2005 Prius 106k miles

If worried about the Inverter make sure you have movement of Inverter coolant in the coolant reservoir. That makes sure the invert coolant pump has not died and the Invert is overheating.

But your problem is you have a 13 year old battery pack. If you continue to drive the car in limp mode that really hammers the battery.

Don't know what you mean by replacing it cold. Only way to make pretty sure the battery is working ok is to load test every individual battery module and make sure they all have pretty close charge/discharge behavior. They charge up correctly and will hold a charge properly. You really cant just buy a module off ebay and slap it in. that will not last long. That is known as whack a mole battery repair. If the battery ecu see's one module tanking it will fail the battery. It has too as all the battery's are in series. The battery as a whole is only as good as the weakest module. Every module has to be pretty close to proper capacity.

In a 13 year old pack i would suspect many modules are not performing well.

 

It's simply not worth swapping used battery modules mixed with old ones. It's an endless cycle that you will forever keep doing as long as you keep driving the car. It's only considered a temporary fix like putting a bandaid on rather than permantely fixing the problem. Brand new batteries is the only permanet solution if you plan on driving these cars for another 10+ years. With tbe age of the battery, it's already too late to try to save a dying battery chemistry.

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