How long will your battery last? find out with our easy-to-use battery runtime calculator. Calculator Assumptions Limitations Of This Calculator How To Use Our Battery Runtime Calculator? 1. Enter battery capacity in amp-hours (Ah): If the battery capacity is mentioned in watt-hours (Wh), Divide the watt-hours by battery voltage (V) to find out the battery capacity in Ah. 2. Enter your battery voltage (V): Do you have a 12v, 24, or 48v battery? For a 12v battery, ENTER 12. 3. Select your battery type: For lead acid, sealed, flooded, AGM, and Gel batteries select “Lead-acid” and for LiFePO4, LiPo, and Li-ion battery types select “Lithium”. 4. Enter your battery’s state of charge (SoC): SoC of a battery refers to the amount of charge it has relative to its total capacity. A fully charged battery will have 100% SoC. 5. Enter your battery’s recommended depth of discharge (DoD) limit: Battery depth of discharge (DoD) measures the used capacity of your battery from its total capacity. Lead-acid, AGM, sealed, flooded, and Gel batteries should not be discharged below 50%, while only lithium (LiFePO4, LiPo, and Li-ion) batteries can be safely depleted to 100%. Ask your manufacturer or have a look on at your battery’s specs sheet for a more accurate value. 6. Is your load connected through an inverter? Select yes, if you’re using an inverter to run the AC appliances. Or select no, if the appliance is directly connected to the battery without an inverter (usually not recommended). 7. Enter total output load in watts: If your appliance has an output load mentioned in amps, convert it into watts by multiplying the amps by the given volts of the appliance. 2 Ways To Calculate Battery Runtime Ready for calculation? Let’s dive in! I’ll share 2 methods to estimate battery life from basic (least accurate) to advanced (most accurate). Formula 1 Formula: Battery capacity in Ah ÷ Output amps (A) Or, Battery runtime = Battery capacity in watt-hours (Wh) ÷ Total Output in watts Accuracy: Lowest Dividing the battery capacity (in amp-hours – Ah, or milliamp-hours – mAh) by the output load (in amps – A, or milliamps – mAh) is the least accurate way to calculate the battery runtime. Because it doesn’t take into account for battery’s discharge efficiency rate, recommended depth of discharge, and state of charge. Mostly, the battery capacities are mentioned in amp-hours (Ah) but our appliances are in watts. Which makes it confusing in the first place. You can convert the battery capacity in watt-hours or the appliance input capacity into watt-hours to make it work. Battery capacity in watt-hours = Battery Ah × Battery voltage AdvertisementsOutput load in load in amps = Load in watts ÷ volts Example Let’s say you have: To calculate 50ah battery lifetime using this formula, divide 50ah by 10a.50ah ÷ 10a = 5 hrs According to this formula, a 50-ah battery will run a 10-amp load for 5 hours. Formula 2 Formula: Battery runtime = (Battery Ah × Battery volts × discharge efficiency × DoD limit × SoC × inverter efficiency) ÷ load Accuracy: Highest This formula takes into account for battery’s discharge efficiency rate, recommended depth of discharge, and state of charge. Based on directscience.com data: Example Let’s continue with the previous example and find out the most accurate runtime estimate. Let’s say you have: Now let’s put this info into our 2nd formula. (50ah × 12v × 85% × 50% × 100% × 90%) ÷ 120 watt (229.5) ÷ 120 watt = 1.9 hours Turns out, in actuality a 50ah battery will run a 10A load for about 1.9 hours. Why None Of These Methods Guarantee 100% Accuracy? Battery runtime formulas aren’t fully accurate due to the complexity of battery discharge, as there are other factors that can impact discharge time. Which are… 1. Batteries Discharge Unevenly If you discharge a battery too quickly, it won’t provide as much energy as the label claims, known as Peukert’s law. This is because more energy turns into heat instead of powering devices when discharged quickly. The effect of Peukert’s law on Lead-acid vs. Lithium: Lead acid batteries lose energy faster when discharged quickly, while lithium batteries can be discharged at up to 50% of their capacity without losing power. Here’s an example of how discharge time affects the usable capacity of a 100ah lead acid battery. Usable 100ah lead acid battery capacity Hours of discharge 100ah 20 hours 90ah 10 hours 87ah 8 hours 82ah 6 hours 80ah 5 hours 70ah 3 hours 60ah 2 hours 50ah 1 hour 2. Battery Life Cycles Matter Batteries can only be charged and discharged for a limited number of times, which is called the life cycle. Lead-acid batteries last for a few hundred cycles if they are maintained properly. Lithium batteries can last for thousands of cycles. But as batteries are used and charged more, they hold less charge capacity. After about 500 cycles, a lead-acid battery will lose about 20% of its capacity, while a lithium battery will 20% of its capacity after about 2000 cycles. Check your battery’s data sheet for more accurate numbers. 3. Effect Of Temperature On Batteries Performance Temperature affects how well your battery works. Usually, batteries work best when it’s between 68°F and 77°F (20°C to 25°C). A higher or lower temperature (than what’s recommended) will affect the battery’s performance. A rule of thumb is “an increase or decrease in temperature to 77ºF or 25º C can reduce battery performance by 50%”. To get the most out of your battery, keep it at the right temperature by using a cooling system or putting it in a spot where it won’t get too hot or too cold. How Long Does A 12v Battery Last? Here’s a chart on how long 12v different amp-hour (Ah) batteries will last running a 10-watt load. Battery Size (Ah) Runtime (Lead-acid) Runtime (Lithium) 6ah 3 hrs 6 hrs 7ah 3 hrs 7 hrs 12ah 6 hrs 12 hrs 18ah 8 hrs 19 hrs 20ah 9 hrs 21 hrs 36ah 17 hrs 37 hrs 50ah 23 hrs 52 hrs 60ah 28 hrs 62 hrs 70ah 32 hrs 73 hrs 80ah 37 hrs