Lithium batteries have become immensely popular in recent years. From smartphones to electric vehicles, lithium-ion batteries power some of our most essential devices. But how much of a lithium battery’s stated capacity is actually usable? What percentage should you rely on for real-world usage? In this comprehensive guide, as a professional lithium battery packs manufacturer, we’ll cover everything you need to know about usable lithium battery capacity.
Why Usable Capacity Matters
When selecting a lithium battery, you’ll typically see two capacity measurements:
- Rated capacity
- Usable capacity
The rated capacity refers to the battery’s maximum possible charge. However, for reasons we’ll cover shortly, you can only access a percentage of a lithium battery’s total storage. This usable portion is the key number to pay attention to.
Understanding usable capacity helps ensure you get the runtime you expect from a battery. It also prevents premature wear by avoiding over-discharging. Later in this guide, we’ll explore usable capacity percentages for different lithium battery chemistries.
Factors That Limit Total Capacity
While manufacturers rate batteries at a certain capacity, several variables prevent accessing 100% of that charge capacity. The two main limiting factors are:
Depth of Discharge Restrictions
Due to lithium battery chemistry, fully draining them can damage cell components. To maximize cycle life, system designers enforce cut-off voltages that leave a portion of charge unused. This buffer protects batteries but limits the operational capacity.
Typical maximum depth of discharge ratings are:
- Lithium cobalt oxide (LCO) – 80%
- Lithium manganese oxide (LMO) – 80%
- Lithium nickel cobalt manganese (NCM) – 80%
- Lithium nickel manganese cobalt oxide (NCA) – 90%
- Lithium titanate oxide (LTO) – 100%
- Lithium iron phosphate (LFP) – 100%
So in an LFP battery rated for 100Ah capacity, for example, you can theoretically access the full 100Ah. However, an NCM battery with the same rating would max out around 90Ah.
Voltage Cutoffs
In addition to depth of discharge limits, lithium battery management systems enforce voltage cut-offs. This protects cells from dropping below safe voltage levels during discharge.
For most lithium batteries, the low voltage cutoff sits between 2.5V to 3.0V per cell. This further limits access to maximum capacity to ensure safe operation.
Usable Capacity By Battery Chemistry
Now let’s examine usable lithium battery capacity for common lithium battery types. Keep in mind, these numbers are general guidelines that may vary between battery models. Always check specifications to confirm precise usable capacity ratings from manufacturers.
Lithium Cobalt Oxide (LCO)
Lithium cobalt oxide offers high energy density but has known stability issues. Most LCO cells restrict maximum depth of discharge to 80% to maximize cycle life.
Usable Capacity: 80% of rated capacity
Lithium Manganese Oxide (LMO)
With higher thermal stability than LCO, lithium manganese oxide also limits discharge to 80% to avoid cell damage.
Usable Capacity: 80% of rated capacity
Lithium Nickel Manganese Cobalt (NMC)
One of the most popular electric vehicle battery types, NMC offers a good balance of energy density, stability, and cost. Usable capacity varies by exact cathode composition but expect around 80%.
Usable Capacity: 80% of rated capacity
Lithium Nickel Cobalt Aluminum Oxide (NCA)
Used in Tesla electric vehicle packs, NCA cathodes enable very high capacity and high discharge rates. Cost and relative thermal instability balance those capabilities with a typical restriction of 90% max discharge.
Usable Capacity: 90% of rated capacity
Lithium Titanate (LTO)
LTO batteries offer the highest usable capacity thanks to exceptional stability and longevity. Maximum depth of discharge is typically listed at 100%.
Usable Capacity: 100% of rated capacity
Lithium Iron Phosphate (LFP)
The emerging star for renewable energy storage, LFP batteries also safely enable 100% depth of discharge for maximum usable capacity.
Usable Capacity: 100% of rated capacity
When Usable Capacity Changes
Keep in mind that battery capacity and depth of discharge capabilities will change over time.
Most lithium batteries rate lifespans covering thousands of charge cycles. However, usable capacity slowly decreases over those cycles as batteries wear.
Extreme operating temperatures also reduce how much of a charge is accessible on each use. Colder temps inhibit chemical interactions while heat accelerates component degradation.
Finally, the discharge rate impacts usable capacity. Faster discharge speeds limit capacity by exacerbating molecular instability. Slower discharge cycles enable accessing more of a battery’s maximum rating.
Key Takeaways On Lithium Battery Usable Capacity
While manufacturers state peak lithium battery capacity, engineering restrictions prevent accessing the full rated charge:
- Depth of discharge limits protect batteries but decrease usable capacity
- Voltage cutoffs also help avoid cell damage from over-discharge
- Usable capacity percentages vary from 80% to 100% by battery chemistry
- Real-world capacities decrease over battery lifespans
- Extreme temps and fast discharge rates also lower usable capacity
Understanding these dynamics helps you properly size lithium power systems and set runtime expectations. Consider usable capacity the key specification to pay attention to when comparing solutions.
Lithium Battery Management Maximizes Usable Capacity
Advanced battery management systems balance protecting lithium cells with optimizing usable capacity. For example:
Cell Balancing
To ensure safe, consistent performance, a BMS actively balances voltages between cells in a pack. This prevents individual cells from exceeding limits.
Thermal Controls
Whether air or liquid-based, thermal management regulates battery temperatures within safe operating ranges. This enables consistent access to maximum capacity.
Charge/Discharge Current Regulation
Actively controlling charge and discharge rates gives a BMS more headroom to safely access a battery’s full usable capacity. Slow speeds maximize capacity while fast speeds restrict it.
Accurate SOC/SOH Tracking
By closely monitoring state of charge and state of health metrics, a BMS knows exactly how much capacity remains usable. This prevents unexpected shutdowns from over-discharge.
Modern battery management technology balances usable lithium battery capacity with safety and longevity. That optimizes runtimes for superior real-world performance.
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