An apparatus for use with an assembly for warming an intravenous fluid to be administered to a living body, which includes a case through which an the intravenous fluid line extends, the apparatus comprising: a housing configured for attachment to the case, the housing including: one or more lithium polymer cells (110, 112, 114, 116; 210, 212, 214, 216; 310, 310A, 312, 312A, 314, 314A, 316, 316A; 410); a charger (232, 332, 432) connected to the one or more lithium polymer cells; a heater configured to heat the intravenous fluid and attached to a thermal fuse (224, 324, 424); a fluid warmer microcontroller (222, 322) connected to the charger and the heater; a spread spectrum oscillator connected to the fluid warmer microcontroller; a battery condition indicator and controller connected to a fluid warmer cover closure signal and to the fluid warmer microcontroller; a first order battery protection circuit (250, 350) connected to elements selected from the group consisting of: the battery condition indicator and controller, at least one sensor (256, 258, 260, 264, 266; 358, 372) connected to the lithium polymer cell or at least one of the lithium polymer cells, a cell balance circuit (254, 354) connected to the at least one of the one or more lithium polymer cells, and a battery disconnect switch; a second order battery protection circuit (252, 352) connected to the at least one of the one or more lithium polymer cells and the thermal fuse (224, 324, 424); and a fluid warmer overtemperature protection circuit attached to the case and connected to the thermal fuse (224, 324, 424). a housing configured for attachment to the case, the housing including: one or more lithium polymer cells (110, 112, 114, 116; 210, 212, 214, 216; 310, 310A, 312, 312A, 314, 314A, 316, 316A; 410); a charger (232, 332, 432) connected to the one or more lithium polymer cells; a heater configured to heat the intravenous fluid and attached to a thermal fuse (224, 324, 424); a fluid warmer microcontroller (222, 322) connected to the charger and the heater; a spread spectrum oscillator connected to the fluid warmer microcontroller; a battery condition indicator and controller connected to a fluid warmer cover closure signal and to the fluid warmer microcontroller; a first order battery protection circuit (250, 350) connected to elements selected from the group consisting of: the battery condition indicator and controller, at least one sensor (256, 258, 260, 264, 266; 358, 372) connected to the lithium polymer cell or at least one of the lithium polymer cells, a cell balance circuit (254, 354) connected to the at least one of the one or more lithium polymer cells, and a battery disconnect switch; a second order battery protection circuit (252, 352) connected to the at least one of the one or more lithium polymer cells and the thermal fuse (224, 324, 424); and a fluid warmer overtemperature protection circuit attached to the case and connected to the thermal fuse (224, 324, 424). one or more lithium polymer cells (110, 112, 114, 116; 210, 212, 214, 216; 310, 310A, 312, 312A, 314, 314A, 316, 316A; 410); a charger (232, 332, 432) connected to the one or more lithium polymer cells; a heater configured to heat the intravenous fluid and attached to a thermal fuse (224, 324, 424); a fluid warmer microcontroller (222, 322) connected to the charger and the heater; a spread spectrum oscillator connected to the fluid warmer microcontroller; a battery condition indicator and controller connected to a fluid warmer cover closure signal and to the fluid warmer microcontroller; a first order battery protection circuit (250, 350) connected to elements selected from the group consisting of: the battery condition indicator and controller, at least one sensor (256, 258, 260, 264, 266; 358, 372) connected to the lithium polymer cell or at least one of the lithium polymer cells, a cell balance circuit (254, 354) connected to the at least one of the one or more lithium polymer cells, and a battery disconnect switch; the battery condition indicator and controller, at least one sensor (256, 258, 260, 264, 266; 358, 372) connected to the lithium polymer cell or at least one of the lithium polymer cells, a cell balance circuit (254, 354) connected to the at least one of the one or more lithium polymer cells, and a battery disconnect switch; a second order battery protection circuit (252, 352) connected to the at least one of the one or more lithium polymer cells and the thermal fuse (224, 324, 424); and a fluid warmer overtemperature protection circuit attached to the case and connected to the thermal fuse (224, 324, 424).