It's very unlikely that removing your battery will cause apple to deny you warranty service, as long as you don't do any damage in the process.
The reason the battery is tampered down is because the connector on the board is NOT meant for many uses, and will break if you get into a habit of plugging and unplugging your battery frequently. A connector like that is rated for maybe 500 operations tops before failure becomes expected. Apple tampers it down to discourage people from buying a spare and swapping out as needed, wearing out the connector on the logic board. (which if you DID do, would NOT be covered by warranty)
Several times I've encountered cheap knockoff batteries. The most recent for 15" mbp. The IT head at the univ had bought all the staff with MBPs new batteries, on the cheap. They looked VERY similar, too similar for patent purposes probably, but were VERY light. Maybe 1/3 the weight. And they had about a 20 minute (really) runtime, new. They were "rated" the same watt hours (85?) as the original battery, but were obviously lying. So maybe you can find something like that for yours?
