RV Battery bank/solar stock answer stuff

ShadowcatcherShadowcatcher New UserPosts: 202Solar Expert ✭✭✭✭
I am on a number of RV forums and the level of end product of hay through a male bovine is truly amazing and since My key boarding speed is some what glacial I have a folder of stock answers. Perennial questions regard solar and battery banks, This is my attempt to simplify and de-BS the whole thing. So please check this for accuracy and comment.

Size a battery bank and PV array properly
Rule of thumb SWAG (scientific wild ass guess), about 75 to 130 watts of panel for every 100 AH of house battery, or about one watt per AH that you have. Nothing substitutes for knowing your actual needs or wants (not necessarily the same thing). Know the actual measured loads and do the math.

After 1 year of service (and maybe not even then), it is NOT advisable to enlarge a battery bank by adding new batteries to it, because a batteries voltage response changes with age. Internal resistance changes causing losses and failure to equalize. A weak cell will "steal" from the surrounding cells. The battery “chain” is only as strong as its weakest link).

Solar if it is the primary energy source, should be sized to produce more energy than the load requires, that stormy day, shady camp site, solar charging battery loss thing.

Buy high-quality batteries, you get what you pay for. Good deep-cycle batteries can be expected to last for 5 to 15 years, and sometimes more particularly with proper care and feeding. Cheap batteries can fail in half that time, and good batteries can fail with poor care and feeding. 6V batteries are not better than a good quality 12V (but may be easier to lift).

The ideal battery bank is the simplest, consisting of a single series of cells that are sized for the job. Fewer cells will reduce the chance of defects (parts left out don’t go wrong).
According to the Concord/Lifeline battery company based on their research you can mix battery sizes but not age and type i.e. AGM to AGM is OK AGM to Wet Cell is not, (lower internal resistance in AGM).

When wiring a battery bank the goal is to maintain all of the cells at an equal state of charge. Cells that receive less charge are likely to fail prematurely. This can take years off of the life of the battery bank. A fraction of an ohm of added resistance in one battery string can reduce the life of the entire string.

(1) Connect the two main cables to opposite corners of the battery bank, and maintain symmetry in wire size and lengths. This will help to distribute current evenly through the bank.
2) Arrange batteries to maintain even temperature distribution throughout the bank. Avoid uneven exposure to heat sources. Leave at least 1/2 inch of air space around each battery, to promote even cooling.
(3) Apply a finish charge at least every 3 weeks (bring every cell to 100% charge).

Use temperature compensation
When batteries are cold, they require an increase in the charge voltage limit, in order to reach full charge. When they are warm, they require a reduction in the voltage limit in order to prevent overcharge.
Bring the batteries to a full state-of-charge (SOC) at least every 3 weeks. This reduces internal corrosion and degradation, and helps to insure equalization, so that any weaker cells do not fall continually farther behind.

Install a System Monitor
Would you drive a car with no dashboard? Metering is not just "bells and whistles". It is necessary to help you to read the status of the system. Many charge controllers have indicator lights and readouts built-in. Their effectiveness for the most part very from creating false security with essentially meaningless ‘idiot lights’ to it gives “some” idea of what is happening. They are not a substitute for a system that actually measures what goes into and comes out of your battery(s) using a shunt. Two well respected systems Victron http://www.victronenergy.com/battery-monitor and the Trimetric from http://www.bogartengineering.com
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