You can't upgrade soldered RAM — so don't worry about battery life.

The question doesn’t apply because you can’t upgrade soldered RAM. It’s physically attached to the motherboard and not meant to be replaced or removed. If a service claims to swap it out (like reballing BGA chips), that’s a motherboard replacement in disguise — and any power behavior differences come from the new board, not the RAM itself.

If you’re talking about a laptop that has both soldered RAM and an accessible SODIMM slot, adding RAM to the slot is fine. Modern DDR4/DDR5 is efficient enough that the extra stick uses negligible power in idle or sleep states — we’re talking maybe a few milliwatts. Your battery life won’t change in a meaningful way.

If you need more RAM than what came soldered, buy a laptop with fully socketed RAM next time.