What happens when battery goes to zero bars?

You shouldn't really ever see anything less than two bars with regular use. Perhaps shutting down w/ two bars and the next morning see one... perhaps. In any case, even the meter does not display the full capacity of the HV battery. I think it was about 20-80% capacity is displayed.
Unless there is a major malfunction, the car will take care of itself... what you saw is normal... disconcerting but normal.

 

The HV battery will take care of itself. The only thing you need to know is it hates extreme heat so if you are uncomfortable so is your battery. Turn on the AC in those situations. Other than that don't worry about it. Then engine will turn on to recharge the HV battery when needed. The Prius is a pretty smart cookie.

 

The gas engine (ICE) will automatically turn on to charge the battery, assuming you're not in neutral.

 

No worries, mate!

The engine will start and send power through the smaller electric motor-generator (MG1) to recharge the high voltage batteries. You'll likely never see 0 bars, but if you do, the battery is not depleted, just the useable portion is depleted. The system will protect your batteries.

The battery maintainer is just for your 12-volt, a tiny 12-volt that basically just boots the system (the high voltage system spins the engine at start-up). You shouldn't need to use a maintainer in normal use. I do so sometimes on account of our many short trips.

I use a BatteryMinder Plus. You can connect under the hood, by the way, or you can access the 12V in the hatch area and connect directly. Under the hood, you open the fuse panel, flip open the red plastic cover of the positive post there, and ground over to the negative jump point on top of the engine on the passenger side, or any bare metal (I use a bolt head on the inverter).

 

The car takes care of itself. Don't worry unless it throws a warning message or light. The HV battery will go to 2 bars for a while and then the engine will fire up automatically, charge it upto 3 bars and switch off again - assuming you're still stuck in this heavy traffic. It very very rarely goes below 2 bars (only ever done it for me once in 65000 hard taxi miles).

Another thing not to worry about is the battery will usually sit 2 bars from the top. Sometimes in traffic it will go to 1 bar from the top and will only go to completely full after you have gone down a very long and steep hill. The car doesn't like the HV battery to be completely full so will 'burn' off this extra battery power by using the engine less or by revving the engine or a combination of the two. Don't worry about it, it's normal too.

The other weird thing the car does is make a loud humming noise when you first switch on the a/c on a hot day. This is normal too and it soon dies down.

Any other questions, please ask.

 

Outside of ordinary driving it is possible to drain the big high-voltage battery, and that would be bad. Read the owner's manual. (Short answer: don't use Neutral, and don't leave the car in IG-ON.)

 

Either model 12117 or the AGM specific model 12118 will work fine. The list for $50 and $55 respectively, but are available on the web for a little less $. I prefer these over the BatteryTender brand as the Battery Minders will perform desulfating.

JeffD