Jama

the book has absolutely no useful guidance on this one way or the other. downtime actions imply in the text of the action that you should walk away with a single reserve, but the examples suggest a set of interactions that could result in two or three. reserves are given no explicit deadline, but if it's a keep-til-you-use-it, a frugal player can save up enough core batteries to run through any mission that sounds like it might be a bit tough with 100% core power uptime. batteries are an instructive example here, too, because they're batteries, there's no reason you're just spilling them everywhere and losing them if you don't take them, but they specifically pose severe balance challenges if left unchecked.

in other words, just don't... don't worry about it. my recommendation is to use reserves very loosely, narratively as suggested by misha, remember that the listed reserves are suggestions and not a gearlist, and to ensure players have prep and gathering time (where appropriate) to acquire reserves for missions so they don't try to spend their downtime actions gaming out what they think is coming next. tom was clearly inspired by Blades, so i recommend a Blades approach of "it's not really interesting to litigate how you get this thing you strictly need to do this mission, just figure out what it cost." and don't be afraid to be more conservative with toys like core batteries or artillery strikes that are just drop-in boosts.

on that note!!! the Power at a Cost downtime action (p. 53)!!! remind your players it exists early and often mid-mission! it is the source of many of the good things in a lancer's life and they should use it liberally, especially to give you ways to fuck with them later.