There's only one "traction battery". The indicator with 8 bars available is actually not an actual measurement of the battery's true state of charge.
Because NiMH batteries have poor tolerance for being 100% charged or fully discharged, Toyota has built in some protections. Essentially you are using that battery in a range from 40% to 80% state of charge, and this is what that "battery gauge" is showing you. 8 bars equals about 80% SOC, and 2 bars is about 40%. The system will never let it go below 2 bars, it will start the engine to charge.
Toyota is managing this battery perfectly, you don't need to worry about it. (Although continually being low or high SOC might eventually take its tole on the battery.)
In normal driving you should see that battery gauge between 4 and 6 bars. This corresponds to roughly a 50-60% SOC operating range. (each bar is approx 7% SOC) It won't go above 6 bars except on long downhill slopes or lots of regenerative braking. Its a good thing to be using that battery instead of burning gas, but its not a good thing to use it so much that the SOC drops too far. 3 bars is about as low as you want to see it in normal driving if your driving technique is maximizing its use. Once it hits 2 bars the ICE will run.
The traction battery is a 440 volt high-current battery. I recommend you don't try to mess with that. :mrgreen:
