One of the many unknowns about Toyota's partnership with Tesla was whether it would result in a jointly produced car.

That won't be known for many months yet, but Toyota has already taken a first step: It's building Tesla's battery into a test car so it can compare the Silicon Valley startup's technology to its own lithium-ion pack.

Toyota president Akio Toyoda told journalists at a briefing today in Nagoya, Japan, that Toyota is building an electric vehicle with a Tesla battery pack.

The Japanese company's in-house lithium-ion pack uses fewer, larger-format cells than the 6,831 commodity cells (essentially like mobile-phone batteries) that Tesla uses in the battery pack fitted to its 2011 Tesla Roadster 2.5.

Toyota's cells are specifically designed for automotive use, but the company may feel it's a bit behind the curve in lithium-ion adoption.  The all-new 2010 Toyota Prius was meant to have a lithium-ion pack, rather than the carryover nickel-metal-hydride technology it's used since 1997.

But Toyota bet on the wrong battery chemistry, meaning it had to start from scratch. And now it clearly wants to see whether the very different Tesla Motors approach could be used in a lower-cost, higher-volume vehicle.

Most analysts have concluded that the Tesla approach is simply too complex to be cost-effective when scaled up for mass production. The basic message: It's fine for low volumes of $109,000 Roadsters, but too costly for Corollas.

As Autoblog Green notes, test vehicles like the one Toyota is now building--known as 'mules' for their sometimes unpredictable behavior--may never be seen by the public.

Instead, they may spent a short, hard life being pounded around a test track or running for hours on dynamometers to give test engineers all the data they can wring out, before the hapless car is finally dismantled or crushed.

[Autoblog Green]