If your instrument is set up to use both alkaline or Ni-Cad batteries, either type should work well. You need to determine which type of batteries would best suit your needs. One advantage of alkaline batteries is they can be purchased just about anywhere and can be easily replaced when they become discharged. Ni-Cad's advantage is that they can be recharged many times over, and be recycled thus reducing the amount of batteries entering our landfill.

Since you are using alkaline batteries, there are several things that can affect your instrument's performance. First, check the type of batteries that you are using. I would strongly recommend that you stick with high quality alkaline batteries. Some batteries marked "Industrial Strength" will not provide the current capacity that your instrument needs for proper operation. Second, in colder months, if your instruments are left outside in trucks or in unheated warehouses, the cold temperature can reduce the operating life of the batteries. A size D alkaline battery supplying 750mA current at 20° C (68° F) will operate for approximately 12 hours. If the temperature decreases to -10° C (14° F), it will only power the load for about 3 hours. During cold temperatures, you may want to keep your instruments in a heated warehouse or building to reduce this effect and to keep additional batteries on hand for those cold weather jobs. 

BATTERY TIPS: 

1. Never mix battery types; such as alkaline with carbon zinc or Ni-Cad   
2. Never mix old batteries with new batteries.   
3. Never attempt to charge an alkaline battery as the batteries could explode.   
4. Always discard batteries in an environmentally friendly manner   
5. Always remove or replace discharged batteries from your instrument to protect against leakage.   
6. Never attempt to charge alkaline batteries or discard them in a fire. They can explode!   
7. If you are going to store your instrument for a long time, remove the batteries. When you are ready to use the instrument, install fresh batteries and allow the instrument toxic sensors to acclimate for an hour before calibration and use.