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VoltCalcs

Battery Calculators

Calculate battery runtime, charge time, capacity, and more. Every calculator works for all major battery types — LiFePO4, lead-acid, AGM, and lithium-ion.

Most people land on this page with one of three questions in mind: how long will a battery actually run my load, how long will it take to recharge, and how big a bank do I need for the days I want to go without sun or shore power. The calculators here answer those head-on, with the chemistry-specific math that generic spreadsheets skip. LiFePO4 holds 80–90% of its rated capacity at high discharge rates; flooded lead-acid loses 30–50% under the same load. AGM sits in between. The tools below account for that — depth-of-discharge limits, Peukert effects, inverter losses, and real-world temperature behavior — so the numbers you walk away with match what you'll measure on a meter, not what a marketing spec sheet promises.

The audience is mixed: weekend RV owners sizing a single drop-in to keep a fridge cold, full-time van-lifers building 600Ah banks, sailors planning a multi-day passage, and DIY off-grid builders putting together prismatic-cell packs. Pick the calculator that matches the question you're asking — start with the battery runtime calculator if you know your load and want hours, the charge time calculator if you've got a charger and want to know when you'll be full, or the capacity calculator if you're sizing from scratch. The chemistry-specific runtime tools (LiFePO4, AGM, lead-acid) are the right pick when the brand or chemistry is already locked in. The voltage-percentage and reserve-capacity tools answer the gauge-and-state-of-charge questions that come up mid-trip — “is this 12.4V reading still safe to discharge?” or “what does an 80-minute reserve-capacity rating mean for my actual fridge load?” Every calculator runs in your browser instantly, with no account wall, and every page cites the formula and source it's built on so you can re-derive the answer yourself.

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Battery Runtime Calculator

Calculate how long your battery will last under any load. Enter capacity, voltage, and wattage for an instant runtime estimate.

Propane Generator Run Time Calculator

Calculate how long a propane generator runs on a given tank size. Enter tank weight, load, and generator efficiency.

12V Battery Runtime Calculator

Calculate how long a 12 volt battery lasts under any load, with flooded, AGM, gel and LiFePO4 each corrected for discharge rate.

LiFePO4 Battery Runtime Calculator

Estimate LiFePO4 battery runtime with accurate DoD and efficiency defaults. Compare lithium iron phosphate vs lead-acid.

Portable Power Bank Runtime Calculator

Find how long your portable power bank charges your phone, tablet, or laptop. Enter mAh, voltage, and device wattage.

Battery Charge Time Calculator

Calculate how long a battery takes to charge from capacity, charger amps, and chemistry. Returns the bulk stage and the absorption stage separately.

Battery Charge and Discharge Calculator

Work out both halves of one battery cycle. Pick the chemistry and the tool follows the discharge and charge behaviour the makers actually print.

Cold Cranking Amps (CCA) Calculator

Size a starting battery from engine displacement and fuel type, using the original-equipment minimum CCA figures for cars and light trucks.

Battery Size for Inverter Calculator

Find the right battery capacity for your inverter. Enter load, runtime, voltage and chemistry: lead-acid banks need more amp-hours than lithium.

How Long Will a 12V Battery Run a Refrigerator?

Calculate how long a 12V battery powers your refrigerator. Works for power outages, camping, and off-grid setups.

Deep Cycle Battery Runtime Calculator

Estimate deep cycle battery runtime for marine, RV, and off-grid use. Accounts for depth of discharge and system efficiency.

Battery Capacity (Amp Hours) Calculator

Calculate battery capacity in amp-hours from watt-hours and voltage. Convert between Wh, Ah, and watts easily.

UPS Battery Backup Time Calculator

Work out how long your UPS runs on battery from the string inside it, the load you plug in, and the battery class it uses.

Laptop Battery Runtime Calculator

Estimate your laptop battery life based on battery Wh rating and actual power consumption by usage mode.

RV Battery Runtime Calculator

Calculate how long your RV battery bank powers your camper. Enter your loads, battery size, and get real runtime.

Marine Battery Runtime Calculator

Estimate trolling motor and boat electronics runtime from your marine battery. Built for anglers and boaters.

Car Battery Runtime Calculator

Calculate how long a car battery powers accessories with the engine off. Know the limits before you drain your starter.

Reserve Capacity to Amp Hours

Convert battery reserve capacity minutes into amp-hours two ways: the exact capacity at the 25 amp test rate and an estimated 20-hour rating.

Watt Hours to Amp Hours Calculator

Convert watt-hours to amp-hours at any voltage. Use this to compare batteries rated in different units.

Battery Voltage to Percentage Calculator

Turn a resting multimeter reading into a state of charge percentage for flooded lead-acid and AGM batteries, from the makers' own printed voltage tables.

How Many Watts in a 12V Battery?

Calculate total watt-hours stored in any 12V battery from its amp-hour rating. Includes common 12V battery examples.

Lead-Acid Battery Runtime Calculator

Work out flooded, AGM or gel runtime using a rate correction fitted to maker discharge tables, and see how lithium compares on the same load.

AGM Battery Runtime Calculator

Estimate AGM battery discharge time with chemistry-specific defaults. Compare AGM vs flooded lead-acid for your use case.

How Long to Charge a 100Ah Battery?

Calculate charge time for a 100Ah battery from solar, a mains charger, or a DC-DC charger. Lead-acid and LiFePO4 compared, stage by stage.

Battery Pack Calculator

Size a custom battery pack using series and parallel cell configurations. Enter target voltage, capacity, and cell specs.

CPAP Battery Runtime Calculator

Calculate how many nights your battery powers a CPAP machine. Enter battery capacity, voltage, and CPAP wattage for instant runtime results.

E-Bike Battery Range Calculator

Estimate your e-bike range based on battery size, assist level, terrain, and rider weight. Get instant results for any electric bicycle.

Drone Battery Flight Time Calculator

Calculate drone flight time from battery mAh, voltage and average power draw. Presets carry DJI printed pack energy and hover times.

Electric Scooter Battery Range Calculator

Calculate your electric scooter range from battery voltage, capacity, speed mode, terrain, and rider weight. Free, instant results.