Battle fatigue plays a key role in all battles. As you take hits and use ki blasts, your power level decreases. Here are some basic methods of calculating the rough loss of power level that you will suffer as battle rages on.

Please note that since battles are largely based on roleplaying with stats used as support, there is no actual mathematical formula which can determine how much weaker you will get over time. You only need to know that as a general rule of thumb, you will get weaker as you are damaged and as you expend energy (unless you are an Android) and that having a high vitality and a high stamina will dampen the effects battle fatigue has on your power level.

Ki Usage: All energy attacks take a percentage of your ki. When your ki reaches 0%, according to the statistical formula on the how to play page, your power level would be halved. This is not actually true as your stamina acts as a "buffer" for your other stats when calculating battle fatigue. Any ki drain first drains your stamina thus reducing your power level by just one stat instead of all of them.

You might be thinking that having a lower stamina would then be more beneficial as you would be losing a smaller number and thus your power level would go down less. This is incorrect. It is important to note that the percentage lost is based on your ki and not the stamina stat itself. Therefore the drain will be the same no matter how high or low your stamina is. Once your stamina has been "used up", any future ki drain will decrease all of your stats. This would obviously reduce your power level very quickly. The benefit of having a high stamina is that it allows you to keep this buffer in place for longer and protect your other stats from being affected.

Taking Damage: Ki usage is not the only way your power level decreases during battle, taking damage also lowers it. This type of battle fatigue works in the same way as ki drain does, only it is based on your vitality stat instead of your stamina stat. As you take more and more damage, vitality acts as another buffer for your other stats. Once your vitality is used up, taking any more damage will start to wear down all of your other stats, thus severely reducing your power level. Having a high vitality will keep this buffer in place longer and is very important for long drawn out battles.

It is a bit trickier to calculate battle fatigue based on vitality because there is no ki stat to base the percentages on so a battle referee is needed to tabulate it. It is more of a comparison between how much punishment a character has taken and how high their vitality stat is. There is no set formula and it is largely based on what has happened in the battle. Keep in mind this is not meant to be a measure of your characters hit points or life remaining. As the how to play page says, you can have 0 vitality left and still be fighting.

Damage Report: If at any time in the battle you want a "damage report," you may ask the battle ref to state a percent damage that you have taken. This will be a judgment call by the ref based on what has happened in the battle thus far and can be used to calculate your fatigue from damage.

Androids: Androids are not affected by battle fatigue when it comes to ki drain. Their power level will not go down no matter how much energy they expend. Damaging them will still wear them down, however, it is much harder to do so because an Android's stamina acts as a second vitality stat. You will need to inflict enough damage on an Android to drain their entire Stamina statistic before it even starts to take away from vitality (in which case the same calculations used in the above section on vitality would take effect).