The tracker is powered by an integral rechargeable battery. A fully charged battery lasts around 5 to 7 days.
Battery life may be shorter if you synchronise your tracker several times a day or set several alarms.
*credit to Sylvain of iHealth Labs EU
https://helpcenter.ihealthlabs.eu/hc/en-gb/articles/205909741-iHealth-AM3-Battery-life