Apple Watch battery at 82% health is not lasting all day

Unless the battery is defective, you will have to wait until the Battery Health reaches those 80%, that's what I had to do, when my old Series 5 had to be recharged twice a day. The Apple Support checked my watch online and only after the diagnostic report showed the 80%, they could initiate the battery service.

Part of the issue is that Apple does not really replace the battery. They send you a new watch of the same model. This happened with both my series 4 and my wife’s series 7 when the battery health was at 80%. Our watches both notified us that the battery needed servicing. The gotcha was that both watches were under extended AppleCare+ and there were no charges for the service.

you might like to check the battery for internal leakage; if you could demonstrate it was self discharging they should "agree" to you spending money with them, crazy though the situation is

How to check your Apple Watch battery fo… - Apple Community

you can see the batt health in settings->battery->battery health

this is the link to their battery replacement procedure Apple Watch Service and Repair - Apple Support