Battery Life Calculator

A battery runtime calculator estimates the amount of time a battery will power a device before it is discharged. The basic math principle is simple: the amount of energy stored in the battery is divided by the rate at which the device consumes electricity. This version supports two methods. For smartphones, sensors and small electronic devices, milliamp hours (mAh) and the device's power consumption are entered. For inverters, solar batteries and off-grid systems, you can switch to the energy mode where amp hours, system voltage and load in watts can be entered.

What is battery life?

The battery life defined here is the time a fully charged battery can power a device continuously. This is distinct from the lifespan (number of charge and discharge cycles) which indicates how many cycles a battery can endure before it loses its performance. The runtime depends on two adjustable factors; the amount of available energy stored in the battery, and the power consumption of the device. Real losses also play a role reducing the ideal value.

How to use this calculator:

Select a mode. In the Current Mode you enter battery capacity and load current and the tool will calculate runtime. In the Energy Mode you enter ampere-hours, system voltage and load in watts. Both modes support conversion: if you don't know one value leave it blank and enter the other two values and the tool will calculate the result. If you leave runtime blank you can find out how long a battery lasts. If you leave capacity blank you can determine the specification of the required battery to achieve a given runtime. By enabling "real losses" you can set discharge depth and system efficiency, which reduces estimated runtime to a level useful for planning purposes.

Formula for calculating battery life:

In the current mode, run time (in hours) is the result of dividing capacity by load current, provided that units for capacity and load current are consistent.

If a battery with 2000 milliamp hours is powering a load of 150 milliamps, the approximate operating time would be:

In the energy mode, the energy has to be calculated. The capacity is multiplied by the voltage to convert it into watt hours, then the available depth of discharge is taken into account and finally divided by the load power.

If a battery with 100 amp hours and 12 volts of electricity is powering a load of 100 watts, the available energy at an 80% depth of discharge would be approximately 960 watt-hours, and the approximate runtime would be:

The table below shows the individual input parameters with examples.

Factors that affect actual battery life.

This calculation equation is based on ideal conditions. The actual operating time may be lower than the calculated values due to various factors and is usually about 10 to 30% less.

The rate of discharge is important. A high current discharge reduces the available capacity. This phenomenon is explained by Peukert's law and is particularly pronounced in lead-acid batteries. Temperature also plays a role, as low temperatures can reduce the effective capacity of the battery by 20 to 30%. Additional losses may occur due to the inverter and wiring if DC power is converted to AC or if inadequate wiring is used, which can result in an additional loss of 5 to 15%. Additionally, all batteries age over time, and their capacity gradually decreases as they undergo more charge cycles. Therefore, batteries that have been in use for several years may not be able to deliver the rated power listed on the data plate.

Discharge depth and battery life

How deep a battery is discharged in each charge cycle represents a tradeoff between runtime and lifespan. Lithium iron phosphate (LiFePO4) cells can have lifespans well over 3000 cycles if operated with an 80% depth of discharge. However, the same cell run to 100% depth of discharge may only achieve about 2000 cycles. Lead acid batteries are more sensitive in this regard, and frequent use beyond half capacity can reduce their lifespan by as much as half. By adjusting your depth of discharge you can find a balance between extending current runtime and prolonging the life that will last for many years to come.

Tips to get closer to reality

Start with the nominal capacity listed on the data plate and account for actual losses to make the result more realistic. Set the depth of discharge based on the chemical makeup of the battery; Lithium batteries should use values close to 80-90%, while lead acid batteries should use a value closer to 50%. Account for inverter and wiring efficiency when determining power output. If the load varies, such as with devices that repeatedly go into sleep mode and then wake up again, estimate the runtime for each state separately and weight them according to how much time each state occupies.

This calculator provides estimates for planning and learning purposes only. Actual runtime will vary with battery age, temperature, discharge rate, and load. For mission critical systems, you should test the results under actual conditions before relying on them.