Join Date: Jan 2003
Location: Maine Coast
Thanked 295 Times in 228 Posts
Rep Power: 17
Might I suggest that you first install a battery monitoring system on your existing house battery. A Xantrex XBM or LINK 10 is all you need. This is the only way you'll be able to accurately know how much power you use per 24 hours.
Once you know your average power usage you can then build a system around those parameters. Fridge/freezers are notorious consumers and there is NO average as every installation is entirely different. Things like insulation, ice box cu/ft, door position is it top or side opening, the amount of warm food or beer you toss in, did it run at the dock all week or are you firing it from warm up every Friday night etc. etc. all play into the fridges consumption and efficiency.
Some fridges, when properly cooled, will run as little as 50% of the time but some others will still run 70% of the time. When you fire it up for the first time and load it with food it may run as much as 80-90% of that first 24 hours.
I hope your getting my point that only a dedicated battery monitor will tell you how efficient or inefficient your fridge is.
For grins let's say you guess your fridge draws 5.5 amps per hour and runs 80% of the time. That is 4.4 amps per hour over 24 hours or 4.4 X 24 = 105.6 amp per day for just the fridge. Now lets throw in an extra 70 amp hours of use for everything else and your at about 175 amp hours per day.
Now you want to go for three to four days, will figure four, and you look like this 175 X 4 = 700 amp hours of use!!
Considering that a battery bank should never be discharged bellow 50% of capacity, and you'll rarely ever get it charged back beyond 90% or so running the engine, you'll need a bank of about 1500 to 1600 amp hours. That is freaking HUGE!!! Actually it's 14 6V golf cart batteries or 14 group 31 12v Deep cycle batts..
Or you could buy a monitor and find out for real that in 24 hours you only really use 90 amp hours per day and your bank gets drastically smaller..
A battery monitor should be the first step as it can save you HUGE money. My buddy Charlie spent well over 2k on a wind turbine, Balmar alternator and 4 six volt batteries with associated wiring and gear.
His last purchase was a battery monitor. To say the least he was very surprised to find out that he only used about 25-30 amp hours per day not the 100 he had guessed on. He could have saved a lot of money by buying the LINK 10 first not last...
Figuring current draw on some stuff such as lights is simple but variable items such as fridges, autopilots, stereos, TV's and laptop computer chargers are tough to guess on.
My Mac, while plugged in, burns anywhere from a low of about 2.2 amps to over 7 running through a small inverter and it's very dependent on what I'm doing on the computer as to how much current it draws. If I'm running PhotoShop it rarely ever dips bellow 5 amps but if I'm surfing the net it hovers in the low 2 range..
Get in about five or six average days, with the monitors ah consumption re-set each 24 hours, and monitor those numbers and base your system on that..
-Maine Sail / CS-36T
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Last edited by Maine Sail; 09-02-2008 at 10:38 PM.