I used to use a Mugen battery cover and a double-cell battery on several N900s.
I discovered that the pressure from the doubled up BL-5Js was flexing the system board and breaking the modem connection, leading to the no-SIM of death. It was really too bad as the phones were in great shape otherwise. I repurposed them as wifi tablets with a bunch of N64 and webOS games and gifted them out to kids.