Doug:
Battery Tender is actually the brand name of a smart charger.
As for an easy to read technical explanation, I'll quote an article on aircraft batteries from Aviation Consumer. As this quote is not for profit and is not physically reprinted for distribution, I believe this is a legal use of the material. I might add that Aviation Consumer is a wonderful magazine. It's pricey because it accepts no advertising ($57/yr or so) but that's what keeps it objective. The reason I love it is that it covers everything from high dollar stuff to things everyone uses, such as spark plugs, oil, oil filters, batteries, cleaners and waxes, flashlights....you get the idea
Jonathan
Battery Physiology
While most owners are familiar with the typical lead-acid wet-cell or flooded-cell battery, sealed batteries are far less well understood and that may be the reason they haven’t really caught on in light aircraft. Aviation wet cells are nearly the same as the typical modern auto battery, with the exception of a higher specific gravity electrolyte (1.285 versus 1.265 for the car battery), positive gas venting, lower capacity to save weight and non-spill vent caps to prevent the loss of electrolyte with aircraft attitude changes. These things haven’t changed much in the past four decades.
The new technology is called absorbed glass mat (AGM) and it’s not exactly cutting-edge new, either. This technology was introduced in 1985 by Concorde Battery Company to support a military contract for a high-reliability, rugged, maintenance-free and powerful (for its size) leak-proof battery. The AGM was to have none of the shortcomings of the messy wet-cell battery or the labor-intensive nature of nickel-cadmium aircraft batteries used in heavier aircraft where high-capacity is critical. The AGM battery is also called a valve-regulated, sealed lead-acid battery (VRSLAB), a starved-electrolyte battery or a recombinant-gas (RG) battery. All of these things mean the same thing so we’ll just call them AGM batteries.
While the AGM has been well received in the marine world, there has been little market penetration in the single-engine aircraft arena, although operators of heavier aircraft have embraced the AGM battery to a larger degree. Why is this so? We’re not so sure, since the prices are competitive but the performance should be much better, thanks to higher capacity and better temperature range.
One of the purposes of our head-to-head testing is to see just how much better AGMs perform. These days, both Concorde and Gill are in the AGM battery business but Gill came to the party much later. AGM technology has continued to advance to the point that gel cells—one of the few improvements in aircraft lead-acid batteries—are now passé. Concorde stopped making gel cells in 1987.
The AGM battery is so powerful that it can serve as a replacement for high-current demand—and potentially dangerous—NiCad batteries used in heavy twins and jets. The AGM also has superior cold-weather starting performance. This is so because of the internal design, which gives this battery low internal resistance to current flow and no liquid electrolyte to thicken up at cold temperatures. It’s capable of safe, repeated high discharge and charge currents—far greater than an equivalent size wet cell.
AGM batteries are also sometimes called starved electrolyte batteries because there is no electrolyte liquid to spill. There’s just enough electrolyte to saturate the glass mats. The aviation VRSLAB uses the same lead-acid chemistry as the wet cell, but it’s internally packaged in a much more usable way. When a wet-cell battery is charging, explosive hydrogen gas is vented to the atmosphere. Conversely, the sealed AGM battery uses one-way pressure relief vent valves. It operates with a positive internal pressure, which forces the gasses produced during charging to recombine and remain within the battery, hence the term “recombinant gas.â€
