With the proliferation of lights, lasers, red dots and a variety of other electronics it opens up the supply chain to a potential fault. Electrons. Some people have the ability to conduct an 8 hour operation, then change out all their batteries. Night in and night out. This is a convenience afforded to very few of us. As times change, resources will likely only get tighter and tighter so we must learn to make the most of whats afforded to us, while still ensuring we can make mission.
Not to jump too deep into the science of things, electronics require a specific voltage to run properly. Various batteries are designed to work at various voltages. Voltage is like the pressure of the electricity. When there is not enough voltage (pressure) coming from the battery, the optic/light/etc may not function anymore.
The normal opcheck of an electronic device is turn it on, hit the activator, and hopefully there is light or lazer energy emitted from it. When you are changing your batteries every night, this is more that suitable because the energy source is straight from the package. However, in less than ideal supply circumstances you may have to stretch your batteries.
A tool that everyone should own is a multimeter. With this, you can determine the voltage of a battery. Knowing the voltage will give you a good idea of where the life of a battery is at.
To test the voltage of a battery:
First, move the selector switch of the multimeter to the below setting. This represents measuring DC voltage.
Then hold the red probe on the positive end of the battery, and black on the negative end of the battery. You should now get a voltage read out. On a typical CR123 battery, that is a 3 volt battery, its actual volt reading will usually be around 3.1-3.25
This is an example of a fresh Energizer Lithium CR123
This is an example of a fresh Surefire Lithium CR123
This is an example of a fresh Streamlight Lithium CR123
If the output on the multimeter reads negative, you have the terminals reversed.
This is an example of a Surefire that has been moderately used. The light still powers on. However its voltage is slightly diminished from a fresh battery. I would reserve this battery for training, but not life supporting equipment (unless absolutely required)
This is an example of a reading on a battery that has been significantly depleted and no longer works.
There are new styles of lights coming out that use the 18650 or 18350 battery. There are huge numbers of these types of batteries. Most of these offer the ability to be recharged. When storing them, ensure you mark the last time they were checked or charged.
A resource for a variety of inexpensive (compared to when purchased directly from the weapon light manufacture) is https://www.18650batterystore.com/