I've rigged up 2 dummy AA batteries, cut a tiny section from the battery cover for external power wire, and using an IMR18650B Li-ion battery. I get a shocking 14.5 hours with the backlight on constant mid brightness and alert LED on the 436HP. Scanning conventional and trunking systems. The battery save option turned on.
Don't even think of doing this unless your schooled and knowledgable with dealing with unprotected Li-ion batteries. An IMR or SDI or NCR unprotected battery can discharge 20-50 amps if shorted!!!! If using unprotected Li-ion cells, make sure they are Sony, Panasonic, Sanyo, AW brands. Don't worry, the Li-ion batteries in your cell phones and everything else are using protected batteries. I've tried protected Li-ion batteries in the Uniden scanners, but they never last very long before reaching the limit of the scanners cut off voltage.