2007 Prius Hybrid Drain

Yes those are signs your battery is going.

 

It's not uncommon to lose bars when sitting but when it's several and the rapid charge/drain cycles appear, those are signs the battery is getting weaker. Eventually you will lose a module or two and the pack will fail (and car will only run in turtle mode).

I'd suggest you research "grid charging" and see if this is something for you. Also, check into new battery costs vs. how long you intended to keep this car.

 

Agree with others - the computer showing a large change in state of charge over night is a sign of the battery weakening. Losing a bar when parked overnight isn't rare, and two bars isn't too rare if parked for multiple days. Nickel Metal Hydride batteries self discharge 1-4% per day, even more on the first day, even more above room temperature, and even more once heavily used (which is true for you at 193k). Also keep in mind that the computer is estimating the state of charge of the battery - it estimates it based on voltage and current readings from the battery and is not spot-on accurate. When you have a very rapid drop in shown battery display capacity (without using a high amount of power - i.e. AC and accessories off) what's often occurring is a "negative calibration" - the computer is re-calibrating to a loss in battery capacity. As your battery will be colder in the morning and warmer during operation (which affects battery capacity chemically - colder batteries have less capacity), a little drop is okay, but more than a little is a sign of battery degradation. Using a grid charger to help even out the charge across the cells of your battery will likely extend your battery life. Of course at some point the batteries will fail and the cost and time of using a grid charger should be weighed against how long you intend to keep using the car and the cost of replacing the batteries and the car etc.

 

NOOOOOOOOOOOOOO!!!! That happened to me today. I let it sit for 5 days and it went to 2 bars.

 

This is a classic sigh of hybrid battery deterioration. If you do nothing, the red triangle is in your future. If you perform preventative maintenance reconditioning and start caring for the battery, you can recover lost capacity (less shallow cycling), restore regular EV mode, increase your MPGs, and keep the battery running for a few more years. Here is a before and after discharge test on an 05 Prius with a weak battery like yours. After 3 years now, it is still running strong:


You can read more about the symptoms of hybrid battery deterioration and how our products can help here:
Prolong Battery Systems FAQ - Hybrid Automotive