EVs Could Soon Cost Same as Gas Cars as Battery Costs Drop

Anyone know if there are technical restrictions on third-party battery packs? As in, if you don't buy the cryptographically-signed (or whatever) OEM approved battery pack, it won't work? So if you buy a used electric vehicle today, what are the odds you could buy a new, lower cost battery pack in say 5 years, when the current one reaches its end-of-life?