ADC Mismatch

Despite the implementation of three-level protection systems in Lithium BMS, several major brands have experienced fires within the first six months of operation. This issue has occurred regardless of the UPS brand or the type of Lithium chemistry used.

The question is whether a 12-bit ADC is sufficient to measure small voltage variations in high-voltage lithium battery packs (500 V to 800 V) used in data center UPS systems, where charging decisions are typically based solely on voltage values. While it's possible to integrate additional data, this is seldom done due to practical challenges in the field

Understanding Voltage Operating Windows

Side note: this analysis is based on theoretical values without real-world testing or data. We are not claiming to have scientifically evaluated these findings, but it's a thought worth considering.

Lithium-Ion Batteries:

• Per-Cell Voltage Range:
• LiFePO₄: 2.5 V (fully discharged) to 3.65 V (fully charged)
• NCM: 3.0 V (fully discharged) to 4.2 V (fully charged)
• Voltage Window per Cell: Approximately 1.15 V to 1.2 V
• Total Voltage Variation: When cells are connected in series, the total voltage variation is the per-cell voltage window multiplied by the number of cells.

Lead-Acid Batteries:

• Per-Cell Voltage Range: 1.75 V (fully discharged) to 2.4 V (fully charged)
• Voltage Window per Cell: Approximately 0.65 V
• Total Voltage Variation: Larger voltage swings due to fewer cells needed to reach high voltages.

Calculating ADC Resolution

To determine if a 12-bit ADC is sufficient, we'll calculate the voltage resolution and compare it to the required measurement precision.

ADC Resolution Calculation:

Resolution (ΔV):

ΔV = (Vmax - Vmin) / (2n - 1)
                            

Where:

• Vmax = Maximum measurable voltage
• Vmin = Minimum measurable voltage
• n = Number of bits in the ADC

Parameters:

Total Battery Voltage Range: Let's assume the battery pack operates between 500 V and 800 V.

Voltage Window (Battery Operating Range): Vmax - Vmin = 800 V - 500 V = 300 V

Number of ADC Levels: For a 12-bit ADC, 2¹² = 4096 levels.

Voltage Resolution with a 12-bit ADC:

ΔV12-bit = 300 V / 4096 - 1 ≈ 300 V / 4095 ≈ 0.073 V (73 mV)
                            

Assessing Measurement Requirements

Voltage Variation Sensitivity:

Per-Cell Sensitivity: For LiFePO₄ cells, a voltage change of 0.01 V per cell can be significant.

Total Pack Sensitivity: For a pack of 200 cells in series:

Total Voltage Change = 200 cells × 0.01 V = 2 V
                            

Critical Events: Overcharging by even a small voltage can lead to safety risks.

Required ADC Resolution:

To detect a total voltage change of 2 V within a 300 V range, the ADC resolution must be finer than:

ΔVrequired = (2 V) / (Desired Number of Levels)
                            

Let's aim for at least 100 discernible levels within the 2 V range for adequate precision:

ΔVrequired = 2 V / 100 = 0.02 V (20 mV)
                            

Comparison with 12-bit ADC Resolution:

12-bit ADC Resolution: Approximately 73 mV per step.

Required Resolution: 20 mV per step or finer.

Conclusion: A 12-bit ADC cannot discern voltage changes smaller than 73 mV, which is insufficient for detecting critical voltage variations in the battery pack.

What about 14bit, 16bit or higher?

14bit or 16bit seems to be better choice. See the calculation.

• 14-bit ADC:
  
  $2^{14} = 16,384$ levels $$\Delta V_{\text{14-bit}} = \frac{300\,\text{V}}{16,384 - 1} \approx 0.0183\,\text{V (18.3 mV)}$$
  • This meets the required resolution of 20 mV.



• 16-bit ADC:
  
  $2^{16} = 65,536$ levels $$\Delta V_{\text{16-bit}} = \frac{300\,\text{V}}{65,535} \approx 0.00458\,\text{V (4.58 mV)}$$
  
  
  • Provides even greater precision and safety margin.

Summary

• 12-bit ADC Resolution: Approximately 73 mV per step over a 300 V range.
• Required Resolution: At least 20 mV per step to detect critical voltage changes.
• Recommendation: Use higher-resolution ADCs (14-bit or 16-bit) and integrate advanced battery monitoring systems for safe and effective UPS operation with lithium batteries.