The biggest issue with Plasma technology was that economies of scale was against it. Plasma was great for TVs but not so great for PC monitors. On top of that there was the lighting market which benifited from LED. So it was too tempting for manufacturers to go in on LED. The power consumption was also another important factor.
In comparison, Batteries are more straight forward. There is no real competing technology other than other lithium ion batteries. If there is an improvement to anode or cathode, generally the process stays the same.
To add to that, Tesla is covering half the cost and Tesla and SolarCity are supplying you guaranteed demand.
For Panasonic, which is eager to transform itself from a AV manufacturer (TVs, DVDs and smartphones) to an industrial company (read "automotive"), the partnership with Tesla presents a very attractive opportunity.
And yet, how much Tsuga wants to share his company's future with Musk 's big dream (and big talk) remains a question. Whatever decision Tsuga makes will define what Panasonic will become (or won't become) in the next five years.