    2) Current drain from 2 8A fridges = 16A
    3) Hours until drained = [Approx real world capacity]/[Current drain] = 55Ah/16A = 3.4Hrs.

    Which statement don't you like? Whichever it is, see if you can disagree with it without making it personal...
    Roaringgirl, your calculations above do not take into account Peukets equation. This is often incorrectly ignored, but does need to be taken into account, particularly when the discharge is high relative to battery size.

    Pvb is trying to point this out in his post.

    Unfortunately, we cannot do an exact calculation taking into account Peukets equation unless we know the specifics of the battery, which needs to be supplied by the battery manufacturer.

    A rough “back of the envelope” calculation suggests the duration for a battery rated at 110Ahr @C20 would be about 2 -2.5 hours. Pvb’s estimation of 1.5 hours is I think incorrect. This sounds way too low a capacity that is estimated from a discharge rate than puts the capacity above the C5 rate. So I think something has gone wrong with the maths in making this estimation.

    So therefore I think you are both wrong and the correct answer is somewhere in the middle of both of your estimations .
    Last edited by noelex; 23-08-19 at 10:18.

  5. #35
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    May 2001
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    UK East Coast
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    36,280

    Default Re: 12 volt Electrical Battery Problems. Where is all the power going?

    Quote Originally Posted by noelex View Post
    A rough “back of the envelope” calculation suggests the duration for a battery rated at 110Ahr @C20 would be about 2 -2.5 hours. Pvb’s estimation of 1.5 hours is I think incorrect. This sounds way too low a capacity that is estimated from a discharge rate than puts the capacity above the C5 rate. So I think something has gone wrong with the maths in making this estimation.

    So therefore I think you are both wrong and the correct answer is somewhere in the middle of both of your estimations .
    The 1.5 hours is the time to reach 50% discharged (the notional level which it's safe to go to on a regular basis). This is for an average flooded battery with a Peukert number of 1.3. If you think this is incorrect you should take it up with the people who have the online calculator I linked to in post 23, but you'll be wasting your time because all the online calculators will come up with the same sort of figure. A different sort of battery will give a different 50% discharge time; for example an AGM battery with a Peukert number of 1.1 would last 2.6 hours.

  6. #36
    Join Date
    Nov 2014
    Posts
    68

    Default Re: 12 volt Electrical Battery Problems. Where is all the power going?

    Quote Originally Posted by pvb View Post
    Statement 3 is completely incorrect. I explained why in post 23, and said that the battery would be drained in about 1.5 hours, not 3.4 hours.

    As you seem not to understand battery theory, it would be better if you didn't post incorrect advice.
    My approximation comes to the same conclusion as yours - the batteries are performing as expected by being emptied quite quickly. You seem a bit upset about something, I hope you manage to get better



    The next question then, is why are both fridges drawing 8A each all the time - are the thermostats broken?

  7. #37
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    May 2001
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    UK East Coast
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    Default Re: 12 volt Electrical Battery Problems. Where is all the power going?

    Quote Originally Posted by roaringgirl View Post
    My approximation comes to the same conclusion as yours - the batteries are performing as expected by being emptied quite quickly.
    If you think 3.4 hours is the same as 1.5 hours, you're taking "approximation" to a new level!

    Quote Originally Posted by roaringgirl View Post
    The next question then, is why are both fridges drawing 8A each all the time - are the thermostats broken?
    They're not drawing 8A each, the OP said "..together they take 8 amps approximately". It always helps to read the OP.
    Last edited by pvb; 23-08-19 at 11:02.

  8. #38
    Join Date
    Jul 2005
    Posts
    3,313

    Default Re: 12 volt Electrical Battery Problems. Where is all the power going?

    Quote Originally Posted by pvb View Post
    If you think this is incorrect you should take it up with the people who have the online calculator I linked to in post 23, but you'll be wasting your time because all the online calculators will come up with the same sort of figure.
    Sorry, I did not see your link. Unfortunately, the calculation in the link is incorrect (or at least it assumes the wrong starting parameters). It assumes a battery is rated with a 1 amp discharge. All marine batteries are rated at C20 (or very occasionally C100).

    For example: If you put into the table a 110 Ahr battery with a 5.5A discharge, the table gives an answer of 66AHrs when of course the correct answer is 110Ahrs because this is how the battery is rated.

  9. #39
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    May 2001
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    Default Re: 12 volt Electrical Battery Problems. Where is all the power going?

    Quote Originally Posted by noelex View Post
    Sorry, I did not see your link. Unfortunately, the calculation in the link is incorrect (or at least it assumes the wrong starting parameters). It assumes a battery is rated with a 1 amp discharge. All marine batteries are rated at C20 (or very occasionally C100).

    For example: If you put into the table a 110 Ahr battery with a 5.5A discharge, the table gives an answer of 66AHrs when of course the correct answer is 110Ahrs because this is how the battery is rated.
    OK I just tried another calculator, which came up with 2.5 hours to 50%, same as your guesstimate but still way less than the 3.4 hours believed by roaringgirl. The key point for people to understand is that the capacity of a battery reduces quite significantly when discharged at higher currents than the C20 rate; you can't just divide capacity by current and get an accurate run time.

  10. #40
    Join Date
    Aug 2002
    Location
    Gone Sailing -on the move exploring SE Asia
    Posts
    1,821

    Default Re: 12 volt Electrical Battery Problems. Where is all the power going?

    Forget about the % shown. What is the voltage measured at the batteries when you are seeing the 8amp draw. Also what else are you running? STERRO lights what is the actual amp draw and what voltage do you see at the batteries?

    Start there...

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