Life is indeed grand!
How many nights can you go without plugging in? My 12vdc frige will deplete my 2 4d's in about 4 days if I'm not plugged in (Don't ask-cocktails may have been involved). I don't see how yours is much different except the 15 watt solar which can only put out about an amp per hour (watts/12 volts). For all day maybe 10 ah's. What am I missing?
My C-310 fridge used between 60 & 80 Ah's/day..
The life of batteries is directly proportional to the number of discharges as well as the DOD or depth of discharge.
Cycling the batteries to a 50% DOD is the generally 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.
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..
Here's some math:
The generally accepted max discharge depth for deep cycle batts is 50% of rated Ah capacity.
The average group 4D has only 160Ah's. With two @ 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 Ah's (Based on Exide Nautilus 4D)
160 Ah's use @ a 50% DOD = 160 Ah's left
160 Ah - This is your usable capacity if you can guarantee a 100% charge every day.
If recharging while away from the dock, with an alternator, the best you can usually get back to is about 80% SOC.
80% SOC of a 320 Ah bank = 256 Ah's of usable capacity
256 Ah's (80%) - 160 Ah's (to a 50% SOC) =
96 usable Ah's while out cruising!
96 Ah's - This is your usable Ah capacity while away from the dock and only using the alternator to re-charge.
An AB fridge can draw 5+/- amps. If you figure it runs for 50% of the 24 hour day, on our 310 it often ran a lot more than 50%, 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. It seems that RP can do one night without charging at this point if he wants to follow the 50% - 80% guideline..
As I mentioned before consider a battery monitor as it will tell you how efficient your fridge really is. Some fridges cycle as low as 25% and some as high as 80% of the time. That can be a big difference and could be a costly mistake if it is a high cycler vs. a low cycler.
A
Victron BMV-600 Battery Monitor (LINK) will be the best $158.10 you've spent..
Based on two Nautilus Gold 4D's you certainly can run your fridge over night depending upon your other consumption needs. Other needs might include anchor light, cabin lights, plotter, radar, stereo, VHF, depth, wind speed, water pressure etc. etc. etc.. Going for multiple days will lead to short battery life as the math above shows..
