Time taken to charge batteries VS alt size.

Discussion in 'OnBoard Electronics & Controls' started by DennisRB, Apr 16, 2010.

  1. DennisRB
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    DennisRB Senior Member

    My friends yacht has a 4 battery bank, plus one motor battery. The engine is a MD22-A. I'm not sure on the alternator, but it doesn't look any bigger than a passenger cars. We ran the batteries flat and after 3 hours with the engine running the battery voltage was only 12.9 V under charge and they didn't last long at all. When you run such a large bank of batteries, is one small alternator sufficient to get a decent charge current into 5 batteries in parallel?

    The batteries went to 10V so they were really flat. When we started the engine with a spare battery the voltage only went to 11.9V. Would this mean the alternator would be taxed to the extreme? Would changing to a truck alternator make a large difference in charge time and raise the charge voltage closer to 14V? Also how long does a flat battery usually take to charge from almost flat if you can maintain 14V up to it?
     
  2. DennisRB
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    DennisRB Senior Member

    Also, for example a 60A alternator wont automatically put in 60A in, and a 300A truck alt wont put in 300A, it will depend on how flat the battery is. I cant see how a 300A would ever put 300A into 4 batteries as they would overheat and explode. But surely our alt is too weak if it cant raise voltage close to 14V when the batteries are flat. I heard that each battery can only handle about 25A max before it will gas anyway, but we must be way off that figure ATM.
     
  3. marshmat
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    marshmat Senior Member

    Hi Dennis,

    These little car-like alternators are meant to top off the cranking battery and to handle a few light loads (a radio, some lights, etc.) while the engine is running. They rarely run at more than a small fraction of their rated output. If you're only getting 11.9 V with the engine running (does this increase if you rev the engine a bit?) then your alternator is almost certainly undersized and overloaded, possibly also faulty.

    For charging a big bank, you need a big alternator. One good idea I've seen is to keep the stock alternator for the cranking circuit, and add a second, large heavy-duty alternator for the house bank. Your big bank needs to be charged by a big alternator whose regulator is set appropriately for that type of battery. A small, overworked car alternator simply can't put out enough power to make up the amp-hours you've drawn from that battery in a reasonable time frame (of course, if you leave the motor idling for eight hours, you'll probably come close).

    If your batteries are cranking types, not deep-cycle, you may have also damaged them by running them flat. Good deep-cycle batteries shouldn't be too badly harmed by this; still, don't ever let them get down to 10 volts again.

    If you can find a copy of Nigel Calder's "Boatowner's Mechanical and Electrical Manual", you'll find it quite useful for this problem and many others.
     
  4. CDK
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    CDK retired engineer

    A 60A alternator can supply 60A @ 12V to all consumers in a passenger car simultaneously. That would be a small car with a fairly low consumption: a larger car with A/C, seat heating etc. has a 90-120A alternator.
    The charging current of an alternator solely depends on the internal resistance of a battery, which depends on size, condition and temperature. Because alternators are limited to 14V they cannot restore the full capacity of a discharged battery in just a few hours.

    Normal charging rate is 0,1 times the battery rating for 12 hours.
     
  5. TollyWally
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    TollyWally Senior Member

    CDK,
    Is that .1 x battery amp hour rating for 12 hrs? I think that's what you meant but the comma is throwing me and I'm fuzzy enough on all of this to need to ask simple questions sometimes.
     
  6. marshmat
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    marshmat Senior Member

    Charge profiles aren't flat. CDK's figure of 0.1 x rating (ie, 1 amp of charging current for every 10 amp-hours of capacity) is roughly a "normal" charge rate. But this can vary considerably depending on battery voltage and state of charge.

    An automotive regulator, fixed at 14.0 volts or so, will have a hard time properly charging a deep cycle bank. It's designed for cranking batteries and doesn't work well with deeply discharged deep-cycle batteries (rising surface voltage on the plates will cause the regulator to cut out well before the inner parts of the plates are properly charged; this can happen at as low as 50% of full charge). A heavy duty alternator with a proper multi-step regulator will have a much easier time with typical deep-cycle banks.
     
  7. CDK
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    CDK retired engineer

    That is correct.
    It applies to constant current chargers and new, unformed batteries.
    With constant voltage chargers the initial current is higher, approaching 0 at the end of the cycle, the average value is also .1C.
     
  8. DennisRB
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    DennisRB Senior Member

    I can see how a constant current charger can be set to 0.1C.

    If you drain the batteries to a voltage of 11.5V then apply 14V there will be a 2.5V potential to push current through the battery. My understanding is that the amount of current will vary wildly depending on the internal resistance of the battery. The larger the battery the more current will flow. As it charges up the difference in the voltage of the battery and alternator will get smaller until they are both equal at which point only a small amount of current will flow. You will never get zero current as a 12V battery always wants to sit at below 14V at rest, but the more charge it has the lower the current will be.

    Wouldn't a alternator supply a fair bit more than 0.1C to a flat battery (assuming the alternator has a surplus of available current? I guess it would be difficult to say an average, as this would depend on how flat the batteries were? Are you pretty much saying with the 0.1C thing that it will take 10h to charge a dead flat battery? And that would be with an alternator which could supply 14V the whole time? It must take ours forever then if it can only manage 11.9V on a flat battery bank.

    What I want to know is what sort of currents might I expect when the batteries are dead flat if I can maintain 14V on them via an alternator. (the one we have seems too small if it can only do 11.9V on dead flat batteries). This will help me choose the correct alternator. Also can 2 alternators safely charge one battery bank? I realize this would be pointless if one alternator can already supply 14V on its own. I think having 2 alts would give good redundancy.

    Would a truck alternator be more suitable?
     
  9. pistnbroke
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    pistnbroke I try

    I did not read all of the above but my experience with canal boats tell me this ...you need 25 A of alternator output for every 100 AH in your bank. However big your alternator you will not get more than a 25A max charge current into each battery and this will be falling as the batteries come up. some systems ues one alternator for the starting and one for the house bank . others bring in the start alternator to help the house alternator once it has charged the starting battery. You need thick thick thick cables every where .
    Your voltage regulator should bring the batteries up to 14.4 to 14.7v when they are fully charged . If they have been down below 10.8v then you need a mains charger to get them back up as 14.4v only represents 80% of the untimate full charge...If you need diagrams let me know ...avoid sealed batteries.
    forget the .1c its only related to bench charging a battery for 14 hrs.
    charge times can be reduced to a minimum if you have 25A per 100ah of battery capacity..you will not charge faster.
    one alt for the start and one for the house is the simplest/ best soluton ie original alt for start and a new 100A/120A output one for the house . Gearing the alt to at least 10000 rpm is essential at engine max rpm.
    5 batteries in parallel is a bad idea....you should run the engine every day and if you leave it for more than 10-14 Days disconnect the batteries as they run each other down. all should be the same age ...one oldbattery will pull the others down.
     
  10. CDK
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    CDK retired engineer

    My old Mercruiser setup included Ammeters for both engines, scale 50-0-50 Amps. Immediately after starting, the current shown was approx 30 A to a 100 Ah battery. One engine needed a lot of cranking due to a minor leak in the fuel line, that side showed nearly 40 A.
    The alternators were Motorolas, 90 A.
    Within 2 minutes, the charging current dropped to 10-15 A and after say one hour the needles were between 5 and 0 A.

    I later installed new engines, same brand and type, obtained from a US boat builder (Renken). The alternators were marked (15V!) and kept charging their batteries until they were ruined, which took only a few weeks. After installing diode bridges and new batteries, the problem was solved.

    I hope this answers your question.
     
  11. capt littlelegs
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    capt littlelegs New Member

    If you can't get the charge voltage up the regulated voltage then it would seem that one or more of your paralled batteries has an internal short circuit. Try disconnecting them all and charge them individually to locate any faulty battery.

    The quickest you can charge lead acid cells is in about five hours from 50% discharge, going below that will shorten the life and that is with a full bulk charge for about 2 to 3 hours of that time after which the current drops off progressively as the battery voltage rises, thats not the charge voltage BTW.

    In calculating the alternator output you also need to add the normal or average load being use so that what's left is for charging. At just under 15 charging volts the battery will take it's own fill of available current according to the state of charge without undue overcharge and gassing over extended periods.

    Often engines are not run for long enough to provide sufficient charge so it's important to frequently fully charge from the shore supply to help maintain a heathy battery. Maybe these batteries are past their best now.
     
  12. pistnbroke
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    pistnbroke I try

    I always told my students that batteries were a bit like women......how old is it ,does it look in good conditon ? But the only true test is to charge it (with alcohol) and then dip it ( with an hydrometer) . Stand it overnight and see how it is in the morning ....
    Often you can be supprised that one you thought was old and worn out came up good when dipped....
     
  13. RoyHB
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    RoyHB Junior Member

    Running batteries down to 10 volts is a good way to kill them. The battery manufacturers often have charts that show depth of discharge vs. number of discharge/charge cycles before the battery is worn out. Once you discharge beyond about 50% the life expectancy of most batteries reduces noticeably.

    The best resource for learning about marine electrical systems, as someone else mentioned before, is Nigel Calders book. I've had mine for years and I still learn something new every time I pick it up.

    Another resource is Balmar, who make regulators and alternators. There is some good informative info on their site www.balmar.net. Another company with informative material is Ample power http://www.amplepower.com/

    Balmar is just one of several companies that make aftermarket voltage regulators. The manufacturers seem to recommend from 25 to 33 % of battery capacity as the target for alternator size.

    As mentioned earlier, the limitation in time taken to recharge is battery acceptance rate. This number changes as the battery ages and it depends initially on the battery type and then on how the batteries are treated. Even if you have a 500 amp alternator you won't be able to push amps into the batteries faster than they are willing to accept it. So long as the alternator is rated at or preferably above the charge acceptance of the battery bank you'll get the best possible charge rate by using a good 3 or 4 stage regulator. If you artificially tweak the charge voltage (alternator field current) up to accelerate the charge rate all you'll really be doing is destroying the batteries by overheating them and boiling off / displacing the liquids inside.

    Any batteries that have been repeatedly discharged to 10-11 volts, particularly if they haven't been restored to full charge pretty quickly, are very likely to be toast.
     
  14. F3M4
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    F3M4 Junior Member

    Alternators aren't build so that they put out a certain amount all the time, they are made to reach their output by a certain RPM, for instance most high output alternators claim to put out max amperage at 1200 RPM.

    If your voltage measurement at the alternator isn't about 13 volts, I would consider replacing the alternator with a bigger unit.
    If you need help being pointed in the direction of a good replacement, or add on unit, let me know.
     

  15. pistnbroke
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    pistnbroke I try

    oh boatdesign .net is a wounderfull place but you do get so many people who tell you what they think rather than what they know ....

    Be told from 30 years experience .......
    25A alternator output per 100 AH of battery bank
    Voltage must rise to 14.4 -14.7v when the charge current has dropped to a minimal level ..... below a 10A load on the alternator
    your alternator must run at 12000 rpm + ( not 1200) with your motor at wide open throttle or you wont get the rated output .......
     
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