This probably won't suit everyone, though it's worth knowing anyway if you have what it takes.
Tesco currently have a deal where 8 packs of Duracell Ultra AA and AAA batteries are buy one free so straight off that's 16 batteries for £8.
Tesco have been handing out £2 off battery packs as vouchers with my shopping quite regularly, it seems to be £2 off one pack. Tonight I tried 2 vouchers as I was buying 2 packs, the till accepted it. Net result £4 off £8 resulting in £4for the 16 batteries. I went for one AA and one AAA to stock up.
If you have 2 or more vouchers this is a deal otherwise feel free to ignore it.
Oh the vouchers are valid till Christmas Eve I think, though not sure how much longer the BOGOF is on for. An alternative is they have 12 packs for £6, voucher makes it £4.
This was an Extra store, not sure if it's in smaller stores.
Edit: Sorry I'd said £3.99 it was actually £4, I guess the extra 1p will sink the iceberg ;)