The Battle Potion is a buff potion which grants the Battlebuff when consumed. The buff doubles the spawn rate of both enemies and critters, and doubles the maximum amount of enemies that can be on-screen at once. This lasts for 7 minutes, but can be canceled at any time by right-clicking the icon (), by selecting the icon and canceling it in the equipment menu (), by double-tapping the buff icon (), or by canceling the buff from the buffs screen ().
The effect stacks with those of other items which allow a manual alteration of spawn rates, such as Water Candles. Similarly, items like Sunflowers or Calming Potions moderate the effect. For example, having both a Battle and a Calming Potion active will result in a spawn rate increase of only 54% (instead of 100%, due to the -23% of the Calming Potion) and an increase in the maximum number of enemies on-screen of only 40% (instead of 100%, due to the -30% of the Calming Potion).
Spawn rate effects like Battle Potions (and Water Candles) can be used to increase the spawn rates of events that do not have a set spawn rate, like the Blood Moon and Solar Eclipse.
Battle Potions and Water Candles cannot be used to increase the spawn rates of events like the Pumpkin and Frost Moon. The spawn rates of these events override spawn rate effects.
The Calming Potion has the opposite effect of the Battle Potion but as they are both potions, they are both considered buffs and as such can be canceled early. In contrast, the Water Candle effect is classified as a debuff.
The effect stacks between players, because each player spawns their own NPCs. It also increases the maximum amount of enemies that can be spawned, allowing multiple players to drink them and spawn mobs at a much faster than intended rate.
When using this potion, beware when entering Dungeons, and even more so Hardmode Dungeons. Spawn rates can get out of control and difficult to handle even for the best of players.