Originally Posted by comfun1
A couple of questions for Lou or some of you.
1. What is the advantage of drawing off of one battery instead of two?
2. If I draw off of one until it's exhausted and then switch to the other and use it until it is exhausted, will I get the same amount of energy as I would if they would have both been connected in parallel to start with?
I guess this is a "philosophy" issue so I will preface my answer with a big giant "In My Opinion" ...
It all has to do with YOUR battery life.
Deep cycle batteries are designed from the "Git Go" with a number of useful "cycles" in mind. A cycle is a full charge to 10.5 volt (dead) discharge. Manufacturers use 500 - 1200 cycles for that "number." Cheaper DP or JPJ (Just plain Junk) batteries are at the lower end of that scale and higher quality batteries at the upper.
The "useful life" is defined as when the "stated capacity" is reduced to 80 percent. So a "used up" 70AH battery will only deliver 56AH when it should be replaced IAW the manufacturer's design parameters.
You may still get more than the designed number of cycles even as the battery ages and you might be happy with the performance even though it won't last very long on its own past that number, and most will get disgusted with its performance when it is about 2000 cycles (~60 percent of original capacity or 45AH in the example (my OEM battery).
Worse yet, as the battery ages, it's internal resistance increases to almost double its original factor. This means as the battery ages it will "self discharge" twice as fast as a new one even with a cut out switch.
Battery Life and How To Improve It
If you use the "individual battery till dead then switch; method of battery management, you will get what the manufacturer designed; a battery "good for about 4-5 years before it needs replacement.
The main advantage with this method is that you can use two completely different batteries. "twinning" them does not matter. When one no longer works like you want you just need to replace the bad one.
If on the other hand you use the "paired" method of "bank" battery management, you must start with two new identical batteries and wire them so that the total resistance from the charge/discharge source is as identical as you can make it, so current flow in and out of both batteries is the same.
With bank management you can deliver the same AH to the rig, but it is "shared" between the two batteries. You can then manage the batteries better by recharging them together at a much higher state of charge.
By recharging at 80% discharged instead of dead (for example) you over triple the designed battery life.
The main disadvantage is that when it DOES hit that "magic number" both batteries must be replaced as a set.