I was fortunate enough to be given (for free!) two barely-used 105 AH/20 hour rate AGM deep cycle batteries. They were
gorgeous. How often does $400 worth of batteries just fall into your lap?
The problem: I already have a string of flooded batteries for my solar set up, and you can't just add random mismatched batteries to the end of the string. The quick solution was to create a new string with the new batteries and place a simple single pole, double throw switch between them on the + side.. It's very easy and inexpensive, but then I limit myself to manually switching between strings for both charge and discharge functions. It would work, but there's got to be a better way.
One evening I was wandering around my favorite solar supplier's website
http://www.solar-electric.com and by accident found the Magnum Energy battery combiner. It looked like the answer to my problem.
The ME-SBC goes between two batteries and allows them to charge each other without being directly connected, with user-adjustable high/low. If the voltage in either battery exceeds your set points, the device disconnects. It really is a set-and-forget system.
Photo: My ME-SBC install.For prepping/survival purposes, you could have one string of "good" batteries and use the ME-SBC to add miscellaneous salvage batteries to your system without pulling down your good ones.
I see two main disadvantages: First, it is an electronic device that adds a layer of complexity to your system. If we get into a long term survival situation where spare parts are extinct, I may wish I had that low-tech SPDT switch. Second, this device does not allow you to power a load directly from the secondary battery. All it does is allow the batteries to charge off each other. The MB-SBC does not increase your battery capacity, it increases your battery
charging capacity. It's a subtle difference, but it matters.
The ME-SBC is not a complete solution, but at only $120 it's a relatively inexpensive way to add versatility to my solar power system.
I might write up a more detailed review and post it on my blog.
Available from Northern Arizona Wind & Sun.
http://www.solar-electric.com/me-sbc.html