Your e-bike range has dropped, or your solar bank runs dry by 2 AM. Is the battery dying, or is it just the weather? In this comprehensive diagnostic guide, we detail the equipment and protocols for performing a true Discharge Capacity Test, calculating SOH percentages, and deciding when to retire a lithium battery bank.

The Reality of Chemical Degradation

Batteries are consumable items. Unlike a solar panel which might degrade by 0.5% a year, a lithium-ion battery lives a life of chemical violence. Every time ions shuttle between the anode and cathode, the structure of the active material degrades slightly. The electrolyte oxidizes. The SEI layer thickens. This manifests in the real world as two symptoms: Capacity Fade (The tank gets smaller) and Power Fade (The pipe gets narrower).

Understanding the State of Health (SOH) of your battery is critical for reliability planning. If you are relying on a battery for medical backup or long-range commuting, "guessing" your range based on voltage is dangerous. You must measure the electrons. In this guide, we will walk through the rigorous procedure of benchmarking a used battery pack to determine if it is ready for a second life or the recycling bin.

1. The Metric: What is SOH?

State of Health is a percentage comparing the current reality to the factory specification.

Formula: $SOH = (Measured Capacity / Rated Capacity) imes 100$

The Industry Standards:
- 100% - 90%: Excellent. Like new.
- 90% - 80%: Acceptable aging. Noticeable range reduction, but safe.
- 80% (EOL): End of Life. In automotive and industrial applications, a battery is considered "dead" at 80%. Why? because below 80%, the degradation curve often becomes non-linear. It might take 5 years to get from 100% to 80%, but only 1 year to drop from 80% to 50%. The risk of internal dendrite formation and plating increases significantly.

2. The Tool: You Need an Electronic Load

You cannot test capacity by running your e-bike and watching the odometer. Too many variables (wind, hills, tire pressure) affect the result. You need a controlled laboratory discharge.
Required Gear: An Electronic Load (like the EBC-A20 or EBC-A40L) or a specific battery capacity tester (like the DL24P).
Why not a resistor? Light bulbs and resistors change their current draw as voltage drops (Ohm's Law). To get an accurate Amp-Hour reading, you need a Constant Current (CC) load that adjusts its resistance dynamically to keep the amperage steady as the battery drains.

3. The Testing Protocol

Follow this procedure strictly to get a scientifically valid number.

Step 1: The Saturation Charge

You must start at 100%. Charge the battery until the charger cuts off.
Crucial Detail: Allow the battery to "balance" on the charger for at least 4-6 hours after the light turns green. If your pack is unbalanced, the discharge test will be cut short by a single low cell, giving you a false failure reading. The starting voltage must be perfectly balanced.

Step 2: Thermal Normalization

Bring the battery to room temperature (25°C / 77°F).
Physics: Lithium ion mobility decreases in the cold. A battery tested at 10°C will show ~10% less capacity than one tested at 25°C. To compare your result against the manufacturer's datasheet, you must match their temperature conditions.

Step 3: Setting the Discharge Rate

If you discharge too fast, voltage sag will cut the test early.
Standard SOH Test: 0.2C (Capacity / 5).
Example: For a 20Ah battery, set the load to 4 Amps.
Performance Test: 1C (Capacity / 1).
Example: For a 20Ah battery, set load to 20 Amps.
For general health monitoring, use the 0.2C standard. This minimizes voltage sag and gives you a pure measurement of chemical capacity.

Step 4: The Cutoff Voltage

Set the electronic load to stop at the BMS cutoff voltage (usually 2.8V per cell for Li-Ion, 2.5V for LiFePO4).
Example for 13S (48V): $13 imes 2.8V = 36.4V$.
Start the test. The load will drain the battery until it hits 36.4V and then automatically stop counting.

4. Analyzing the Data: Ah vs. Wh

Your tester will give you two numbers: Amp-Hours (Ah) and Watt-Hours (Wh).

• Amp-Hours: Use this to compare against the Datasheet. If the cells were rated for 3000mAh and you got 2500mAh, your SOH is 83%.
• Watt-Hours: Use this to calculate Range. Range is energy. If your e-bike consumes 20Wh per mile, and your battery delivered 500Wh, you have a 25-mile range.

5. The False Positive: The "Unbalanced" Pack

If your test comes back surprisingly low (e.g., 50% SOH on a 2-year old battery), do not trash the battery yet.
Diagnosis: Monitor the individual cell group voltages near the end of the test.
If 12 groups are at 3.5V, but one group plummets to 2.8V causing the test to stop, your capacity is being bottlenecked by that single weak group. The rest of the battery might still have energy.
The Fix: Perform a Manual Balance on that low group and re-test. Often, you can "recover" massive amounts of capacity simply by realigning the cells.

6. Decision Time: Retire or Repurpose?

• > 80% SOH: Keep in primary service.
• 70% - 80% SOH: Degraded. Use for shorter trips. Monitor closely for self-discharge.
• < 60% SOH: Retire from high-current use. The internal resistance is likely too high for e-bikes or tools. These cells can be harvested and repurposed for low-drain applications like LED lighting or USB power banks, where they can live for another 5 years safely.

Summary

You cannot manage what you do not measure. An annual capacity test is the battery equivalent of a blood test. It gives you the data to predict failure, plan for replacements, and ensure that your critical backup systems will actually last as long as you need them to.