Battery Capacity & Energy Calculator

Calculate battery energy, runtime, and charge cycles from capacity and voltage specifications.

Battery capacity (mAh) (mAh)

Nominal voltage V (V) (V)

Device power consumption P (W) (W)

Depth of discharge DoD (%) (%)

80% = don't discharge below 20%

Please check your inputs and try again.

Capacity, Energy and Why mAh Alone Is Misleading

Capacity in mAh counts charge, not energy. Two batteries with identical mAh ratings store different amounts of energy if their voltages differ, which is why comparing mAh across chemistries is meaningless.

Energy (Wh) = Capacity (Ah) × Nominal voltage (V)

Chemistry	Nominal voltage	1,000 mAh equals
Li-ion / LiPo	3.7 V	3.7 Wh
LiFePO₄	3.2 V	3.2 Wh
NiMH	1.2 V	1.2 Wh
Alkaline	1.5 V	1.5 Wh
Lead-acid cell	2.0 V	2.0 Wh

A 1,000 mAh lithium cell therefore holds three times the energy of a 1,000 mAh NiMH cell. Power banks are usually rated at cell voltage (3.7 V) but deliver at USB voltage (5 V), which is why a “10,000 mAh” bank charges a 3,000 mAh phone fewer times than the arithmetic suggests — conversion losses take a further 10 to 20%.

Depth of Discharge and Cycle Life

Depth of discharge	Typical Li-ion cycle life	Total energy delivered
100%	300–500 cycles	Baseline
80%	1,000–1,500 cycles	Higher
50%	2,000–3,000 cycles	Much higher
20%	> 10,000 cycles	Highest per unit capacity

Shallower cycling extends life disproportionately, which is why electric vehicles and grid storage reserve buffer capacity at both ends rather than using the full range. A pack cycled to 50% depth delivers far more total energy over its lifetime than one cycled to 100%, despite each cycle giving less.

C-rate expresses current relative to capacity: 1C discharges the full capacity in one hour, 2C in half an hour. High C-rates generate heat and reduce both usable capacity and cycle life.

Worked Examples

Example 1: Phone battery: 3000mAh, 3.7V, DoD=80%

Energy=11.1Wh, usable=8.88Wh

Result: At 2W consumption: 4.44 hours runtime

Typical smartphone daily use

Example 2: EV battery: 75kWh=75000Wh at 400V

C=75000Wh/400V×1000=187,500mAh

Result: Range at 200Wh/km: 375km

Tesla Model 3 LR approximate

Example 3: Power bank reality

10,000 mAh at 3.7 V, output at 5 V, 85% efficient

Result: About 6,290 mAh usable at 5 V

Energy is 37 Wh; at 5 V and 85% efficiency that gives roughly 6.29 Ah — enough for about two full charges of a 3,000 mAh phone, not three.

Example 4: Cycle life trade-off

Same pack at 100% versus 50% depth of discharge

Result: 50% depth delivers more lifetime energy

400 cycles at full depth gives 400 capacity-units. 2,500 cycles at half depth gives 1,250 — over three times the total energy from the same pack.

Common Mistakes

⚠️

Comparing mAh across different chemistries

mAh measures charge, not energy. Multiply by nominal voltage to get watt-hours before comparing a lithium cell with a NiMH one.

⚠️

Expecting a power bank to deliver its full rated mAh

Ratings are usually at 3.7 V cell voltage but output is at 5 V, and conversion loses 10–20%. Usable capacity is typically 60–70% of the printed figure.

⚠️

Discharging fully to maximise use

Deep discharge shortens cycle life sharply. Cycling between 20% and 80% delivers far more total energy over the battery's life.

⚠️

Ignoring temperature effects

Capacity falls significantly in cold conditions and high temperature accelerates degradation. Rated capacity assumes moderate temperature.

Frequently Asked Questions

Why DoD matters for battery life?

Lithium batteries degrade faster when fully discharged. DoD=80% means use only top 80% of capacity. Most EVs and phones protect the bottom 20%. Extending DoD from 100% to 80% can double cycle life.

C-rate?

C-rate = current / capacity. 1C charges in 1 hour. 2C charges in 30 min (faster degradation). 0.5C (gentle, best for longevity). Fast chargers use 2-4C for first 80%, then slow down.

Why can't I compare mAh between battery types?

Because mAh measures charge, not energy. A 1,000 mAh lithium cell at 3.7 V holds three times the energy of a 1,000 mAh NiMH cell at 1.2 V.

Why does a 10,000 mAh power bank not fully charge my phone three times?

The rating is at 3.7 V cell voltage while output is at 5 V, and conversion is 80–90% efficient. Usable capacity is typically 60–70% of the printed number.

Why does shallow discharging extend battery life?

Deep discharge stresses the electrode structure. Cycling between 20% and 80% can multiply cycle life several times over, delivering more total energy despite less per cycle.

What does C-rate mean?

Current relative to capacity. 1C discharges the full capacity in one hour; 2C does it in thirty minutes. Higher rates generate heat and reduce usable capacity.

Formula Explorer connections

Interpretation: This formula links electron transfer, charge, potential, current or ionic transport in an electrochemical system. Assumption: Balance electron count and half-reactions, preserve sign conventions, and use consistent concentration, temperature and electrical units. Real cells include losses and overpotential.

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