Battery drain

The original ion lithium batteries died long ago and i've been using the external battery attachment with the screws on the rare occasions when I still use the MZ-R50, usually for recording purposes.

Yesterday I dug it up to go record a live concert and new batteries were drained after 10 minutes recording. It kept giving a low bat message and turning off, each time turning on again(when I slide the rec button) showing the bat lines as 2/3rds, untill it started happening too frequently to try and record anything.

I also tried with a friends NiMH bats, the AA type, same effect.

What can I do about this? Is there anywhere I can get the original special shaped Lithiums that go inside the player? will it make any difference?

Thanks