Battery Life Calculator - Runtime From mAh, Ah or Wh

Why a 100 Ah battery rarely gives you 100 amp-hours

The label on a 12 V deep-cycle battery says 100 Ah, the fridge on it draws 60 W, and a quick division says twenty hours. In practice the battery reaches the level a lead-acid battery should not go below after about ten. The division was right; what went into it was not: a lead-acid battery is usually run to about half its rating to last, an inverter eats part of the rest, and a heavy load shrinks what the label promised.

This battery life calculator turns a capacity and a load into a runtime you can plan around. A 5,000 mAh phone-size cell at 3.7 V feeding 500 mA runs for 10 h.

What the calculator does about it

Converts everything to watt-hours first. Milliamp-hours, amp-hours and watt-hours all become energy, so a load in watts and a battery in milliamp-hours compare honestly.

Counts only the part you use. Both limits are optional; with no depth of discharge the result names both the lithium and the lead-acid runtime.

Shows the spread, not one number. A table repeats the runtime from a quarter to twice your load, and a note warns when the current is high for the capacity.

Filling it in, from the label to the load

Battery capacity and its unit - the rating on the battery: mAh, Ah or Wh.
Battery voltage - the nominal voltage: 1.5 V for an alkaline cell, 3.7 V for lithium-ion, 12 V for lead-acid, 12.8 V for a LiFePO4 pack.
Load and its unit - the draw in mA or A, or the power in W from the nameplate.
Usable depth of discharge (optional) - the share of the rating you are willing to use, for example 50% for lead-acid. Left empty, the result gives two runtimes: the full rating for lithium and 50% of it for lead-acid.
Inverter or converter efficiency (optional) - for a 120 V appliance on an inverter or a device behind a voltage converter. Leave it empty when the current was measured at the battery.
Read the result - the runtime in hours and minutes, days or years, the energy behind it and the table for other loads.

How much of the rating to count, by battery type

These are common rules of thumb, not one manufacturer's limits; a datasheet figure beats them.

Battery	Usually used	What to enter
Lead-acid, flooded or AGM	about 50%	Depth of discharge 50; deeper cycles shorten its life
LiFePO4 (lithium iron phosphate)	80 to 100%	80 to stay conservative, empty for the full rating
Phone, laptop, power tool packs	the rating	Leave empty: the built-in electronics already stop short of empty
Alkaline AA, AAA, 9 V	the rating at low drain	Leave empty for small loads; expect much less at a few hundred mA
Lithium coin cells (the CR series)	the rating at µA loads	Only for tiny continuous currents

Five setups, run through the calculator

A phone-size lithium-ion cell. 5,000 mAh, 3.7 V, 500 mA: 18.5 Wh and a runtime of 10 h. Halve the draw: 20 h.

A camper fridge on lead-acid. 100 Ah, 12 V, 60 W average, depth of discharge 50%: 600 Wh usable of 1,200 Wh, so 10 h, half of the naive twenty.

A LiFePO4 pack behind an inverter. 100 Ah, 12.8 V, a 500 W load on a 90% inverter: 1,152 Wh usable and 2 h 18 min.

A trolling motor. 100 Ah, 12 V, 30 A, depth of discharge 50%: 1 h 40 min. That is 360 W; at a quarter of it, 6 h 40 min.

A coin cell in a sensor. 220 mAh, 3 V, 20 µA continuous: 1.25 years, from 0.66 Wh spent at 60 µW.

Hours from a 12 V, 100 Ah battery

The same 1,200 Wh battery at six loads, used three ways: all of the rating, half of it as for lead-acid, and all of it through a 90% inverter.

Load	100% used	50% used	100%, 90% inverter
10 W	5 days	60 h	4.5 days
50 W	24 h	12 h	21 h 36 min
100 W	12 h	6 h	10 h 48 min
300 W	4 h	2 h	3 h 36 min
500 W	2 h 24 min	1 h 12 min	2 h 10 min
1,000 W	1 h 12 min	36 min	1 h 5 min

Treat the bottom rows as a best case. At 1,000 W a 12 V battery supplies over 80 A, close to its full amp-hour rating every hour, and at that rate a lead-acid battery delivers well under its label.

Questions readers send in

How do I convert mAh to hours?

Divide the capacity by the current in the same unit: 5,000 mAh ÷ 500 mA = 10 h. If the load is in watts, first multiply mAh by the voltage and divide by 1,000 to get Wh, which is what the calculator does.

Why does the voltage matter if I already have mAh?

A figure in mAh counts charge, not energy. A 3,000 mAh cell at 3.7 V holds 11.1 Wh, and the same number of mAh at 1.5 V holds only 4.5 Wh.

Two 12 V 100 Ah batteries: 200 Ah or 24 V?

Both, depending on the wiring. In parallel they make 12 V, 200 Ah; in series 24 V, 100 Ah. Either way it is 2,400 Wh; enter the pair as one battery.

What efficiency should I use for an inverter?

The inverter's datasheet gives it; many are rated around 85 to 95% near their best load and worse at a small fraction of their rating. Without one, 85% is cautious.

My device lists a range, like 200 to 800 mA. Which number?

The average over the time it runs. A device that spends three quarters of the time at 200 mA and a quarter at 800 mA averages 350 mA.

What does the C-rate warning mean?

C-rate is the current divided by the amp-hour rating. 30 A from a 100 Ah battery is 0.3C; the calculator warns above 0.5C, where the delivered capacity typically drops below the label. Lead-acid ratings are usually quoted at a 20-hour discharge, only 0.05C.

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