The life of batteries is directly proportional to the number of discharge cycles as well as the "DOD" or depth of discharge.
Cycling your batteries to a 50% DOD is the generally industry accepted low you want to regularly discharge to. If you can keep your state of charge to 60% or 70% you're doing even better and your batteries will last longer and give a longer cycle life. Less DOD = longer life!
One other thing that is often overlooked, is that when away from shore power, and charging off an alternator, you will rarely get back to any more than 80-85% of capacity so your real usable Ah's from a 200 amp bank quickly become 80-85 amp hours of usable battery life before you hit 50% DOD..
Weekend Warrior: (tied to dock charger each night and charging back to 100%)
I will use two 4D batteries for this example but the math works on all of them.
The average group 4D has about 160Ah's. With two 4D's @ 160 Ah you will get a max of 160 Ah of usable capacity for two batteries in parallel before hitting the 50% threshold.
Two Group 4D's:
160Ah + 160 Ah = 320 TOTAL Ah's (Based on Exide Nautilus 4D)
160 Ah's use @ a 50% DOD = 160 Ah's left in bank or a 50% bank DOD
160 Ah - This is your usable capacity if you can guarantee a 100% charge every day.
Mooring Sailors or Cruisers: (Assuming no solar or wind generation)
If recharging while away from the dock, with an alternator, the best you can usually get back to is about 80% SOC. This is due to battery acceptance.
SOC = state of charge
80% SOC of a 320 Ah bank = 256 Ah's of total bank capacity
256 Ah's (80%) - 160 Ah's (to a 50% SOC) = 96 usable Ah's while out cruising!
Looked at another way;
320 Ah @ 80% SOC = 256 Ah - 96 usable Ah's = 160 Ah's or a 50% bank DOD
96 Ah's - This is your usable Ah capacity while away from the dock and only using the alternator to re-charge.
As an example an Adler Barbour fridge can draw 5+/- amps when running. If you figure it runs for 50% of the 24 hour day, you've burned about 60 Ah's with just the fridge or 62.5% of your total usable bank capacity while away from the dock using the 80% rule.
Ideally you should do an energy budget to know what your other draws total. With two 4D's and a 12V fridge you can do just one night without charging. 320 Ah's may seem like a lot but it vanishes quickly when you do the math. At best you should only ever use 50% of any house bank or battery life gets short. This of course is only if you want to follow the 50% - 80% rule and get long life out of your bank.
A battery monitor is a good investment and will tell you how efficient your fridge really is and how many Ah's you actually consume daily. Some fridges cycle as low as 25% and some as high as 80% of the time. Unless you want to sit in the bilge with a stop watch for 24 hours and time its cycling a battery monitor can make this much easier on you. The cycling range of variable load or cycling devices can be a rather big difference and could be a costly mistake if yours is a high cycler vs. a low cycler.
A Victron BMV-600 Battery Monitor (LINK) is very reasonable at $158.10..
So in summary, two 4D house bank of 320 Ah's will give you:
Weekend Warrior:
160 Ah's of usable capacity when sailing off the dock and charged every day.
Moored Boats or Cruising:
96 Ah's of usable capacity when charging off an alternator.
Cycling your batteries to a 50% DOD is the generally industry accepted low you want to regularly discharge to. If you can keep your state of charge to 60% or 70% you're doing even better and your batteries will last longer and give a longer cycle life. Less DOD = longer life!
One other thing that is often overlooked, is that when away from shore power, and charging off an alternator, you will rarely get back to any more than 80-85% of capacity so your real usable Ah's from a 200 amp bank quickly become 80-85 amp hours of usable battery life before you hit 50% DOD..
Weekend Warrior: (tied to dock charger each night and charging back to 100%)
I will use two 4D batteries for this example but the math works on all of them.
The average group 4D has about 160Ah's. With two 4D's @ 160 Ah you will get a max of 160 Ah of usable capacity for two batteries in parallel before hitting the 50% threshold.
Two Group 4D's:
160Ah + 160 Ah = 320 TOTAL Ah's (Based on Exide Nautilus 4D)
160 Ah's use @ a 50% DOD = 160 Ah's left in bank or a 50% bank DOD
160 Ah - This is your usable capacity if you can guarantee a 100% charge every day.
Mooring Sailors or Cruisers: (Assuming no solar or wind generation)
If recharging while away from the dock, with an alternator, the best you can usually get back to is about 80% SOC. This is due to battery acceptance.
SOC = state of charge
80% SOC of a 320 Ah bank = 256 Ah's of total bank capacity
256 Ah's (80%) - 160 Ah's (to a 50% SOC) = 96 usable Ah's while out cruising!
Looked at another way;
320 Ah @ 80% SOC = 256 Ah - 96 usable Ah's = 160 Ah's or a 50% bank DOD
96 Ah's - This is your usable Ah capacity while away from the dock and only using the alternator to re-charge.
As an example an Adler Barbour fridge can draw 5+/- amps when running. If you figure it runs for 50% of the 24 hour day, you've burned about 60 Ah's with just the fridge or 62.5% of your total usable bank capacity while away from the dock using the 80% rule.
Ideally you should do an energy budget to know what your other draws total. With two 4D's and a 12V fridge you can do just one night without charging. 320 Ah's may seem like a lot but it vanishes quickly when you do the math. At best you should only ever use 50% of any house bank or battery life gets short. This of course is only if you want to follow the 50% - 80% rule and get long life out of your bank.
A battery monitor is a good investment and will tell you how efficient your fridge really is and how many Ah's you actually consume daily. Some fridges cycle as low as 25% and some as high as 80% of the time. Unless you want to sit in the bilge with a stop watch for 24 hours and time its cycling a battery monitor can make this much easier on you. The cycling range of variable load or cycling devices can be a rather big difference and could be a costly mistake if yours is a high cycler vs. a low cycler.
A Victron BMV-600 Battery Monitor (LINK) is very reasonable at $158.10..
So in summary, two 4D house bank of 320 Ah's will give you:
Weekend Warrior:
160 Ah's of usable capacity when sailing off the dock and charged every day.
Moored Boats or Cruising:
96 Ah's of usable capacity when charging off an alternator.
