Originally Posted by katsailor
I am replacing my house bank of batteries as part of my refit and I have have a lot of room. Can anyone provide a rule of thumb for sizing a house battery bank on a 33' sailboat. Also what type of battery?
My biggest load will probably be the cold plate in the ice box, second load would be the freshwater pump but it would only run intermittently. All lighting is led and I do not have a microwave or ac. If I add AC it will only used at the dock so that should not affect the size of the bank.
The new beta 20 motor is equipped with a 75 amp alternator on serpentine belt and I have an echo charge that will be installed between he house bank and the starting battery.
Thanks in advance
Start with a battery monitor so you know your daily averages. A Victron BMV-600 is a very good value.
Once you have a good idea of consumption you can then size a bank for your intended use. 1 day on the hook, 2 days on the hook, 3 days on the hook before recharging..?
Remember you should only use 50% of the capacity so as to get the most cycles per $$ spent. 50% is a guideline however and shallower cycling will simply mean longer life. How do you cycle less deeply? Size the bank larger and re-charge at 65%, 70% or 75% SOC vs. 50% SOC...
there is always a but......
When off cruising you can really only use about 30-35% of the capacity due to declining acceptance during the last 15-20% of the charge cycle. No real sense in charging beyond 80-85% SOC because you just waste fuel and lightly load the engine.
So in reality you need a bank that is about 70-75% larger than your daily use.
If always tied to a dock after each use you can get away with 50% larger than daily use...
If you use say 50Ah per day then a 130Ah - 150Ah bank would be about right if you only want to go 1 day on the hook without re-charge. If you want to go two days then a bank of about 300Ah's would be much better...
Also the larger the bank, compared to daily use, the shallower the cycles and the longer the bank will last.
50Ah per day takes a 100Ah battery to 50% SOC
50 Ah per day takes a 200Ah bank to 75% SOC
50Ah per day takes a 300Ah bank to 83% SOC
The less deeply you cycle a bank the longer it will last.....
That Peukert guy...
Also there is the Peukert effect. 50Ah per day is approx 2Ah every hour or, a 2A average load.
A 2A load on a 100Ah 1.25 Peukert bank is a 1.59A effective load or makes it effectively a 125Ah bank (a 25% increase)
A 2A load on a 200Ah 1.25 Peukert bank is a 1.3A effective load or makes it effectively a 300Ah bank (a 50% increase)
A 2A load on a 300Ah 1.25 Peukert bank is a 1.2A effective load or makes it effectively a 500Ah bank (a 67% increase)
A 2A load on a 400Ah 1.25 Peukert bank is a 1.1A effective load or makes it effectively a 710Ah bank (a 78% increase)
The larger the bank is the better the cycle life will be and the shallower the effective cycles.........