Yes, it's the batteries, they depletes themself on their own, rather than the body drains the batteries. I have 10 pcs LP-E6Ns with Canon LC-E6E chargers (came with the 5D4s + spares) and all of the batteries fully exhausted about 4 months due to COV...
Lithium based cells storage is optimal on 20% charge which means 7.4 volts on a two cells 7.2 volts pack, like the LP-E6 [N]. Professional chargers / dischargers (Like IMAX B6AC - beware of it's clones!) have "storage mode" which discharges / charges...
