The voice-chat battery system is configured though the following cvars:
Voicechat battery
ttt_voice_drain (def. 0):
Enables the voicechat battery feature. Voicechatting reduces a battery meter, when it's empty the player can't voicechat and must wait a few seconds for it to recharge.
ttt_voice_drain_normal (def. 0.2):
Battery drain per tick of voicechat for normal players. "Tick" refers to a game tick, ie. 1/66th of a second.
When full the battery meter is at 100. With the power of math, we can see that the default setting allows you to speak continuously for 100 / (0.2 * 66) = 7.6 seconds before any recharging is needed.
ttt_voice_drain_admin (def. 0.05):
Battery drain per tick of voicechat for both admins and Detectives. Set to 0 to allow infinite mic usage.
ttt_voice_drain_recharge (def. 0.05):
Battery recharge rate per tick of not voicechatting.
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Source: http://ttt.badking.net/config-and-commands/convars#TOC-Voicechat-battery