On Sunday, April 15, 2018 at 7:05:10 AM UTC-7, John-Del wrote:
.... My brother lost some keys last fall and has a pretty good idea where
they are. He was going to buy a detector but I told him I have one so that's
what prompted my inquiry.

If you only need it once for a single job, I would just use alkaline batteries for a quick and easy fix. If necessary, use a couple battery holders and mount them externally. I assume that the circuitry will tolerate the slightly higher voltage, but if you are concerned, you could use one or two fewer cells in each pack.