I have a 94 Grand Cherokee, the charging voltage down below 13 vdc in hot
weather, but is charging normal 14.5 in cool weather. Is voltage controlled by
the BCM, or the ECM?

 

Neither, it's controlled by the PCM. To some extent it is regulated by the ambient temperature sensor, which is under the battery on my 97. Mine did the same thing and concerned me that I wasn't charging enough, so replaced it with a 51K ohm resistor. Big mistake.
While it did ram the charging voltage right up to the 14's in all weather, when I went for inspection it failed for numerous 'monitors not ready'. Spent 2 weeks chasing it until I found out the sensor is actually also used by the OBDII to set the monitors, which will only 'run' in a certain ambient temp range. Reconnected the sensor, it worked and got it inspected that same morning.

 

is that the same ambient temp sensor that reports the outside temp on the interior display?

 

To be honest, I don't know without looking it up. My OH display I think was reporting some flaky numbers while I had a resistor instead of the sensor, and my obd2 failed inspection because the 4 of the monitors would not run because it will only sample them in a certain ambient temp range. So it is critically important.
I do know it controls to some extent the battery charging and the OBD2 and suspect it reports to the OH display too.

 

The sensor in the battery tray is only for the battery temperature, there is a separate sensor for the ambient temperature monitor. The battery temp sensor tells the PCM to reduce the charge rate of the alternator if the battery gets too hot.

 

With all due respect, while that sensor does help the PCM regulate battery charge, it is not only for battery temperature.

 

As far as I can recall. Every vehicle that I have ever owned that had a gauge and not a light showed a higher charge in cool weather, than warm. I'd check the charging system to see what is going on as the gauge can be off as well. My 88 Chevy PU constantly shows charging well below 14. But I don't have battery issues and a test showed it charges at the norm.