Battery Life Calculator
Runtime from capacity and load, plus power bank refills.
What it does
Battery runtime divides capacity by load: a 3000 mAh pack feeding 300 mA lasts ten ideal hours, trimmed by converter losses and age to about eight and a half at 85% usable.
The tool above takes capacity, average load, usable share, and an optional power bank size. It returns runtime, hourly drain, and bank refills.
Average load decides more than capacity. Radios transmitting, screens blazing, and motors starting draw multiples of idle, so mixed use needs honest averaging across the day.
Cold steals capacity temporarily while age steals it permanently. Winter field gear and two year old phones both underperform nameplates by a quarter or more.
How to use
- Read capacity in mAh off the pack or spec.
- Estimate average milliamps across real use.
- Set usable share: 85 for decent gear, lower when old.
- Add the bank size to count refills.
Formula
- capacity / load
- Ideal hours at perfect conversion.
- efficiency
- Usable share after conversion and age.
Peukert effects make fast drains worse than proportional on lead acid. Lithium follows the simple ratio closely.
Worked example
Example: 3000 mAh pack, 300 mA load, 85%, 20000 bank
Phone size battery with moderate load.
- Ideal: 3000 / 300 = 10 hours.
- Real: 10 x 0.85 = 8.5 hours, shown as 8 h 30 min.
- Drain: 100 / 8.5 = 11.8 %/hr.
- Refills: 20000 x 0.85 / 3000 = 5.67, shown as 5.7 charges.
Reference table
| Load | Ideal | Real |
|---|---|---|
| 100 mA | 30 h | 25 h 30 min |
| 300 mA | 10 h | 8 h 30 min |
| 1000 mA | 3 h | 2 h 33 min |
| 2000 mA | 1.5 h | 1 h 17 min |
Limits
- Steady average assumed. Spiky loads differ.
- Nameplate capacity fades with age and cold.
- Self discharge ignored over short spans.
- Voltage conversion losses folded into one share.
Common mistakes
- Dividing by peak instead of average load.
- Expecting nameplate hours in freezing cold.
- Ignoring converter losses in USB gear.
- Counting bank nameplate without its efficiency.
- Running cells to zero regularly, aging them.
Try it yourself
Quick checks. Answers sit tucked under each question.
5000 mAh at 500 mA, 80%?
10 x 0.80 = 8 hours even.
Drain per hour there?
100 / 8 = 12.5 %/hr.
Questions
What does mAh mean?
Milliamps for one hour. A 3000 mAh pack feeds 300 mA for ten ideal hours.
Why does my phone die faster?
Screen, radio, cold, and age all raise real drain above simple math.
Do power banks deliver full rating?
No. Conversion losses keep about 85%, less on cheap units.
Does fast charging harm batteries?
Heat ages them faster. Cool slow charges preserve lifespan.
How do I extend runtime?
Dim screens, limit radios, and keep cells warm in cold.
When is a battery worn out?
Near 80% of original capacity, usually after hundreds of cycles.