If you meant that a rechargeable nimh will hold it's voltage very steady for more of it's discharge span, yep...They may start out at a lower voltage but will flat line that voltage to near the end, while a non-rechargeable will drop it's voltage as it drains much faster. The lipo I use in my machine also holds it's voltage very high as it drains, and in that case even though I used a smaller capacity pack than the stock nimh pack, the lipo gives me equal or longer run times, since it holds it's voltage even higher than a nimh and keeps it up there higher as it flat lines until near the end.
Faster voltage drop hits the low battery alarm faster, in some respects even with a higher capacity pack, unless the difference in pack sizes is greater than a certain point to make up that difference. At least based on my run time impressions, without timing them to be exact, that's the impression I've had of my 750ma lip 3 cell versus a 1000ma nimh pack. On the other hand, the lipo, like any rechargeable in healthy shape, will hold a good bit higher capacity when charged than the label says. I have a charger that displays this and that's one of the ways I monitor pack health. Of course they have to be drained near the low voltage limit (9V in this case) to see how much capacity is put back in when charged back up.
As for the faster drop off to dead right at the end, yep...rechargeables do that, where as a non-rechargeable will tend to have a more steady rate of decline right near the end, but in some respects the non-rechargeable gets near that end faster with it's more steady drop of the voltage as it drains. Lithium based batteries are even more know for this quick "I'm dead" type of drop off right near the end. You can see this with those Energizer lithium non-rechargeable AAs. If you have a voltage monitor on your detector, I've seen a friend check his for a hunt and it was showing very high voltage still, yet when we got out in the field within under an hour the low battery alarm sounding due to it being near that end from all the prior hunting he did with those cells before hand.
Those Energizer lithium cells are well worth the bit of extra cost for the much longer run times. I haven't crunched the numbers but I bet they more than save you the extra money you spent. I know in a camcorder I used them in they seemed to give me about 3 times the run time of regular batteries in it when filming video. The extra weight savings of them is also well worth the price. Very light. Not sure if Energizer makes lithium 9V non-rechargeables but if they do I'd try those and see how much run time versus cost they have, if you don't want to go the rechareable nimh or other route...
