If you are using alkaline batteries, you will have very short battery life. The MM400 and the SM transmitters use a single AA battery to reduce size and weight. Since they are 100 mW transmitters, the load on the single battery is unusually high, about 450 mAh. Alkaline batteries are not designed for that high a current and will last less than 2 hours at room temperature. In cold weather, the run time is much less and can be on the order of minutes.
Lithium batteries can provide higher currents and will run slightly more than 6 hours at room temperature. If run a few hours at a time, they can provide a maximum of 8 hours of life. However at freezing temperatures they have shorter run times. (See FAQ I don't get much battery life in my transmitters when they are cold. What kind of batteries should I be using this winter?)
NiMh batteries are now available with capacity ratings of 2500 mAh and will operate the MM400 or SM transmitters for slightly over 4 hours. The above times were made with very fresh batteries; as the ads say "Your mileage may vary".
With 15 minute chargers available from several large and reliable battery manufacturers, they are a very viable alternative. Some of these chargers (RayOVac and Eveready) will operate from a 12 VDC source. Finally, the NiMh batteries are not nearly as sensitive to low temperatures as other battery types.