Battery Calculator
Runtime
Capacity printed on the battery label, in milliamp-hours.
Power draw of your device. Switch unit (mW/W/mA) using the dropdown.
Reserve capacity to prevent deep discharge. 80% is recommended for Li-ion to extend cycle life.
Enter Battery Details
Fill in your battery capacity, voltage, and device load to see runtime estimates, charge time, pack specs, or unit conversions.
Runtime, charge time, pack builder, and unit converter for any battery chemistry
Batteries power almost every aspect of modern life — from the smartphone in your pocket to the electric vehicle in your driveway, and from the tiny IoT sensor on a factory floor to the emergency backup system in a hospital. Yet despite their ubiquity, battery behavior can seem mysterious. Why does your phone last eight hours one day and only four the next? How long will it take to charge a depleted power bank? What happens when you wire cells in series versus parallel? This battery calculator answers all of these questions with precise, chemistry-aware formulas used by engineers and electronics enthusiasts alike. The Runtime Calculator is the most commonly used mode. You enter your battery capacity in milliamp-hours (mAh), the nominal voltage, and the power draw of your device in milliwatts (mW), watts (W), or milliamps (mA). The tool then applies a usable-capacity percentage — typically 80% to protect battery longevity by preventing deep discharge — and a system-efficiency factor to account for voltage conversion losses in the device's power management circuitry. The result is a realistic runtime estimate shown in hours and minutes, plus a three-scenario breakdown for light (50% load), normal, and heavy (150% load) usage patterns. Built-in device presets for smartphones, tablets, laptops, power banks, IoT sensors, and AA-battery devices let you get results in seconds. The Charge Time Calculator models how long it takes to replenish a battery from a given State of Charge (SoC) to a target level using a specified charger current. Critically, it accounts for battery chemistry: lithium-based cells (Li-ion, LiPo, LiFePO4) use a two-phase CC/CV charging protocol where the constant-current (CC) phase fills roughly 80% of capacity quickly, followed by a constant-voltage (CV) top-off that adds approximately 20% extra time. NiMH batteries suffer significant heat losses during charging — only about 66% of the energy delivered actually enters the cell — so they require a correction factor. Lead-acid cells sit at around 85% efficiency. The calculator shows both the CC phase time and the total CC+CV time for lithium cells, displayed as a stacked bar chart so you can visualize exactly where your charging time is spent. The Pack Builder mode is essential for anyone designing multi-cell battery packs for electric bikes, RC vehicles, solar storage, or custom electronics. You specify the individual cell's voltage and capacity, then choose how many cells to connect in series (S) and in parallel (P). Series connections increase voltage while keeping capacity constant; parallel connections increase capacity while keeping voltage constant. The tool outputs the pack's total voltage, total capacity in mAh and Ah, total energy in watt-hours, cell count, and the industry-standard configuration label (e.g., 3S2P). One-click cell presets for popular formats — 18650, 21700, LiPo 1S, LiFePO4, and AA — speed up the design process. The Unit Converter makes it effortless to translate between the three common ways to express battery energy. Milliamp-hours (mAh) is the most familiar unit, typically printed on battery labels. Amp-hours (Ah) is the same quantity scaled by a factor of 1000, preferred in automotive and industrial contexts. Watt-hours (Wh) is the true energy unit, obtained by multiplying Ah by voltage — and it is the only unit that allows fair comparison between batteries of different voltages. The converter also shows a visual bar chart comparing your battery capacity against common reference batteries (AAA through D cell) so you can immediately understand where your battery sits on the scale. For all four modes, results can be exported to CSV for use in spreadsheets, project documentation, or engineering reports with a single click. The calculator auto-recalculates as you type, so adjusting any input instantly updates all outputs without needing to press a button. Whether you are a student, hobbyist, electronics engineer, or simply a curious consumer trying to understand the battery in your device, this tool provides the accurate, chemistry-aware calculations you need.
Understanding Battery Calculations
What Are mAh, Ah, and Wh?
Battery capacity is most commonly expressed in milliamp-hours (mAh) — the amount of charge a battery can deliver, measured as current (milliamps) multiplied by time (hours). A 3000 mAh battery can, in theory, supply 3000 mA (3 A) for one hour, or 300 mA for ten hours. Amp-hours (Ah) is the same measurement scaled up: 1 Ah = 1000 mAh. Watt-hours (Wh) convert this to a true energy unit by accounting for voltage: Wh = Ah × Voltage. Because a 3.7 V Li-ion cell and a 1.5 V alkaline cell with the same mAh rating store very different amounts of energy, Wh is the only reliable basis for cross-chemistry comparisons. For example, a 3000 mAh Li-ion cell at 3.7 V stores 11.1 Wh, while a 3000 mAh alkaline AA at 1.5 V stores only 4.5 Wh — less than half the energy. Always check both mAh and voltage when comparing batteries.
How Is Battery Runtime Calculated?
Runtime estimation requires converting both battery energy and device load to the same unit — watt-hours and watts respectively. The formula is: Runtime (h) = Usable energy (Wh) ÷ Device load (W). Usable energy accounts for a depth-of-discharge limit to protect battery health — typically 80%, meaning only 80% of total capacity is drawn before recharging. A system efficiency factor (usually 85–95%) models the losses in the device's power regulator or DC-DC converter. For charge time, the basic formula is: Charge time (h) = Capacity to charge (mAh) ÷ (Charge current (mA) × Charging efficiency). Lithium-ion cells add a CV phase multiplier of approximately 1.2× because the final 20% of capacity is replenished slowly at a tapering current. NiMH batteries require dividing by roughly 0.66 to account for the ~34% of charge energy lost as heat during the charging process.
Why Does Battery Chemistry Matter?
Different battery chemistries have fundamentally different characteristics that affect how they should be charged and discharged. Lithium-ion and LiPo cells offer the best energy density and near-100% coulombic efficiency, but they require careful voltage management — overcharging above 4.2 V or discharging below 2.5 V causes permanent damage or safety risks. LiFePO4 (lithium iron phosphate) is safer and has a longer cycle life (2000–5000 cycles) at the expense of slightly lower voltage (3.2 V vs 3.7 V). NiMH batteries are robust, environmentally safer, and tolerant of overcharging, but their charging efficiency is only about 66% because much energy is lost as heat — this is why NiMH chargers can get warm. Lead-acid batteries, used in cars and UPS systems, have the lowest energy density but exceptional surge current capability and very long calendar life when properly maintained. Choosing the right chemistry for your application — and using chemistry-correct charging parameters — dramatically impacts both performance and safety.
Limitations and Real-World Factors
All battery calculations are theoretical estimates based on rated specifications. Real-world performance typically falls short due to several factors. Temperature has a major effect: batteries lose 15–20% capacity in cold weather (below 0°C/32°F) and suffer accelerated degradation in heat above 40°C (104°F). Battery age matters too — after 300–500 full cycles, a Li-ion cell typically retains only 80% of its original capacity, and the degradation continues with each subsequent cycle. The C-rate (ratio of charge/discharge current to capacity) affects effective capacity: high discharge rates draw more current than the battery can efficiently supply, reducing the usable energy. Peukert's Law describes this effect mathematically for lead-acid and NiMH batteries. Self-discharge is another loss mechanism: most batteries lose 1–5% of charge per month even when not in use (NiMH loses more, around 20% per month without low-self-discharge technology). These factors mean real runtimes are typically 10–30% lower than calculator predictions, and real charge times can be longer than estimated, especially for aged batteries.
How to Use the Battery Calculator
Choose Your Calculation Mode
Select one of the four tabs at the top: Runtime for estimating how long a battery will last, Charge Time for how long it takes to recharge, Pack Builder for designing multi-cell battery packs, or Unit Converter to translate between mAh, Ah, and Wh.
Enter Battery and Device Details
For Runtime mode, use the device preset dropdown to auto-fill values for common devices like smartphones or laptops, or enter your own capacity (mAh), voltage (V), and device load. Select the load unit (mW, W, or mA) that matches your device's spec sheet. Adjust the usable capacity percentage (default 80%) and system efficiency (default 90%) for more accurate real-world results.
Select Battery Chemistry (Charge Time Mode)
In Charge Time mode, select your battery chemistry from the dropdown. This automatically applies the correct charging efficiency factor — 99% for lithium cells with CC/CV modeling, 66% for NiMH, 70% for NiCd, and 85% for lead-acid. Enter the starting and target State of Charge percentages for partial-charge calculations. Li-ion results show both the fast CC phase and the total CC+CV time as a stacked bar.
Read Results and Export
Results update automatically as you type. The Runtime tab shows a ProgressRing for usable capacity plus a three-scenario bar chart for light, normal, and heavy usage. Pack Builder shows a donut chart of series vs. parallel cell contribution. Click Export CSV to download all results as a comma-separated file for use in spreadsheets or documentation.
Frequently Asked Questions
What is the difference between mAh and Wh?
Milliamp-hours (mAh) measures electric charge — how many milliamps a battery can deliver per hour. Watt-hours (Wh) measures energy, which factors in voltage: Wh = mAh × V ÷ 1000. A 3000 mAh Li-ion cell at 3.7 V stores 11.1 Wh, whereas a 3000 mAh AA alkaline at 1.5 V stores only 4.5 Wh. mAh is convenient for comparing batteries of the same voltage, but Wh is the only unit that allows fair comparisons between different chemistries and voltages. Most laptop batteries and power tools now list Wh on the label for this reason. Always use Wh when trying to determine whether a power bank can charge your laptop, since their voltages differ significantly.
Why does my phone battery die faster than the calculator predicts?
Several real-world factors cause actual runtimes to fall short of theoretical predictions. Battery aging is the biggest factor — after 300–500 charge cycles a Li-ion cell retains only about 80% of its original capacity, and the decline continues. Temperature plays a major role: cold weather below 0°C reduces available capacity by 15–30%. Screen brightness, cellular signal strength, GPS use, and background app activity all vary the actual load. The calculator's usable-capacity and system-efficiency inputs let you model these losses — try reducing usable capacity to 70% for an aged battery, or lower efficiency to 80% if the device has a less efficient power regulator. These adjustments typically bring predictions much closer to observed real-world performance.
What does the CC/CV two-phase charging model mean?
Lithium-ion and LiPo batteries use a two-phase charging protocol called CC/CV (Constant Current / Constant Voltage). During the CC phase, the charger delivers a fixed current (e.g., 1C) and the battery voltage rises steadily. This phase fills roughly 80% of total capacity and is the fastest part of charging. Once the battery reaches its maximum voltage (4.2 V for standard Li-ion), the charger switches to CV mode, where it holds the voltage constant while the current tapers off. This top-off phase replenishes the remaining ~20% but takes additional time — typically adding about 20% to the CC phase duration. Our charge time calculator shows both phases separately as a stacked bar and applies the 1.2× multiplier to give you the true total charge time, not just the misleading fast-charge time some manufacturers advertise.
How do series and parallel cell connections work in a battery pack?
In a battery pack, connecting cells in series increases the total voltage while keeping capacity the same. Three 3.7 V cells in series gives a 11.1 V pack, which is why many laptop batteries are 3S or 4S configurations. Connecting cells in parallel keeps the voltage the same but multiplies the capacity — two 3000 mAh cells in parallel give 6000 mAh at the same voltage. Most practical packs combine both: a 3S2P pack has three groups of two parallel cells connected in series, giving higher voltage and higher capacity simultaneously. The configuration label (e.g., 3S2P) is an industry-standard shorthand used in RC vehicles, electric bikes, and custom electronics. Our Pack Builder calculates total voltage, capacity, energy, and cell count for any S and P combination.
Why does NiMH take longer to charge than Li-ion at the same current?
NiMH batteries have a charging efficiency of only about 66%, meaning roughly one-third of the energy delivered by the charger is lost as heat during the chemical reaction. To put 1000 mAh of charge into a NiMH cell, you must actually push about 1515 mAh through it. Li-ion cells have near-100% coulombic efficiency — almost all the charge you put in is stored. This makes NiMH chargers slower and warmer than lithium chargers at equivalent currents. NiMH also lacks a clear end-of-charge voltage signature, which is why NiMH chargers use temperature detection or -ΔV (voltage drop) sensing to terminate charging, rather than the clean CCCV cutoff used for lithium. The charge time calculator applies the correct 0.66 efficiency multiplier for NiMH automatically when that chemistry is selected.
What usable capacity percentage should I use?
The usable capacity setting prevents the calculator from assuming you will fully discharge the battery, which degrades cell life. For lithium-ion and LiPo cells, using 80% usable capacity (i.e., discharging to 20% remaining) is widely recommended to maximize cycle life — regularly discharging to 0% can halve the number of charge cycles before capacity degrades to 80%. For LiFePO4, a 90% usable setting is safe due to that chemistry's flat discharge curve and better tolerance for deep discharge. Lead-acid batteries should be kept above 50% state of charge to avoid sulfation damage — use 50% usable capacity for flooded lead-acid, or up to 80% for AGM and gel types. For a rough runtime estimate without worrying about longevity, you can set usable capacity to 100%, though this will overestimate real-world runtime.