Dr Prius and the Gen4 Prime (bad news)

I prefer looking at the road than a phone screen thank you and analyse the data afterwards in a proprietary MS spreadsheet.

20mV is 0.02 which is within that 0.1 to 0.01 you mention btw. The 0.001 is the precision of the data and it's what is reported by the car BMS system, not some off the shelf thingy. If you don't want to believe what the car is giving you as number, then why would you believe what is Dr Prius giving you? Its answer is based on the same data...

 

Update, I just did another test of Dr Prius data gathering but this one, was through he whole battery capacity (from 100% to 1% as displayed in the car, which is roughly 85% to 15% of the full pack) and on hilly terrain with 'spirited' driving. Unlike in 2024 where I manually did the Excel Spreadsheet and did the analysis manually, I gave the CVS files to Google Gemini and told it to analyze and tell me how the pack compares to 2024. Here's the result:

Battery Pack Voltage Variance & Health Analysis (March 2024 vs. July 2026)
A comparative analysis of cell voltage variance (standard deviation and inter-cell $\Delta V$) between the drive log from March 2024 and the drive log from July 2026 demonstrates remarkable stability. The minor differences observed are fully attributable to operating conditions (state of charge and temperature) rather than hardware degradation.

1. Key Voltage Dispersion Metrics
Metric March 2024 Drive (SOC 44% to 66%) July 2026 Drive (SOC 14% to 87%)
Median $\Delta V$ (Max - Min cell voltage) 9.80 mV 9.80 mV
Mean $\Delta V$ 10.64 mV 11.64 mV
Peak $\Delta V$ Observed 23.10 mV 35.30 mV
Mean Cell Standard Deviation ($\sigma$) 2.18 mV 2.52 mV

2. Analysis of Key Differences
A. Identical Median Cell Imbalance (9.80 mV)
The median voltage difference between the highest and lowest voltage cells across the entire driving session is rigorously identical (9.80 mV) over two years apart. This proves that cell balancing (managed by the BMS) remains consistent and highly effective.

B. Depth of Discharge Impact (SOC < 20%)
The slight increase in average $\Delta V$ (11.64 mV vs. 10.64 mV) and the peak spike of 35.30 mV observed in July 2026 are explained by the state-of-charge range:

• March 2024: The battery operated strictly within the flat region of the Lithium Open Circuit Voltage (OCV) curve (44% to 66% SOC).
  
  
• July 2026: The trip discharged the pack down to 14.1% SOC.
  
  
• In Lithium-ion chemistry (NMC/NCA), the OCV curve steepens significantly below 20% SOC. Minor, natural capacity differences between cells manifest more prominently as voltage variations at lower charge levels.

When comparing both datasets within the exact same SOC range (40% to 60%):

• March 2024: Mean $\Delta V$ = 10.38 mV
  
  
• July 2026: Mean $\Delta V$ = 9.06 mV (slightly lower/better in 2026)
  

C. Response to High Current Load

• The peak $\Delta V$ in 2024 (23.1 mV) occurred during a heavy acceleration event drawing 182.6 A.
  
  
• In 2026, under similar or higher load demands (> 200 A), cell voltage variance remained well-controlled, averaging 11.8 mV under heavy load.
  

Summary & Conclusion
There are no signs of cell drift, divergence, or degradation. The slightly higher voltage delta observed at the end of the July 2026 drive reflects standard electrochemical behavior for a Lithium pack operating at a low state of charge (< 20% SOC). The pack remains in excellent health.

 

Went for another trip, this one a very gentle one, no abrupt starts or stops. Lasted 50 km before the battery was exhausted. Average speed around 75 km/h with a max around 85 km/h.

Here is the detailed analysis of the voltage spread (Delta V) recorded cell by cell during this last 50.1 km trip:

1. Overall Voltage Spread Summary ($\Delta V$)
State of Charge (SOC) Average Spread (ΔV) Median Spread Maximum Spread
80% to 87% (Trip start) 13.4 mV 13.4 mV 20.7 mV
60% to 80% 9.3 mV 9.6 mV 13.4 mV
40% to 60% (Mid-trip) 7.5 mV 7.3 mV 12.2 mV
20% to 40% 8.2 mV 8.4 mV 13.5 mV
< 20% (Trip end / 13.3%) 23.4 mV 26.8 mV 46.3 mV

2. Key Analysis Highlights
A. Exceptional Stability at Mid-Discharge (40% – 60% SOC)

• Between 40% and 60% SOC, the average difference between the highest and lowest cell is only 7.5 mV (with a median of 7.3 mV).
  
  
• This is an excellent result showing that all 95 cells have extremely consistent usable capacity and balance very well.

B. Behavior at Low State of Charge (SOC < 20%)

• The maximum spread reached 46.3 mV at the very end of the drive (at 13.3% SOC, about 100 meters from arrival).
  
  
• Why this gap? On Lithium-ion chemistry, the discharge voltage curve drops steeply below 20% SOC. Tiny, natural differences in capacity or internal resistance between cells manifest as voltage deviations when the pack is nearly empty. This is entirely normal behavior.

C. Key Cells Identified

• Lowest voltage cells under 20% SOC: Vol#27 and Vol#44.
  
  
• Highest voltage cells under 20% SOC: Vol#3 and Vol#78.
  
  
• Even under heavy load or regenerative braking, voltage drift remains tightly controlled above 20% SOC (averaging 11.3 mV under loads exceeding 50 A).

Conclusion
The overall voltage distribution throughout the trip confirms a very healthy battery pack. The increased spread below 20% SOC is a standard electrochemical characteristic and not a sign of a faulty cell.