Help me understand Dr. Prius App Info

The number in the middle box called "voltage diff" is a good way to predict when you battery pack is going to throw error lights on the dash. A healthy pack will have a voltage difference of approximately 0.09v to 0.24v.

As the pack ages that number will start moving up briefly into 0.30v range and then once it gets higher like an occasional 0.64v the dash lights come on and the pack needs to be rebuilt or replaced.

 

+1 to Priuscamper.
The battery ecu monitors the voltage of each of the 14 "blocks". Each block is made up of 2 "modules". Each sealed module has 6 nickel metal hydride cells at 1.2V each. Since all the cells and modules are connected in series, the entire pack is 1.2V x 6 (for 7.2V per module) x 28 modules total = 201.6V (nominal). The actual voltage for the pack will vary quite a bit as the State Of Charge changes.
As stated one of the most important pieces of data is the Voltage Diff-erential. It displays the difference between the highest and lowest voltage levels of any of the battery blocks. On a new battery that might be around 0.1V. As the battery ages and deteriorates, its capacity decreases. As long as all modules remain fairly even, (say, around 0.3V under charge or discharge) then the pack (and car mpg) will still be reasonably good.
When the difference gets higher for longer periods of time, then the ecu will set codes. A common failure is for a weak cell in a module to "short" and suddenly you have a block that is about 1.2V low under discharge, but with higher voltage when charging. Also the SOC will change from near full to empty and back fairly quickly.

Posted via the PriusChat mobile app.

 

If anyone knows, what are the critical criteria for internal resistance? What is a good level? What is bad and how does one use it to predict failure?

 

internal resistance measure and battery capacity measure are similar... If your charge status screen fluctuates wildly you have low capacity / bad internal resistance and need to do some deep cycling of your battery pack to restore / recondition you NiMH pack: BU-807: How to Restore Nickel-based Batteries - Battery University

 

Is there some absolute value threshold of IR that is an indication of battery decline? What is the IR of a new battery?

 

I just look for values to be fairly even- within a few milli ohms of each other- mostly I don't pay it much attention. If something is way off then I would start looking at the voltage sense harness and the battery ecu connection.

Posted via the PriusChat mobile app.

 

It's more about all the modules still matching and working together as the pack ages because you can have really bad IR and still have a good battery pack that's never failed as long as all the modules are aging in the same way.

 

19 mΩ.

I don't know if there is an absolute value threshold but from what I've seen any value up to the low 30mΩ mark seems to still work. You'd probably start becoming concerned if any starts getting to the mid 30's or higher. The higher the value the more heat will be a problem.

As others have said in and of itself it is not something on which to focus. By the time the IR is getting high, other indicators will be shouting out louder.

 

From screenshots I’ve seen, elsewhere, Dr Prius changes the IR icon from green to yellow in the low thirties, so I was curious if anyone knew what was magical about that threshold.

It appears from these comments that that criterion is not significant in and of itself especially if all cells are within a mOhm or two of each other?