Limiting battery drain by using what?

I recently built a small fan system to dry my wing tanks.

It is 12 volt and runs off of a 9 amp hour glider battery.

I thought that when the battery would get down to about 9 volts
that the fan would stop, this is not the case, the fan keeps working
till
the battery is down into the 5 volt range.

Is there something I can wire in that would make the system quit
working
when the battery reaches 9 or 10 volts so that I can get some more
life
out of my batteries?

Thanks for any advice,
Joe Simmers