Registered users don't see ads

How to determine a battery's REAL capacity

Discussion in 'Ask All Sailors' started by Gene Neill, May 19, 2017 at 5:50 AM. Add this thread to a FAQ

  1. Gene Neill

    Gene Neill

    Joined Sep 30, 2013
    1,678 posts, 291 likes
    Catalina 22, Albin Vega 27
    US central Florida
    Trying to determine what size bank I need ...

    If a battery is rated at, say, 100 Ah reserve capacity, does that mean 100 Ah in total? In other words, does it mean the battery actually has only 50 Ah you can actually USE, before reaching the critical 50% level? Or does it mean there are 100 Ah available for daily use?

    Sorry for the rookie question. I'm reading Don Casey's "Sailboat Electrics Simplified", but have not yet come to this answer. Plus I'm a slow learner. :)
     


  2. dlochner

    dlochner

    Joined Jan 11, 2014
    758 posts, 149 likes
    Sabre 362
    US Oswego, NY
    100Ah is total capacity. Useable is 50Ah. But really about 30 to 40 Ah because getting the last 10-15% of the capacity back into the battery takes a long time. Won't happen unless you motor for hours or plug into shore power with a good charger.
     


  3. jssailem

    jssailem

    Joined Oct 22, 2014
    2,042 posts, 346 likes
    CAL 35 Cruiser
    US Salem, Moored Port Everett WA


  4. Stu Jackson

    Stu Jackson

    Joined Feb 26, 2004
    19,181 posts, 292 likes
    Catalina 34
    US C34 San Francisco
  5. Gene Neill

    Gene Neill

    Joined Sep 30, 2013
    1,678 posts, 291 likes
    Catalina 22, Albin Vega 27
    US central Florida
    Thanks jssailem. Actually, I guess I'm going at it backwards by trying to determine how much battery bank I can HAVE. (It's a very small boat.) What I can have, is going to determine what my energy budget can be. Is this idea fatally flawed somehow?

    My Albin Vega was designed to hold two group 24's down in the keel. I have found that I can build more shelving in there and fit one more. G24's are rated at about 75Ah, so if I understand correctly, this would provide somewhere around 225 Ah, which means in reality I would only have about 112 Ah I can actually USE between charging (?). I'd like to have more, but this means moving the bank to a higher location, and probably further from the center line.
     


  6. jssailem

    jssailem

    Joined Oct 22, 2014
    2,042 posts, 346 likes
    CAL 35 Cruiser
    US Salem, Moored Port Everett WA
    Stu, One of these days I'm going make a favorits list of your great documents so that they are at my fingertips. I currently do a search. And they always come up.:) But if an enterprising computer moderator somehow messes with the search system I will be SOL... (sorry old landlubber)...
     


  7. jssailem

    jssailem

    Joined Oct 22, 2014
    2,042 posts, 346 likes
    CAL 35 Cruiser
    US Salem, Moored Port Everett WA
    Gene, No space is always a consideration. MaineSail will telll you that there is a "greater bang for the buck" using 6 volt golf cart batteries. It is tru in several ways. One they are smaller in physical size yet supply more amp hours. And two they give you more charging cycles (longer life). Problem is you need 2 to get 12 volts. If one dies all are dead. So now you need 4 to have a back up 12 volts. You do get 225 amp hours from two 6 volt batteries.
    I found an alternate rout. I had room for 2 Group27 sized batteries. Then I learned that the Group 31 batteries are the same size (fit the Goup 27 battery box). I checked out Dyno Batteries here on the west coast and found they make a group 31 thick plate battery for aerial lifts - 30TMX
    (http://www.dynobattery.com/products/sweeper-and-aerial-lifts/30tmx.php) 135 amp hours. Maybe not perfect but better than the group 24's I was using.
     


  8. Benny17441

    Benny17441

    Joined May 24, 2004
    4,883 posts, 201 likes
    CC 30
    US South Florida
    Do not think of 50% as the goal to discharge a battery to be able to use maximum a/h. Discharging to 40% will be more beneficial and if you need additional a/h just increase the size of your bank. I try to size the bank to my average usage for a discharge range of 25% to 40%. Too large a bank in relation to usage may not allow a bank to exercise properly. We need to reach a level that triggers bulk rate charging for about an hour to reduce sulfating.
     


  9. Don S/V ILLusion

    Don S/V ILLusion

    Joined Sep 25, 2008
    4,633 posts, 83 likes
    Alden 50
    US Sarasota, Florida
    Anybody know of a reference written by a battery manufacturer or other informed source which verifies the hypothesis that exceeding 50% capacity shortens battery life?

    And if so, how much useful life is sacrificed by going to 30%, 20%, etc...?

    Like others, I've likely exceeded the 'magic' 50% figure probably numerous times on battery banks which have far exceed d my expectation for longevity.
     


  10. Chief RA

    Chief RA

    Joined Nov 26, 2012
    2,066 posts, 30 likes
    Catalina 250
    US Bodega Bay CA
    Most people do not know the relationship to % and DC readings. Such as the perception that 50% is basically 12.2 volts! Chief
     


  11. capta

    capta

    Joined Jun 4, 2009
    1,705 posts, 196 likes
    Pearson 530
    XX Sailing in the Windward or Leeward Islands where ever we are anchored
    A visual aid
    Battery state of charge.JPG
     


  12. Stu Jackson

    Stu Jackson

    Joined Feb 26, 2004
    19,181 posts, 292 likes
    Catalina 34
    US C34 San Francisco
    Gee, Don, I would think Maine Sail is a pretty reputable source.

    http://www.pbase.com/mainecruising/deep_cycle_battery

    Calder also discusses this in detail in his Boatowners Manual.
     


    SFS likes this.
  13. Don S/V ILLusion

    Don S/V ILLusion

    Joined Sep 25, 2008
    4,633 posts, 83 likes
    Alden 50
    US Sarasota, Florida
    Stu

    I've read Calder's manual and looked at the link but neither answer the question as both omit meaningful info, that being actual useful lifetime vs. capacity draw. They speak only in terms of # of cycles.

    More specifically, if one uses more than 50% capacity on a regular basis, the actual time between recharges during which battery drawdown occurs is obviously extended meaning I am using the battery more per recharge interval.

    That additional time isn't considered in the diagrams in Calder's or the Penn data.

    The more time batteries are being used isn't necessarily proportional to life as the diagrams note only cycles, not life measured in practical terms such as hours.

    What am I missing?
     


    JamesG161 likes this.
  14. Chief RA

    Chief RA

    Joined Nov 26, 2012
    2,066 posts, 30 likes
    Catalina 250
    US Bodega Bay CA
    capta: even the charts differ: thats why I said "50% is basically 12.2v". Chief
     


  15. dlochner

    dlochner

    Joined Jan 11, 2014
    758 posts, 149 likes
    Sabre 362
    US Oswego, NY
    Useful hours is dependent upon the current draw on the batteries. A 1 amp draw for 10 hours is equal to a 10 amp draw for 1 hour, however, when charging the total 10Ah draw counts the same towards the recharging cycles.

    The number of recharging cycles is not the number of times the battery is recharged, it is the number times the battery is recharge from a specified state of discharge to a full charge.

    MaineSail no doubt has more specific information on recharging cycles, so let me give you a hypothetical example. If one recharge cycle is returning a battery from a SOC of 0% to 100%, recharging a battery from a 90% SOC to 100% SOC would count as 1/10 of a cycle. A recharge from 50% SOC to 90% SOC would count as 4/10s of a cycle. The cycles are additive. These two recharges would count as .5 cycles.
     


  16. capta

    capta

    Joined Jun 4, 2009
    1,705 posts, 196 likes
    Pearson 530
    XX Sailing in the Windward or Leeward Islands where ever we are anchored
    The colors are soooo pretty, which makes this one better. lol
     


  17. Don S/V ILLusion

    Don S/V ILLusion

    Joined Sep 25, 2008
    4,633 posts, 83 likes
    Alden 50
    US Sarasota, Florida
    Appreciate your response, however, I'm asking about useful life rather than recharge cycling and by useful life, I hoped to imply life at a typical draw, not one amp as your example cites.

    I'm not surprised at the lack of info forthcoming as I suspect there may be none.
     


  18. dlochner

    dlochner

    Joined Jan 11, 2014
    758 posts, 149 likes
    Sabre 362
    US Oswego, NY
    Don, every boater is going to have a different typical draw. If you know what your typical draw is and you know the number of cycles for a battery it becomes easy to calculate. There are just too many variables to get reliable data from a large group of boaters. Even looking at 1 boat, the draw will vary from day to day based on how the boat is used and how the batteries are recharged.

    If I understand your question correctly you are asking for information that would simply be either impossible to obtain or if presented would be not be very precise and thus unreliable.

    If you looking to get the best bang for the buck, you could calculate the cost per rated cycle.
     


  19. Chief RA

    Chief RA

    Joined Nov 26, 2012
    2,066 posts, 30 likes
    Catalina 250
    US Bodega Bay CA
    It seems both of you are right. Battery life depends on a miriad of things but I suspect who made it to be the most major consideration. I have no problem with that, and then, its our use and care of the batteries. Through the years my battery banks life expectancy has been about 5 years. I use Walmart wet cells ($79 ea.) and that cost per year is fine with me. I do draw down my bank to as low as 11.8v on occasion but not a high amperage load at that voltage. My 2a draw TV might be on with 11.8v load reading. Just my observations, Chief
     


  20. Don S/V ILLusion

    Don S/V ILLusion

    Joined Sep 25, 2008
    4,633 posts, 83 likes
    Alden 50
    US Sarasota, Florida
    The phrase "typical draw" was a bad choice of words I used to imply an amount greater than the one amp-hr someone used as an example.

    I don't want to belabor the point that there seems to be no correlation between 50% capacity draw-down cycles and useful battery life without a consideration of actual time under load which none of usual references consider. The generalization about never going below 50% capacity is likely an over-simplification given the lack of real information on how to get the most out of a battery bank.

    In my experience which spans the period in which we used and abused the cheapest batteries to trying to 'baby' (temperature compensated charging and careful recharge management routine) expensive Rolls batteries, they all lasted the typical 5-8 years, the lower end being here in Florida heat and 12 month season vs. New England's 5 month season.
     



Stainless documentation placard
Who said a documentation placard cant be beautiful? Brushed stainless with laser-fused numerals.
Sunbrella lifesling covers
Any color, fits over existing bag to match your existing canvas.
Teak companionway doors
Amazing artisan quality, custom made to fit your boat.
Gray ports and parts
The most popular port on boats built from the 70s into the 90s.