I believe you may have answered your own question there, Hilary.
Why not revert to using whichever battery the T-34 is supplied with ... and then take things from there. That is, stick to
Alkaline (for the time being, at least).
Also ... do you know how long your batteries usually last? And (or)
why your batteries don't last? Who uses them? Can you be sure that users don't "misuse" them?
Have you checked out the excellent
information previously linked to by Robert?
More technical information on batteries
here.
Meanwhile (if it were me) I think I would experiment with rechargeable PP-3's from *Tesco's.
Lastly, I have seen (on eBay) 8.4 V 880 mAh Lithium PP-3's with a mini-USB socket (that is, they can be recharged from a USB connector); something else to consider, perhaps.
*
Other local vendors (and on-line suppliers) are available.
