Interpretation of the relationship between voltage changes and battery quality during the repair process of nickel hydrogen batteries in hybrid vehicles

hi, as you already elaborated to me the difference of each step of 4 cycles discharge & charge to NiMH battery reconditioning, each cycles discharge capacity data represents the stage and quality of the battery,  the relationship between NiMH battery quality during reconditioning cycles, temperature changes, and the factors  including the 4th cycle discharging capacity result represents battery quality etc, hence I have further questions to ask: how is the voltage change in NiMH battery reconditioning process to bring impact to the battery quality, what is the best voltage & current setting to ensure max. good result can be achieved of reconditioning result?

Great to dive deeper into NiMH battery reconditioning with you! Let’s tackle your questions about voltage changes and optimal settings for reconditioning.

Voltage Change and Its Impact on Battery Quality During Reconditioning

Voltage changes during the reconditioning process of NiMH batteries are a key indicator of the battery’s internal state and directly influence the effectiveness of the reconditioning process. Here’s how it works:

Discharge Phase Voltage Drop:  

When you discharge a NiMH battery, the voltage gradually decreases as stored energy is depleted. A healthy NiMH cell typically starts around 1.2–1.4V (fully charged) and drops to about 0.9–1.0V at the end of a controlled discharge.  

If the voltage drops too quickly or stabilizes at a lower-than- expected value during early cycles, it may indicate issues like increased internal resistance, crystal formation (memory effect), or capacity loss due to aging. Reconditioning aims to break down these crystalline structures (dendrites) on the electrodes by fully discharging the battery, which can restore some capacity and stabilize voltage behavior.

Charge Phase Voltage Rise:  

During charging, the voltage rises as the battery accepts energy. For NiMH, the voltage typically peaks around 1.4–1.6V per cell near full charge, followed by a slight drop (the “-ΔV” signal) as the battery reaches saturation and converts excess energy into heat.

If the voltage rises too rapidly or overshoots significantly (e.g., beyond 1.6V), it could indicate overcharging, poor Cell Balancing, or damage, which degrades quality. Conversely, a sluggish voltage rise might suggest high internal resistance or sulfation like effects, reducing the effectiveness of  reconditioning.

Temperature and Voltage Interplay:  

Voltage changes are closely tied to temperature. As the battery charges, especially in later cycles, heat generation increases near full capacity. Excessive heat (e.g., above 45°C) can accelerate electrolyte degradation or damage the separator, lowering overall quality. Proper voltage monitoring helps avoid this by signaling when to stop charging.

Cycle-to-Cycle Voltage Trends:  

Over the 4-cycle reconditioning process, you’d ideally see the discharge voltage plateau (the flat part of the discharge curve) stabilize and extend, indicating improved capacity and reduced internal resistance. A consistent or improving voltage profile by the 4th cycle suggests successful reconditioning, while erratic or declining voltage points to persistent degradation.

In short, voltage changes reflect the battery’s ability to store and deliver energy efficiently. Reconditioning success hinges on managing these changes to minimize stress on the battery while maximizing capacity recovery.

Optimal Voltage and Current Settings for Reconditioning:

To achieve the best reconditioning results for NiMH batteries, you need a balance of voltage and current that fully exercises the battery without causing damage.

Here’s a practical guide based on established practices:

Trickle Charge (Optional): After full charge, a trickle current of 0.02C-0.05C (40-100mA for 2000mAh) can maintain the battery without overcharging, though this isn’t always necessary for reconditioning.

Temperature Control:  

Keep the battery temperature between 20℃ & 40℃during the process. Above 45℃, chemical degradation accelerates, undoing reconditioning benefits. Use a charger with thermal monitoring if possible.

Cycle Recommendation:  

Perform 3–4 full discharge/charge cycles. By the 4th cycle, the discharge capacity should stabilize, giving you a clear picture of the battery’s recovered quality. If capacity doesn’t improve after 4 cycles, the battery may be too degraded to salvage.

Practical Example

For a 2000mAh NiMH AA battery:  

Discharge at 400mA (0.2C) down to 1.0V.  

Charge at 1000mA (0.5C) until the voltage hits 1.45V or the -ΔV signal triggers.  

Repeat for 4 cycles, monitor capacity and temperature each time.

Key Factors for Success

- Charger Quality: Use a smart charger with -ΔV detection and individual cell monitoring to avoid overcharging or imbalance in multi-cell packs.  

- Consistency: Stick to the same settings across cycles for comparable results.

- Battery Age/Condition: Older or heavily sulfated batteries may not respond as well, even with optimal settings.

By carefully controlling voltage and current, you can maximize

capacity recovery while minimizing wear.