Not sure what battery technology your dewalt uses. A home depot listing for Dewalt 18V XRP lists them as NiCad.
Older battery technology used NiCad or NiMH batteries. Those have a self discharge rate of 10-15% in 24h, and then 10-15% per month.
Newer tech uses Lithium-ion, which have a self discharge of around 5% in 24h, then 1–2% per month