Thank you, everybody.

Seems like the problem was in dirty "+" connector.

Facts:
- 14.3 volts when engine is running
- 12.1 volts when it's not
- 11.6 volts when no engine and all lights/fan/light bar are on
- 11.6 volts 5 minutes later

Based on the facts, the battery is fine.

The "+" connector had a lot of build up. I cleaned it up and running for a few days trouble-free.

Thanks for your help