A number of hot battery stocks have cooled off in the last month, offering more attractive valuations.

Clean technology has been one of the hottest sectors in 2009, and as such it is prone to some pretty heavy speculation. Case in point A123 Systems (NASDAQ: AONE - News), the Watertown, Massachusetts-based lithium ion battery maker whose third-quarter losses widened after its well-publicized, late-September initial public offering. The stock slipped on yesterday's earnings report, but the 3% uptick in total revenue and a new contract with BMW gave investors a reason to stay optimistic.

VentureBeat credits the A123's PR team for putting a positive spin on the mixed results, and noted that the company's success largely depends on growth in the electric car market, which could be years down the road. However, many major automakers have an electric offering in the works, so the battery sector's recent pullback could be a buying opportunity to get in on a technology that could eventually be a mainstay of the auto industry.

Electric cars aren't the only thing driving components of the Energy Storage and Battery Technology Stocks Index. This was made evident recently, when Warren Buffett's bet on the sector BYD (OTC: BYDDF - News) slipped on reports that Nokia (NYSE: NOK - News) was recalling 14 million of the company's cell phone chargers.

Other Chinese energy storage plays are down sharply in the last month. China BAK Battery (NASDAQ: CBAK - News) is back to its summer levels after spiking sharply in September. Part of the more recent pullback is attributable to a 5.8 million unit offering worth $20.6 million. China BAK has pulled back by -6.5% in the last five sessions, outperforming only China Ritar Power (NASDAQ: CRTP - News) for the period.

Hong Kong Highpower (AMEX: HPJ - News) was among the sector's one-month laggards until today. Investors are now bidding up shares by 50% after the company doubled the single EPS estimate in the third quarter, earning 18 cents a share on $21.1 million in revenue.

Berkshire Hathaway (NYSE: BRK-A - News, BRK-B - News) is making over 6% on its BYD investment today, and Exide Technologies (NASDAQ: XIDE - News) is up by 3% for the session.

As of this writing, the Energy Storage and Battery Technology Stocks Index is one of the 15 worst-performing tickerspy Indexes over the last month, down by -10%.

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