Answer: Absolute max voltage for supply
HiloV2: “5.1 Vbat”: Vbat MAX 4,5V
URD1_OTL_5635_2_013_72398_ed_02_-_HiLo_V2_technical_specification_14_March_2011.pdf
Hilo3G: “5.1 Vbat”: Vbat MAX 4,4V
URD1_OTL_5696_1_004_72362_Ed_06_-_HiLo_3G_technical_specifications_29_sep_2011.pd
But same nominal value for supply: you can target 3,7V to 3,8V