Olympus brand replacement batteries for E-500 cameras are expensive. The official Olympus BLM-1 battery MSRPs for $90. You can find it online for around $44, but still. If you already have a battery that is a lot of money for a backup. Enter the BLM-1 Replacement battery by Adorama. It’s $4, and judging by the reviews pretty much equivalent to the official one. Mine is in the mail.
Thanks to el chupapadre for the tip.