IFMAR rule (http://www.ifmar.org/pdf/rules/ifmar_wc_electric_track_2010.pdf):
"1/12th cars will be driven by a maximum of 4 NiCd or NiMh cells giving a maximum
nominal voltage of 4.8v., or a 1S LiPo/LiFe battery with a maximum nominal voltage of
3.7v."