What is a good average volt for battery cells?

Nothing to worry about unless you see a check engine light come on and read the code as bad hybrid battery. The voltage can vary from time to time due to a range of factors: temperature, atmospheric air pressure, humidity etc. Keep in mind when you see 15.8 volts or whatever you measure, it's showing the voltage of TWO modules, so divide by 2 to get the actual per module votage.

All that matters really is how much CURRENT is being stored into the modules during regen and how much current is coming out when the engine ecu and battery ecu work together to drive the motors. The battery ecu constantly monitors all that and it will definitely let you know there's a problem by throwing codes and lighting the CEL.

 

The symptoms you are seeing are normal. The AC compressor is driven by an electric motor, which gets power from the inverter and HV battery. If you're sitting in idle, the AC will slowly drain the HV battery until it hits 2 purple bars on the display. The engine will turn on to recharge the HV battery until it shows 3 blue bars, then the engine will turn off and the cycle repeats.

 

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Is this the original battery that shipped with the vehicle? If so, you may want to do a top balance, or charge/discharge cycling to rejuvenate it.

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Your battery - charging behavior is normal for a Gen2 when the State Of Charge is low (gauge at 2 bars). The hybrid control ecu will not "waste" gas by putting more power into the battery than absolutely necessary when the car is stationary.

It's expected that the battery will be charged while driving and braking.

As far as block voltages, the pack on my Gen2 will vary from around 200V (low SOC) to 230V (high SOC), or 14.2 to 16.4V per block. This is READY but "at rest" while the car has near zero amps battery current.

Those voltages will be (momentarily) lower when the battery is discharging, and higher when charging.

The important thing to monitor is any difference in block voltages. Good packs have that difference under 0.3V for 99+% of the time.

Dr Prius is a handy app to monitor HV battery voltages.

Here's a thread that reviews several scantool apps and devices.

https://priuschat.com/index.php?posts/3290690

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