They do have this under tech specs:

Power source: 1 Lithium ion polymer rechargeable battery, AC adapter

Battery lifespan: Will last for approximately 500 cycles, 8 times/day measurements in normal temperatures of 77 °F (25 °C) when new battery fully charged

Battery life: A typical user can expect to charge HeartGuide approximately 2-3 times per week, depending upon the frequency of use of HeartGuide’s features