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20 hour vs. 50 hour battery rating

Discussion in 'Boat Electronics' started by Glenn Ashmore, Dec 25, 2006.

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  1. Interesting question came up this evening perusing the Surrette L16H battery
    specs. The specs say they have a 20 hour rating of 400AH and a 50 hour
    rating of 476AH. Almost 20% more. That is to be expected as total amp
    hours increase with slower draw rates. The 20 hour rate is used to compare
    one battery against another but if in normal good practice a bank is only
    discharged from90% to 40% over 24 hours, wouldn't it be more appropriate to
    use the 50 hour rather than the 20 hour rating when estimating the actual
    usable amp hours?.

    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at:
    Shameless Commercial Division:
  2. Of course it would, but in practise you should not consider that low of a
    discharge. In practise, it should never get below 60%, if you want
    reasonable battery life. It is much better to install extra capacity, as it
    greatly extends battery life.
  3. Larry

    Larry Guest

    Not on our boat.....(c;


    No, you STILL can't run the electric heater off the inverter.....
  4. Matt Colie

    Matt Colie Guest

    At one time (either very long ago or recently depending on your age) I
    did a great deal of battery work.

    Batteries are now rated by:
    Reserve capacity (minutes at 25a to 10.5v(12vnom)
    20 hr (current * 20hrs to discharge to 10.5v)
    50 hr
    Cold Cranking Amperes (30sec current to 7.2v at 32F)
    0CA (0F Cranking current (same but 0F)

    None of these numbers can be used for anything except comparison. If
    you run most any battery to its limit by these numbers, it will be

    If you are going to use the rating to size the bank, go looking for the
    for a set of discharge tables (I'll keep looking for the set that I have
    - too). You might be able to get a set from a real battery supplier,
    but autoparts people will have no idea what you are talking about. This
    set of tables will tell you what capacity you can expect (expressed as
    percent of 20 or 50hr rate) that you might get at another rate and
    temperature to a give terminal voltage.

    My actual advice that seems to work best - put in all the battery that
    you can afford (weight-space then cost).

    How soon to launch?

    Fair wind and Smooth Sea
    Matt Colie
  5. OK, here is one for you. Surrette doesn't seem to have the Peukert exponent
    for the L16H on their site but the spec sheet does have the capacity for
    draw times from 1 hour to 100 hours. Using that and the formula on the site
    you referenced solved for n (Peukert) gives a Peukert exponent of 1.22 which
    when plugged into the original formula tracks almost exactly to the Surrette
    specs until the draw gets extremely large or extremely small so it appears
    that my math was correct.

    However, using the formula in the back of the Link 20 manual working from a
    reserve minutes at 25A of 870 I get 1.44. If you plug 1.44 for Peukert into
    the original formula you do get the stated 870 minutes @ 25amps but the
    calculated capacities do not track the Surrette spec at all. .

    It appears that the Peukert formulas must be taken with a grain of salt.

    Incidentally, I got a price on a set of four Surrette S-530 (L16H) batteries
    delivered to Macon from DC Battery Specialist in Miami for only $20 more
    than the local price for a set of Trojan L16Hs. Actually less with sales
    tax and way less than any internet source with shipping.

    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at:
    Shameless Commercial Division:
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