Battery Life Estimation Guide: What Engineers Check Before Trusting “mAh ÷ mA”
Battery life estimates fail for one reason: the calculator used the wrong model. Real devices don’t draw a steady current, batteries don’t deliver a fixed mAh under all conditions, and “dead” usually means “voltage dropped below what the electronics can tolerate,” not “the battery is empty.”
This guide is a field-friendly workflow for electronics and industrial automation use: define your end-of-life voltage → build a load profile → account for pulses and self-discharge → verify with a measurement method that doesn’t lie. Use it before you commit to a battery choice, a maintenance interval, or a “10-year life” claim.
It’s not a chemistry deep-dive. It’s a decision checklist that prevents the common “it should last a year, it died in two months” surprises.
Step 0 — Define what “battery dead” means in your system
Most devices stop working because the supply dips under a minimum voltage during a load event (radio burst, relay pull-in, motor start, backlight on, etc.). That minimum is your real finish line.
Two numbers you need up front
- Minimum operating voltage (Vmin): the lowest voltage where the system still behaves correctly.
- Worst-case load event: the moment your current draw peaks (and causes the biggest voltage sag).
Assuming you can use “full rated capacity” even when your electronics brown out well before the battery reaches its lowest-voltage endpoint.
Step 1 — Build a load profile (average current is earned, not guessed)
If your load is steady, life ≈ capacity ÷ current. In almost every modern product, the load is not steady. The only honest way is to list the states and time-share them.
A simple state table (copy/paste style)
| State | Current | Duration each cycle | How often | Notes |
|---|---|---|---|---|
| Sleep / standby | Isleep | Tsleep | Every cycle | Include regulator quiescent current + leakage |
| Active sensing | Iactive | Tactive | Every cycle | Sensor warm-up often dominates |
| Transmit / burst | Iburst | Tburst | Every cycle | Peak current drives voltage sag risk |
Average current formula (what engineers actually use)
This looks obvious, but it solves a very common forum-level failure: using “active current” as if it’s continuous, or ignoring sleep current because it “seems tiny.” In long-life devices, sleep current is often the whole story.
If your duty cycle is small, your average current can be orders of magnitude lower than your active current. That’s why the load profile table matters more than the peak number.
Step 2 — Do the “ideal” estimate (then immediately distrust it)
The baseline estimate is still useful as a first pass:
Now the important part: the capacity you used above is usually not the capacity you will get in your application. The next steps explain why.
Step 3 — Know when capacity is not “a fixed mAh”
3A) Discharge-rate effects (especially lead-acid)
Many batteries deliver less usable capacity as discharge current increases. This is the core idea behind the Peukert effect (most relevant to lead-acid, less so to many lithium systems).
- If your battery is rated at a slow discharge rate (like 20 hours) and you pull harder, you often get less than the rated Ah.
- For lead-acid, this effect is significant enough that tools and monitors explicitly model it.
3B) Temperature and internal resistance (the silent killer)
Voltage sag under load is driven by internal resistance. As batteries age or get cold, internal resistance rises, and your worst-case load event becomes the moment your device “dies” even when capacity remains.
3C) Cutoff voltage (your “functional endpoint”)
Usable capacity depends on the minimum voltage your electronics can tolerate. If your system needs 2.0 V but the battery can still deliver energy below that, the device is still effectively done. Plan around the device’s functional endpoint, not the chemistry’s absolute endpoint.
Step 4 — Pulse loads: why coin cells and small packs disappoint on “average current” designs
This is the classic trap in low-power wireless and field sensors: you calculate a tiny Iavg, choose a small battery, then the radio burst causes a voltage dip and resets the system. The math wasn’t wrong — the battery model was.
What changes with pulsed loads
- Peak current can be tens of mA even when Iavg is < 1 mA (radios, modems, burst logging).
- Coin-cell datasheet capacity is often specified at very low currents. If your pulses are far above that, effective capacity and voltage stability can drop.
“It works on the bench, fails in the cold,” or “it resets during transmit,” or “battery says 80% but it dies early.” These are usually pulse + internal resistance + cutoff voltage problems.
Practical mitigations engineers reach for
- Add reservoir capacitance close to the load (to supply the burst locally).
- Lower the peak demand (shorter transmit, lower power, staggered loads).
- Choose a battery built for pulses (or increase size / parallel cells).
- Revisit Vmin and brownout behavior (a reset loop can destroy average-current assumptions).
Step 5 — Long-life devices: self-discharge and “always-on” overhead dominate
If you’re estimating multi-year life, the calculation is rarely about active work. It’s about everything that happens while you think the device is “doing nothing.”
Two real-world contributors people underestimate
- Self-discharge / shelf loss: capacity slowly decreases even with no load.
- Always-on overhead: regulator quiescent current, protection circuits, leakage paths, indicator LEDs, pull-ups.
If your sleep current is not measured, your “years of life” estimate is not real — it’s a guess.
Step 6 — Verification: measure in a way that matches the problem
Measuring “average current” with a multimeter is often misleading because the device’s load is bursty. Engineers typically use one of these approaches:
- Shunt + scope to capture pulses and integrate a realistic average.
- Logging power monitor / coulomb counter when you need long-term integration over real duty cycles.
- Battery voltage under load as a health indicator (especially when internal resistance is the limiter).
Test your worst-case event at low battery voltage and at cold temperature. If it survives that, your estimate is much closer to reality than any “room-temp fresh-cell” calculation.
Use the calculator as the final step (not the first step)
Once you have a realistic average current (from a state table or measurement) and a battery capacity that matches your discharge conditions, use the Battery Life Calculator to convert those into a time estimate.
Key takeaways
A trustworthy battery-life estimate is less about clever math and more about using the right assumptions. If you model the load states, respect pulses and cutoff voltage, and validate with a measurement that matches reality, the “calculator number” becomes something you can actually plan maintenance around.