1985 Pontiac Firebird Low voltage second battery/second ALT

Electrical problem
1985 Pontiac Firebird V8 All Wheel Drive Automatic 55k miles

Hello,

I had problems starting the car ( thank god that's over) and my mech told me I needed a new alternator. I did that and few weeks later my battry went dry. I assume it was pretty old I just bought a new one. Then a month after the new battery did the same, went dry. This time I used my warranty and got another one for free, secured the battery to the car and tighted the cables real good. Everything was working fine during the cold season ( good voltage, no overheating etc) but now it started to get warm again, I noticed the voltage on my dash went to 11 volts at night with my lights on and the dash indicator is showing maybe 12 running with the lights on. Without the lights is a mere 13-14 volts. I replaced the alternator but I still get the same problem. Can you please tell me what it could be the cause of the low voltage during hot days? I was thinking the fan relay is going bad and is draining all the power but I don't know for sure.

Thanks ahead of time for the help!