There are a number of factors that will affect the lifespan of batteries in a flashlight. These include what type of batteries they are (whether they're A, AA, AAA, or 9 volt), how big the flashlight is and how often its used. A number of experiments have been done by organizations such as Consumer Reports to attempt to find out which regular batteries last longer than others not just in flashlights but in all devices. The results have shown that Rayovac batteries worked continuously for 24 hours in a standard sized flashlight. Duracell brand batteries lasted for 15 hours of continuous use, and Energizer lasted for 22 consecutive hours. Eveready batteries came in last, holding a charge for only six hours.
You can improve the lifespan of your flashlight batteries by caring for them properly. Certain batteries will lose a charge if subjected to extreme heat or cold. For example, leaving your flashlight in a hot car is a good way to make sure the batteries don't last as long as they could. However, certain brands of batteries will actually last longer in extreme heat or cold than normal batteries. Energizer's E2 brand of battery is an example. The E2 batteries are designed to operate longer in extreme temperatures. They have an average lifespan under normal temperature conditions.
There are a number of rechargeable batteries that will work well in any flashlight. These rechargeable batteries come in many sizes, so make sure you get the size that fits your flashlight. After a battery's charge has been used up, place it on a charger connected to a wall power outlet. Once it's recharged, use it until the charge again is drained. Rechargeable batteries cost more than regular batteries but can end up saving a person money because of the ability to be recharged.
These batteries will not work infinitely, however. A rechargeable battery loses its overall charge over time and will eventually need to be replaced. On average a rechargeable battery loses around 10 percent of its overall charge per month of use, meaning if they were used heavily a person could use and recharge them for around 10 months.