The best new feature in Apple's [AAPL] new MacBook Pro is the 8-hour, ultra-durable battery that Apple says it custom-designed. But is it really the advanced powerplant it claims to be?

Conventional notebook batteries, or lithium-ion batteries, are made up of tightly-packed cylindrical cells, almost like a bunch of little Duracell Cs in a case. Apple has chosen to use lithium-polymer batteries in the new MacBook Pro, which are widely regarded as more advanced than li-ion batteries; they're lighter, more physically robust, more energy-dense, cheaper to manufacture and don't have to be in the traditional block form-factor.

Because li-poly batteries can be any shape, Apple smartly molded the new MacBook Pro's battery to the inside of its case, packing in a 40% "bigger" battery into the machine. That increase probably has as much to do with the physical size of the cells as the energy density of li-poly storage, which has helped Apple keep weight down. The MacBook Pro tips the scales at 6.6 pounds.

The new li-poly battery allows Apple to claim eight hours on a single charge, or seven with a beefier video card. Each battery should survive longer, too; Apple claims five years, or 200-300 full charges. That longevity owes itself to Apple's new "adaptive charging" system, which uses microchips inside the battery to determine the optimal level of current for each cell.

But put your ear to the ground in the battery community, and the buzz isn't about custom-shaped li-poly batteries, with or without "adaptive" charging. Companies like ZPower are already boasting huge improvements in silver-oxide batteries (also known as silver-zinc batteries), which have a much better energy-to-weight ratio than lithium-based batteries. You can see an animation of how silver-zinc batteries work here.

Not only are they 40% more energy-dense than lithium-based cells, silver-zinc batteries are 95% recyclable, and because they don't contain lithium, they're not prone to exploding and engulfing your precious notebook in flames. Apple's new li-poly battery, to its credit, is arsenic, BFR, mercury, and PVC free, and “highly recyclable,” according to the company -- but it's hard to beat silver for its green cred and safety.

The downside is that silver-based batteries are susceptible to the fluctuations of the price of silver, which has been on a rollercoaster ride since late 2005. [To see a graph of silver prices since 1992, courtesy of TheBullionDesk.com, click here.] That has rendered them useful only in small, if important, devices like the on-board computers inside self-guided weaponry.

But ZPower, for one, has said that it's gearing up to launch its batteries inside a "major notebook computer" this year, suggesting that some of the leakage problems that plagued early silver batteries has been solved. The company plans on using a trade-in recycling program to keep battery prices stable and manageable. California-based ZPower, whose backers include Intel [INTC], claims this will add two hours to the life of an average notebook battery, a jump from five hours to seven hours.

With its monster battery registering at welter-weight size, Apple is ahead of the pack for now. But should a competitor like Dell [DELL] or HP [HPQ] come out with a silver-ion battery in one of their 2009-2010 notebooks -- and should the price of silver fall -- all of Apple's custom in-house engineering might have been for naught.