This popular deal is back.
Copied straight from TA's post -
It's a straight one-for-one replacement, with up to 20 max per customer. These are heavy duty carbon-zinc batteries, that are (according to Battery World) perfectly suited for smoke detectors.
More info in thid Courier Mail article.
Credit to TA for title and description.
Got these last year. While free they didn't seem to last long..maybe 6 months.