Battery Life Calculator Widget

Add a battery runtime calculator to your website. Visitors enter the battery capacity and voltage, the load in watts or amps, how deeply they will discharge it and the inverter efficiency, and get an estimated runtime - with an optional Peukert correction for lead-acid batteries.

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<iframe src="https://a2z.tools/embed/w/battery-life-calculator" title="Battery Life Calculator by A2Z Tools" width="100%" height="800" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>

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How it works

The widget uses the battery engine behind the A2Z Battery Backup Runtime Calculator. The stored energy (Ah x V) is reduced to the usable part by the depth of discharge you allow and the inverter or converter efficiency, then divided by the load in watts. When the load is given in amps drawn from the battery, the efficiency cancels out and the runtime is simply usable amp-hours divided by amps. Lead-acid batteries deliver less than their label at high currents, so an optional Peukert exponent applies Peukert's 1897 law in the form that references the capacity to its rated discharge time: t = H x (C / (I x H))^k, where C is the capacity at the H-hour rate (usually 20 hours). The battery current and C-rate are always shown, and a warning suggests the correction when the C-rate exceeds 0.5 without it. Doerffel and Sharkh's review of the law explains why it suits lead-acid better than lithium chemistries, whose exponent is close to 1.

Calculation method

  • Runtime (h) = Ah x V x DoD x efficiency / load W
  • Load given in amps from the battery: runtime = Ah x DoD / A (efficiency cancels)
  • Battery current I = load W / (V x efficiency); C-rate = I / Ah
  • Peukert (optional): t = H x (C / (I x H))^k x DoD, with H = rated hours (e.g. 20), C = rated Ah, k = 1-1.6
  • Runtime shown in days, hours and minutes (rounded to the minute) and in hours to 2 decimals

Worked examples

12 V leisure battery with a small load

Inputs: 100 Ah, 12 V, 60 W, 50% DoD, 90% efficiency

Result: 9 h 0 min; 1,200 Wh stored, 540 Wh usable; battery current 5.56 A (0.056C)

100 x 12 x 0.5 x 0.9 = 540 Wh; 540 / 60 = 9 h.

Same battery, heavy 300 W load, with Peukert

Inputs: 100 Ah, 12 V, 300 W, 50% DoD, 90%, k = 1.25 at 20 h

Result: 1 h 10 min (1.17 h) with Peukert, against 1 h 48 min without

I = 300 / (12 x 0.9) = 27.78 A; t = 20 x (100 / 555.6)^1.25 x 0.5 = 1.17 h.

Load known in amps

Inputs: 100 Ah, 5 A from the battery, 50% DoD

Result: 10 h 0 min

100 x 0.5 / 5 = 10 h; efficiency does not enter because the current is measured at the battery.

Limitations

  • Assumes a constant average load; motors, compressors and pumps with start-up surges or duty cycles need an averaged figure.
  • Ignores temperature (capacity falls in the cold), battery age and state of health, and the inverter's idle draw when the load is small.
  • Peukert's law is an empirical fit for constant-current discharge of lead-acid cells; it is not accurate for lithium chemistries or for varying loads.
  • Does not model battery voltage sag under load or the inverter's low-voltage cut-off, which can end the run before the chosen depth of discharge.

Where publishers use it

  • Camping, van-life and boating blogs planning 12 V power
  • Solar and off-grid system retailers
  • Drone, e-bike and portable power station reviews
  • Emergency-preparedness guides for home backup

Questions

What depth of discharge should I use?

The figure the battery maker's cycle-life data is based on. Lead-acid batteries are commonly limited to about half their capacity for a long life, while lithium iron phosphate is usually rated for much deeper cycling - the datasheet states the trade-off.

What is the Peukert exponent?

A number that describes how a battery's usable capacity shrinks at high discharge currents. Lead-acid makers publish it or the capacity at several rates from which it can be worked out; lithium batteries are close to 1, so leave it blank for them.

How much does Peukert change the answer?

For a 100 Ah, 12 V battery running 300 W through a 90% inverter at 50% depth, the plain estimate is 1 h 48 min; with k = 1.25 at the 20-hour rating it drops to 1 h 10 min, because 27.8 A is far above the 5 A the label assumes.

Why is my real runtime shorter?

Cold temperatures, battery age, high surge loads and the inverter's own idle consumption all reduce runtime. Treat the figure as an estimate for a healthy battery.

Sources

  1. A critical review of using the Peukert equation for determining the remaining capacity of lead-acid and lithium-ion batteries (Doerffel & Abu Sharkh) - Journal of Power Sources 155(2):395-400, 2006 . Peukert's law, its rated-time form and its limits
  2. Ueber die Abhaengigkeit der Kapazitaet von der Entladestromstaerke bei Bleiakkumulatoren (Peukert) - Elektrotechnische Zeitschrift 20, 1897 . Original statement of the law (print source)

Cite or recommend this tool

If you reference this tool in an article, course or documentation, these formats are ready to copy. They are optional - nothing is added to your site unless you paste it.

A2Z Tools Battery Life Calculator
https://a2z.tools/battery-backup-runtime-calculator

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