Battery System

The off-the-grid movement has done much to educate us on how to set up battery systems for powering homes.  If you're interested in digging into the details, visit Building a Battery Bank for Your Off-Grid Energy Needs.  I would also recommend you look at Battery Basics: A Layman's Guide to Batteries and the discussion below on selecting the type of batteries will make more sense. Its an interesting topic, and I definitely made a few mistakes along the way.

Using my shorthand way of calculating amp usage and 12V Battery needs in the Dreaming About Amps post, my rough estimate of daily energy needs first looked at the 120V appliances.  I estimated 10 amps for HVAC, and 5 amps for cooking and hot water heating.  Converting that means 15 - 120V amps x 12X conversion rate is 180 - 12V battery amps needed.

I also have 12V DC needs of 25 Amps for the refrigerator and some interior lighting.  Add this to the above and I need 205 12V battery amp-hours daily.

There's another battery consideration to account for.  In the Battery Basics link above they talk about deep cycle batteries.  Deep cycle batteries can't stand discharging clear down to zero on a regular basis.  Batteries are an electro-chemical process so they degrade a bit each charge / discharge cycle, and their effectiveness degrades as they discharge closer to zero.  Deep cycle batteries stressed to their limits will last a few months, and batteries regularly discharged to only 50% of capacity will last years.  So its best to size your system for a 50% average discharge, which means whatever total battery calculation you arrive at, you double it.

Doubling our estimated amp-hour needs makes my daily battery storage needs 410 Amps.

Selecting Batteries

I am mounting my batteries inside my trailer in some unused nooks and crannies, because of weight distribution, keeping them and the inverter out of the weather, and to make wiring easier.  You commonly see wet cell batteries in off-grid applications and they have very good price performance characteristics.  But wet cell batteries give off noxious fumes when charging and  if enclosed or inside, they have to be vented - not good for my application.  So I went with an absorptive glass mat (AGM) Sealed Battery.

I am using two 12v 200ah solar power batteries that are 4D Size which is 20.5" x 14" x  12".

These are 150lb each which is why I wanted them sitting closer to the center of gravity of the trailer. They are mounted out of the way in NOCO heavy duty battery boxes.