Business Green: Two US battery startups targeting opposite extremes of the energy storage market secured significant financial backing this week as investors gave a ringing vote of confidence to the fast-expanding sector. Infinity Power Systems raised $20m in series C financing to continue the development of its thin-film battery business, while 24M, a spin-off from battery giant A123Systems, raised $16m to support its work on utility-scale batteries. Infinity, which raised $13m in series B …