Voltage-based State of Charge (SoC) measurements offer a quick and convenient way to estimate battery capacity, but their reliability varies widely among battery chemistries and usage conditions.
Overview
As a battery charges, its voltage rises. Each battery chemistry has distinct voltages that indicate full charge and complete depletion:
| Battery Chemistry | Nominal Voltage (V) | Empty Voltage (V) | Full Voltage (V) |
|---|---|---|---|
| Lithium-Ion (Li-ion) | 3.6-3.7 | 3.0 | 4.2 |
| Lithium Polymer (LiPo) | 3.7 | 3.0 | 4.2 |
| Lithium Iron Phosphate (LiFePO₄) | 3.2 | 2.5 | 3.65 |
| Nickel-Cadmium (NiCd) | 1.2 | 1.0 | 1.4 |
| Nickel-Metal Hydride (NiMH) | 1.2 | 1.0 | 1.4 |
| Lead-Acid | 2.0 | 1.75 | 2.1 |
| Alkaline | 1.5 | 0.8 | 1.6 |
| Zinc-Carbon | 1.5 | 0.8 | 1.6 |
| Lithium Thionyl Chloride (Li-SOCl₂) | 3.6 | 3.0 | 3.67 |
| Lithium Manganese Dioxide (LiMnO₂) | 3.0 | 2.5 | 3.2 |
| Silver Oxide | 1.55 | 1.2 | 1.6 |
By measuring actual battery voltage and relating it to battery chemistry, you can estimate the current state of charge.
Caveats
The reliability of voltage-based SoC measurements largely depends on the battery chemistry and several factors:
-
Discharge Curve: Some chemistries have a very flat discharge curve, meaning that voltage remains nearly constant for a wide range of SoC levels. LiFePO₄ batteries are a prime example. While this stability in voltage is useful for consistent power delivery, it limits the effectiveness of voltage alone for SoC estimation.
-
Load Effects: When a load is applied, internal resistance causes a voltage drop. For accurate SoC estimates, it’s best to measure voltage at rest (after no load for 10-20 minutes). Load-adjusted measurements, which involve measuring voltage under a known, constant load, can also provide useful data for SoC.
-
Temperature: Environmental temperature affects battery voltage. Cold temperatures can lower voltage and reduce capacity, while high temperatures can increase voltage.
| Battery Chemistry | Voltage SoC Reliability | Alternative Methods Recommended |
|---|---|---|
| Lithium-Ion (Li-ion), Lithium Polymer | Moderate to High | Coulomb counting under load |
| Lithium Iron Phosphate (LiFePO₄) | Low to Moderate | Coulomb counting or SoC curves |
| Nickel-Cadmium, Nickel-Metal Hydride | Low | Time tracking, coulomb counting |
| Lead-Acid | Moderate | Resting voltage after load |
| Alkaline, Zinc-Carbon | Moderate | Not commonly monitored |
| Lithium Thionyl Chloride (Li-SOCl₂) | Low | Load test for SoC |
| Lithium Manganese Dioxide (LiMnO₂) | Moderate | Coulomb counting for precision |
Tips for Reliable Voltage-Based SoC Measurements
- Measure in a stable temperature environment.
- Avoid measuring voltage under load; if possible, let the battery rest for 10-20 minutes before reading voltage.
- Use load-adjusted measurement techniques by applying a known load (like a resistor or electronic load) when more accuracy is needed.
Coulomb Counting
Voltage measurement is an indirect way to estimate SoC, and it’s subject to limitations. Coulomb counting is the only highly accurate method for tracking SoC.
With Coulomb Counting, you monitor the energy going into the battery during charging and the energy drawn during discharge. This allows precise tracking of remaining capacity. Many battery management systems (BMS) use Coulomb counting for reliable SoC monitoring, and specialized integrated circuits are available for use in smaller devices, like smartphones.
This is why voltage-based SoC indicators are less common in digital devices that require high accuracy. Voltage-based SoC measurements are more often used in RC modeling and other applications where only a rough estimate of charge status is needed.
Balancing
Balancing is a key function in battery management, often handled by Battery Management Systems (BMS). In battery packs made up of multiple cells, performance is optimized when all cells maintain the same voltage. However, over time, cells can become imbalanced, where some cells hold higher or lower voltages than others. This imbalance reduces overall capacity because charging must stop once any single cell reaches its full charge voltage, even if other cells are not fully charged.
Some advanced battery voltage testers also offer balancing features. These testers measure the voltages of individual cells within a pack, and if they detect imbalances, they can shift energy from higher-voltage cells to lower-voltage cells. This balancing process typically moves energy gradually, over several hours, as only small amounts are transferred at a time. When complete, all cells in the pack reach equal voltage, restoring capacity and improving overall pack health.
Balancing testers are especially valuable tools when building custom battery packs, such as packs assembled from recycled cells, ensuring all cells operate at matched voltages for maximum efficiency.
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(content created Nov 10, 2024)