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Battery Kindergarten (part 3 of 3) Best practices and sources

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Best practices


Discharge Floor
As a rule of thumb, limit discharge to 50% of capacity
don't pull more than 50Ah from a 100Ah battery
Don't allow no-load/rested voltage to drop below 12.0V
Going lower will shorten the life of the battery

Available Amp-hours
As a rule of thumb, assume battery is normally charged up to 90%
Means usable range is 40% of battery's capacity (from 90% down to 50%)
Useful for estimating capacity need: capacity of a bank should be 2-1/2 x anticipated demand between charges.
(e.g., if you anticipate using 40Ah between charges, get a 100Ah battery)

NOTE: Casey says that discharging to 20% / ~12.0V (rather than only down to 50% / 12.2V) can be a viable choice
Reduces the number of cycles in the battery's life-expectancy, but
Also reduces the number of times you need to recharge (so, you "need fewer cycles")
40 usable Ah (90% to 50%) --> 70 usable Ah (90% to 20%) = 75% more Ah per cycle
Sources:

-- "How Boat Things Work" (Charlie Wing)

-- "Boatowner's Illustrated Electrical Handbook" (Charlie Wing)

-- "Simplified Sailboat Electrical (Don Casey)
Note, this book is the same as the "electrical" chapter in "Casey's Complete Illustrated Sailboat Maintenance Manual"

-- "This Old Boat" (Don Casey)

-- "Boatowner's Mechanical and Electrical Manual" (Nigel Calder)

-- (I can't find my copy of "the 12-Volt Bible", will add anything interesting from there when I dig it up)

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Updated 02-19-2018 at 02:08 PM by bgary

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Comments

  1. MMLOGAN's Avatar
    That was easy to follow and reminded me of a few things I had forgotten. Put another log on the fire.
  2. Christian Williams's Avatar
    Very useful. Now I know where to turn when more confused than usual.

    I do admit much enjoying my very expensive AGM batteries (no maintenance). The previous owner destroyed $1200 worth of them by remaining emotionally attached to his 32-year-old charger.

    New batteries should call for a long hard look at the existing battery charger.