How about Nickel-Cadmium Cells?
Some people have written to ask me about Nickel-Cadmium (Ni-Cd) Cells. The larger ones contain a liquid much like flooded batteries while smaller ones used in flashlights are relatively "dry". Nickel-Cadmium batteries are frequently found inside power plants or telephone switches.
Ni-Cd cells tend to be even less power conversion efficient than wet/flooded cell lead-acid technology. For example, to completely re-charge a new Ni-Cd cell, Ni-Cd manufacturer SAFT recommends charging their cells with 140% of the rated capacity (so a 100aH cell requires 140Ah of current to be recharged). Regular flooded cells weigh in at 125% while AGMs can require as little as 105% to be recharged.
Furthermore, Ni-Cds self-discharge faster than AGMs (5% per month @ 25deg C vs. 1-2% for AGMs). The cycle life in deep-cycle applications is only marginally better: SAFT claims about 2,000 cycles at 50% DOD for their Ni-Cd system, while Rolls and others make lead-acid systems with cycle lives in excess of 3,000 cycles at 50% DOD. Lifeline AGMs weigh in at 1,000 cycles under similar conditions. Industrial strength or PV-system lead acid cells can last even longer.
The only logical thing that comes to my mind regarding the use of Ni-Cd cells is that they require virtually no maintenance and have a very long life in float operation. As far as I can tell, they are meant for telephone exchanges and other back-up type applications where grid power is normally available. However, most Marine applications are off grid (i.e. you don't have a shore connection) and require heavy-duty deep cycling - precisely the kind of operation that Nickel-Cadmium cells are NOT built for.
Like all technologies, there are areas where Ni-Cd cells excel cmpared to their peers. For example. Ni-Cd cells are comparatively impervious to temperature fluctuations and high ambient temperatures degrade them less than lead-acid cells. Such resistance is particularly useful in power plants or exchanges which can get very hot. Furthermore, Nickel-Cadmium cells can operate at lower temperatures than lead-acid batteries, making them useful for mountain-peak radio transmitters and the like. However, if properly installed, the operating environment in Marine applications is unlikely to be at either extreme.
Ni-Cd cells tend to degrade slowly with time - they can be more predictable - which is very important in backup-to-the-grid-type applications. Such predictability is sure to make the preventative maintenance crew happy at the power plant, but has little bearing on Marine applications... After all, most users only have a few cells to monitor and system reliability can be ensured by monitoring usage (cycles), overall cell life, and replacing cells before they are expected to fail.
Furthermore, Ni-Cd cells are extremely toxic to the environment and typically require very expensive disposal. Thus, even if you can get some "free" batteries from the local telephone company, they might end up costing you more to dispose of than buying new lead-acid cells. And no, I don't consider dumping your toxic waste in some third world landfill a ethically responsible "solution".
Lastly, recreational boaters do not have excess energy capacity during "off-hours" to recharge "lossy" NiCd cells like power plants or telephone exchanges unless they themselves are tied to the electric grid. Indeed, much of the economic benefit of AGMs hinges on you producing the energy yourself: If your boat is recharged every night from an outlet on the dock, then battery recharging efficiency is pretty much a moot point.