Estimate runtime
Runtime = capacity × efficiency ÷ draw.
About 11 hours 20 minutes.
Sanity check
How hard you are working the cell, and whether the estimate is trustworthy.
The formula
Textbook runtime is simply capacity divided by draw. A 2000mAh cell feeding a 150mA device gives 13.3 hours on paper. You will not get 13.3 hours, which is what the efficiency factor is for.
Why we default to 85%
Rated capacity is a laboratory figure, measured at a gentle discharge rate, at a comfortable room temperature, on a brand-new cell, discharged all the way down to a voltage your device would have given up at long ago. Every one of those conditions is optimistic:
- Cut-off voltage. Your device stops when the voltage drops below what it needs, leaving real capacity stranded in the cell. This is usually the biggest single loss.
- Discharge rate. Pull harder and you get less out. This is the Peukert effect, and it is mild for NiMH and Li-ion but not zero.
- Temperature. Cold cells hold less. Near freezing you can lose a significant chunk of usable capacity, and it comes back when the cell warms up.
- Age. A cell that has been through hundreds of cycles is not the cell on the label any more. Our battery health guide covers how to spot it.
85% is a reasonable middle for a healthy cell at room temperature. Drop it to 70% for a cold or hard-working device, or push it to 90% for a gentle low-drain load like a clock.
What the C rate is telling you
The C rate is your draw divided by the cell's capacity - it is how hard the device leans on the cell, independent of size. At 0.2C or below, a cell delivers close to its rating. Above 1C, high-capacity cells start to underperform their headline number and cheap cells fall apart. This is exactly why a camera flash is the one place where Eneloop Pro earns its price, and why in a wall clock it is a waste of money.
Common questions
How do you calculate battery runtime?
Divide the battery capacity in mAh by the device current draw in mA to get hours. Then apply an efficiency factor, because no cell delivers its full rated capacity in real use. This tool defaults to 85%, so runtime = (capacity x 0.85) / draw.
Why is my real runtime shorter than the calculation?
Several reasons stack up. Rated capacity is measured at a gentle discharge rate and a comfortable temperature, and your device is probably neither. Cells lose capacity as they age. Cold weather can cut usable capacity substantially. Most devices also stop working before the cell is truly empty, because they cut off at a minimum voltage. The 85% default absorbs some of this, but a hard-working device in the cold can do noticeably worse.
How do I find my device current draw?
Check the manual or the label first - many devices state it in mA or W. If it is given in watts, divide watts by the battery voltage to get amps. Otherwise use a USB power meter or an inline multimeter, or start from the typical figures in the table on this page.
Does higher mAh always mean longer runtime?
Not always. Within the same chemistry and size, yes - a 2500mAh AA outlasts a 2000mAh AA in a given device. Across chemistries it breaks down, because mAh ignores voltage. It also breaks down at high drain: high-capacity NiMH cells like Eneloop Pro have higher internal resistance and fewer cycles, so in a low-drain device a standard Eneloop can be the better long-term buy despite the smaller number.
Next: how long to put it back, or convert the same cell to watt-hours.