What
batteries? Most manufacturers have some recommendation for maximum drain.
On the chance you are using the recommended AW's: The AW rep on candlepower forum says not to drain them below 2.5v. There are about a thousand posts on ECF that say 3.5v, but all of them are regurgitating a typo in a post in an old battery function
thread that had some expert conversation in it. AW IMR 18350's = 2.5v is as low as recommended before charging.