I replaced our TST 510 TPMS with the TST 507 after a bit more than 5 years of very satisfied usage when the batteries started going out. However, TST charges $20 to refurbish each 510 sensor when the batteries are dying, and they offered me an excellent deal to move to the 507 with user replaceable batteries instead. I just ordered new batteries for our ten 507 sensors at a cost of $9.15 for two 5-packs, or about $0.91 each. Even rounding off to $1 makes the battery cost just $5 over the course of the 5 year expected life of the non-user replaceable battery sensors. And no need to send them in and wait for the return shipping. That seems like a pretty good deal to me...