# LiPo Batteries: C Rating, Flight Time and the 80% Rule

What the C rating on the pack actually promises, why you should never plan to use the last fifth of the capacity, and how to estimate flight time without guessing.

---

- **Canonical URL:** https://dothecalculation.com/blog/hobby/lipo-battery-guide
- **Category:** Hobbies & Leisure Utilities
- **Author:** Do The Calculation Team
- **Published:** 2026-09-20
- **Reading time:** 16 min read
- **Publisher:** Do The Calculation (https://dothecalculation.com)
- **Methodology:** https://dothecalculation.com/methodology

---

A LiPo pack is labelled with three numbers that decide everything: capacity in milliamp-hours, cell count with an S after it, and a C rating. Two of those are honest measurements. The third is marketing with a kernel of truth in it, and knowing which is which is most of what keeps a pack alive.

## Cell count and voltage

The S number is how many cells are wired in series, and it sets the pack voltage. A LiPo cell has three voltages worth knowing, and the manufacturer Grepow states them plainly: the nominal voltage is "the average voltage of a LiPo cell when it is halfway discharged", 3.7 V per cell; "a fully charged LiPo cell reaches 4.2 V"; and "the safe minimum voltage per cell is typically 3.0 V".

**Pack voltages by cell count**
| Pack | Nominal (label) | Fully charged | Cut-off |
| --- | --- | --- | --- |
| 1S | 3.7 V | 4.2 V | 3.0 V |
| 2S | 7.4 V | 8.4 V | 6.0 V |
| 3S | 11.1 V | 12.6 V | 9.0 V |
| 4S | 14.8 V | 16.8 V | 12.0 V |
| 6S | 22.2 V | 25.2 V | 18.0 V |

> **The number on the label is the middle column** — A "11.1 V" 3S pack reads 12.6 V off the charger and should never be run below 9.0 V. Grepow notes that charging above 4.2 V per cell "is strictly unsafe unless the battery is specifically labeled as High Voltage (HV)", and that discharging below 3.0 V "can cause permanent damage to the cell". Many pilots set their cut-off at 3.2 to 3.3 V per cell instead, trading a little run time for pack life.

## What the C rating actually means

C is a multiplier on capacity, not a current. Grepow define it as the discharge current divided by the capacity, so 1C is whatever current would empty the pack in exactly one hour, 2C empties it in thirty minutes, and 10C in six.

**Maximum continuous current**

```
Max continuous amps = capacity in Ah × C rating
```
- Convert mAh to Ah by dividing by 1000 first.
- Worked: a 2200 mAh pack rated 45C is 2.2 × 45 = 99 A.
- Running it backwards gives the C rating your setup actually demands: Grepow's own example is a 2200 mAh pack pulling 20 A, which is 20 ÷ 2.2 = 9.09C.

**What the label promises**
| Pack | C rating | Maximum continuous current |
| --- | --- | --- |
| 1300 mAh | 75C | 97.5 A |
| 2200 mAh | 45C | 99.0 A |
| 2200 mAh | 20C | 44.0 A |
| 5000 mAh | 50C | 250.0 A |

> **Treat the advertised C rating with suspicion** — C ratings are not independently certified, they are not measured to a common standard, and a pack advertised at 45C will very often sag badly long before 99 A. Leave headroom: sizing the pack so your maximum draw is no more than about 80% of the advertised continuous rating is the usual advice, and our calculator applies that rather than calling 100% "safe".

So the 2200 mAh 45C pack above advertises 99 A and should be planned around 79 A. A motor that peaks at 30 A is comfortable on it. The same motor on a 2200 mAh 20C pack has 44 A advertised and 35 A of planning headroom — still fine, but the margin has gone from enormous to ordinary, and that pack will get hotter.

Tool: [Check a pack against your motor](https://dothecalculation.com/calculators/rc-battery-c-rating-calculator) — Maximum continuous current, pack energy in watt-hours, and whether your motor draw leaves the recommended headroom.

## The 80% rule

The other 80% in this hobby is about capacity rather than current, and it is the more important of the two. The convention is to land with at least 20% of the pack still in it — to plan your flight around 80% of the rated capacity rather than all of it.

The reason is the shape of the discharge curve. A LiPo holds a fairly flat voltage through most of its usable range and then falls off a cliff near the bottom. Flying into that knee means the voltage under load crosses the cut-off suddenly rather than gradually, which is how a pack gets over-discharged and how an aircraft loses power on approach rather than on the ground.

**Flight time**

```
Minutes = (capacity in Ah × 0.8) ÷ average current draw × 60
```
- The 0.8 is the reserve convention, not a property of the battery.
- Average current draw is the hard part. It is not the motor's maximum: a motor drawing 30 A at full throttle draws far less at half throttle, because power rises faster than throttle position does.

**A 2200 mAh 3S pack with a motor peaking at 30 A**
| Average throttle | Average draw | Flight time |
| --- | --- | --- |
| 40% | 5.8 A | 18.3 min |
| 60% | 12.0 A | 8.8 min |
| 80% | 20.1 A | 5.3 min |
| 100% | 30.0 A | 3.5 min |

The fall-off is steep and it is not linear. Going from 40% throttle to 60% halves your flight time; going to full throttle cuts it to under a fifth. This is why a relaxed flight on a park flyer lasts a quarter of an hour and an aggressive one on the same pack is over in three minutes.

> **How the throttle curve is modelled here** — Our calculator estimates average draw as the motor maximum times throttle to the power 1.8. That exponent is a modelling assumption, not a manufacturer figure — it is there because power does not scale linearly with throttle stick position, and a linear model badly overestimates flight time at low throttle. Treat the output as an estimate to plan around and then confirm it with a timer and a charger that reports how much went back in.

Tool: [Estimate flight time](https://dothecalculation.com/calculators/rc-flight-time-calculator) — Flight time from pack capacity, motor draw and average throttle, with the usable-capacity reserve applied.

## The measurement that beats every estimate

Fly, land, and look at how many milliamp-hours your charger puts back into the pack. That single number tells you what you actually used, and it turns every estimate above into a calibrated one.

- If the charger returns about 80% of the rated capacity, your timer is set correctly.
- If it returns much more than 80%, you are flying into the reserve and should shorten the timer.
- If it returns far less, you are landing early and leaving flight time unused.
- Do this on a pack you know is healthy, because a tired pack returns less simply because it holds less.

## Keeping packs alive

- Do not store packs charged. Storage charge is roughly 3.8 V per cell, and most chargers have a storage mode that will take a pack there from either direction.
- Let a pack cool before charging it. Charging a hot pack straight off the aircraft is hard on it.
- Charge at 1C unless you have a good reason not to, and stay with the charger.
- A puffed pack is finished. Swelling means gas has been generated inside the cell, and that is not reversible.
- Balance charge rather than charging the pack as a block, so the cells stay matched. A pack where one cell is consistently lower is a pack on its way out.

## S and P: what the pack label is telling you

Grepow describes LiPo packs as one or more cells "connected in series (denoted as S) or parallel (denoted as P)". The two numbers do different jobs, and confusing them is behind a lot of bad pack choices.

- **Series (S) adds voltage.** Each cell in series adds another 3.7 V nominal. A 3S pack is 11.1 V; a 6S pack is 22.2 V. Capacity stays the same as one cell.
- **Parallel (P) adds capacity.** Each cell in parallel adds its capacity and current capability. Voltage stays the same as one cell.
- **Most hobby packs are 1P,** so the label often shows only the S number. A pack marked 3S2P has three pairs of cells in series.

**Building from 2200 mAh, 45C cells**
| Arrangement | Nominal voltage | Capacity | Maximum continuous current at 45C |
| --- | --- | --- | --- |
| 3S1P | 11.1 V | 2200 mAh | 99 A |
| 3S2P | 11.1 V | 4400 mAh | 198 A |
| 6S1P | 22.2 V | 2200 mAh | 99 A |

Doubling the cells in parallel doubles both the capacity and the current the pack can supply, because each cell only has to deliver half of it. Doubling them in series doubles the voltage instead, which is what makes a motor spin faster for the same current. Your motor and speed controller are rated for a specific voltage range, so the S count is set by your equipment; the P count and capacity are where you have choices.

## Power: why higher voltage packs feel stronger

**Electrical power**

```
Power (watts) = voltage × current (amps)
```
- Use nominal voltage for a quick figure. Real voltage is higher when the pack is full and lower under heavy load.
- A 3S pack at 11.1 V delivering 30 A supplies about 333 W.

The same power can come from different combinations. A 4S pack at 14.8 V supplying 60 A and a 6S pack at 22.2 V supplying 40 A both deliver about 888 W. The 6S setup does it with a third less current, which means less heat in the wires, connectors and speed controller, because resistive heating rises with the square of current. That is the main reason larger models tend to move to higher-voltage packs.

## Energy in watt-hours, and flying with your packs

Watt-hours measure the energy stored in a pack: nominal voltage times capacity in amp-hours. The number matters for comparing packs of different voltages, and it matters a great deal if you ever travel by air with your batteries.

**Energy in common packs**
| Pack | Energy | FAA category for a spare battery |
| --- | --- | --- |
| 1300 mAh 4S | 19.2 Wh | Up to 100 Wh |
| 2200 mAh 3S | 24.4 Wh | Up to 100 Wh |
| 5000 mAh 4S | 74.0 Wh | Up to 100 Wh |
| 5000 mAh 6S | 111.0 Wh | 101 to 160 Wh: airline approval needed |
| 8000 mAh 6S | 177.6 Wh | Over 160 Wh |
| 10000 mAh 6S | 222.0 Wh | Over 160 Wh |

> **What the FAA says about spare lithium batteries** — Under the FAA's PackSafe rules, spare lithium-ion batteries "must be carried in carry-on baggage only", with their terminals protected from short circuit. They are limited to 100 Wh per battery; with airline approval a passenger may carry up to two spare batteries of 101 to 160 Wh. Airlines can set stricter limits, and rules differ outside the United States, so check with your airline and the rules where you are flying before you pack. A large 6S pack can easily exceed 160 Wh.

## Voltage sag: why the reading drops under load

Grepow notes that "under heavy load, the voltage may sag temporarily due to internal resistance, but it recovers when the load is removed", and that older or damaged cells show greater sag. The size of the effect is simple to estimate once you know the pack's internal resistance, which many chargers can measure.

**Voltage sag**

```
Voltage under load = resting voltage − (current × internal resistance)
```
- Internal resistance is in ohms, often shown in milliohms. It rises as a pack ages.
- Illustrative figures only: a pack with 0.015 Ω total internal resistance sags about 0.3 V at 20 A, 0.6 V at 40 A and 0.9 V at 60 A.

**Illustrative 3S pack at 11.1 V nominal, 0.015 Ω total internal resistance**
| Current | Sag | Pack voltage under load | Per cell |
| --- | --- | --- | --- |
| 20 A | 0.30 V | 10.80 V | 3.60 V |
| 40 A | 0.60 V | 10.50 V | 3.50 V |
| 60 A | 0.90 V | 10.20 V | 3.40 V |

This matters for low-voltage cut-offs. A speed controller watching pack voltage sees the sagged figure, so at high throttle it can trip a cut-off even though the resting voltage would be fine. It also explains why a pack's voltage rises again after you land: the load is gone and the sag disappears. For judging how much charge is left, the resting voltage a minute or two after landing is the meaningful reading.

> **Rising internal resistance is an early warning** — Because internal resistance increases with use and age, logging it every so often on the same charger gives you a trend. A pack whose resistance has climbed well above its siblings' will sag more, run warmer and deliver less, and it is a candidate for retirement before it puffs.

## Bigger pack or more packs?

The flight time formula makes it look as though more capacity always means more flying. At the same average current it does: at 12 A average, 80% of a 2200 mAh pack lasts about 8.8 minutes, a 3300 mAh pack about 13.2 and a 5000 mAh pack about 20. But the bigger pack is heavier, and a heavier model needs more current to fly, so the real gain is smaller than the arithmetic suggests.

- **Check the weight your model is designed for.** Past a point, a larger pack costs more flying time in extra current than it adds in capacity.
- **Check the physical fit and balance.** A pack that moves the centre of gravity changes how the model flies.
- **Consider several smaller packs.** They let one charge while another flies, spread the wear across more cells, and keep the model at its designed weight.
- **Measure rather than assume.** Fly both options with a timer and note how much the charger puts back. That is the only reliable comparison.

## Reading pack voltage as a fuel gauge

Grepow describes the practical operating range as "typically between 3.5V and 4.2V per cell", with a relatively flat discharge curve until the "knee", after which voltage drops more rapidly. That flat middle is why voltage is a poor fuel gauge during a flight and a good one at rest. A cell at 3.7 V resting is around half discharged; a cell sagging to 3.5 V under load may have much more left than that figure suggests.

- **Use a timer set from measurement, not voltage, as your main guide in the air.** Set it from the charger's returned capacity, as described above.
- **Use a resting voltage check after landing.** A consistent resting voltage per cell after each flight is a good sign that your timer is right.
- **Store at storage voltage.** Grepow gives the optimal storage voltage as typically between 3.7 V and 3.85 V per cell, often aiming for 3.85 V, which is roughly 40 to 60% of capacity. Most modern chargers have a storage mode that reaches it automatically.
- **Watch individual cells, not just the total.** A 3S pack reading 11.4 V could be three cells at 3.8 V or one weak cell hiding behind two healthy ones. A cell checker on the balance lead shows the difference.

## Common LiPo mistakes

- **Flying to the cut-off.** The cut-off is a last defence, not a timer. Land with capacity to spare.
- **Trusting the C rating alone.** Leave headroom, and judge a pack by how warm it gets and how much it sags.
- **Charging unattended or on a flammable surface.** Charge somewhere you can react, in a container designed for LiPo charging if possible.
- **Leaving packs fully charged for days.** Bring them to storage voltage if they will not be used soon.
- **Flying a damaged or puffed pack.** Retire it safely, following local rules for battery disposal.
- **Packing spares in checked luggage.** Spare lithium-ion batteries go in carry-on only, with terminals protected.

## Charging arithmetic: current, time and charger power

Charging uses the same C notation as discharging. Charging at 1C means a current equal to the capacity: 2.2 A for a 2200 mAh pack, 5 A for a 5000 mAh pack. At 1C an empty pack takes roughly an hour, plus some extra time at the end, because a LiPo charger switches from constant current to constant voltage as the cells approach 4.2 V and the current tapers off.

**Charger power needed**

```
Charger output power ≥ full-charge voltage × charge current
```
- Full-charge voltage is 4.2 V per cell, so 12.6 V for 3S and 25.2 V for 6S.
- A charger's rated wattage has to cover the highest voltage the pack reaches, not the nominal voltage on the label.

**Charger output needed to charge at 1C**
| Pack | 1C current | Voltage at full charge | Power for one pack | Power for two at once |
| --- | --- | --- | --- | --- |
| 2200 mAh 3S | 2.2 A | 12.6 V | about 28 W | about 55 W |
| 5000 mAh 4S | 5.0 A | 16.8 V | about 84 W | about 168 W |
| 5000 mAh 6S | 5.0 A | 25.2 V | about 126 W | about 252 W |

This is the calculation to do before buying a charger. A charger with a 50 W output limit can charge a 2200 mAh 3S pack at 1C with room to spare, but it cannot charge a 5000 mAh 6S pack at 1C at all; it will simply charge more slowly, at whatever current its wattage allows. If you plan to charge two large packs at once, the total adds up quickly. Also check the charger's input supply: many chargers quote their output with a particular power supply, and a smaller supply limits them further.

> **Cold packs deliver less** — Grepow notes that cold temperatures reduce voltage delivery. On a cold field, a pack that has been sitting in a car overnight will sag more and trip a low-voltage cut-off sooner than the same pack at room temperature. Keep packs warm until you fly, and expect shorter flights in cold weather.

## Sources

- Grepow, "What Is the Voltage of a LiPo Battery?" — the 3.7 V nominal, 4.2 V fully charged and 3.0 V cut-off per cell, the HV charging warning, and the 3.2-3.3 V cut-off some users prefer: https://www.grepow.com/blog/what-is-the-voltage-of-a-lipo-battery.html
- Grepow, "What Is The C Rating On A Lipo Battery?" — the C rating definition, the discharge-time table and the worked 2200 mAh at 20 A = 9.09C example: https://www.grepow.com/blog/what-is-the-c-rating-on-a-lipo-battery.html
- The 80% usable-capacity reserve and the 20% current headroom are hobby conventions rather than published standards. The throttle-to-power exponent of 1.8 used in the flight time estimate is a modelling assumption, and the calculator says so.

## Where to go next

The other model-flying calculation worth doing before you get to the field is altitude. [How high will my model rocket go](/blog/hobby/model-rocket-altitude-guide) covers reading motor codes and what mass, drag and body diameter each do to the apogee.

## Common questions

**What does the C rating on a LiPo mean?**

It is a multiplier on capacity that gives the maximum continuous discharge current: amps = capacity in Ah × C. A 2200 mAh pack rated 45C advertises 2.2 × 45 = 99 A. Because 1C empties the pack in an hour, 2C does it in thirty minutes and 10C in six.

**How long will my LiPo fly?**

Take 80% of the capacity and divide by your average current draw. A 2200 mAh pack with a motor peaking at 30 A gives roughly 8.8 minutes at 60% average throttle, 5.3 at 80% and 3.5 at full throttle. Average throttle matters far more than most people expect.

**What is the 80% rule?**

Plan to use only 80% of the rated capacity and land with 20% still in the pack. LiPo voltage stays flat through most of the discharge and then drops sharply at the end, so flying into that last fifth risks over-discharging the pack and losing power in the air rather than on the ground.

**What voltage is a fully charged 3S LiPo?**

12.6 V — three cells at 4.2 V each. Its label says 11.1 V, which is the nominal voltage at roughly half discharge, and it should not be taken below 9.0 V.

**Is a higher C rating always better?**

Not automatically. Higher C packs are usually heavier for the same capacity, and the ratings are not independently certified, so a 45C label from one seller and another are not comparable. What matters is that your maximum draw sits comfortably inside the rating, with roughly 20% left in hand.

**Why did my battery puff?**

Gas generated inside the cell, most often from over-discharging, drawing more current than the pack can deliver, heat, or storing it fully charged. It is not reversible and a puffed pack should be retired rather than flown.

**What does 3S2P mean on a LiPo?**

Three cells in series, and two such strings in parallel. Series adds voltage, so 3S is 11.1 V nominal. Parallel adds capacity and current, so a 3S2P built from 2200 mAh, 45C cells is 11.1 V and 4400 mAh, able to deliver about 198 A continuously.

**Can I take LiPo batteries on a plane?**

Under FAA rules, spare lithium-ion batteries must go in carry-on baggage only, with terminals protected from short circuit, and are limited to 100 Wh each; up to two spares of 101 to 160 Wh are allowed with airline approval. A 5000 mAh 6S pack is about 111 Wh, so it needs approval. Check your airline and the rules for your destination.

**How do I work out watt-hours for my pack?**

Multiply the nominal voltage by the capacity in amp-hours. A 2200 mAh 3S pack is 11.1 V × 2.2 Ah = 24.4 Wh; a 5000 mAh 6S pack is 22.2 V × 5.0 Ah = 111 Wh.

**Why does my pack voltage drop under throttle and then recover?**

Voltage sag from internal resistance. Voltage under load equals resting voltage minus current times internal resistance, so it falls at high current and recovers when the load is removed. Sag grows as a pack ages, which is why an old pack trips low-voltage cut-offs sooner.

**What voltage should I store LiPos at?**

Grepow gives the optimal storage voltage as typically between 3.7 V and 3.85 V per cell, often aiming for 3.85 V, roughly 40 to 60% of capacity. Most chargers have a storage mode that takes a pack there from either direction.

**What size charger do I need for my LiPo?**

Multiply the pack's full-charge voltage, 4.2 V per cell, by the charge current you want. Charging a 5000 mAh 6S pack at 1C, 5 A, needs about 25.2 × 5 = 126 W of output; two at once need about 252 W. A charger below that will still work, just more slowly.

---

_Source: [Do The Calculation](https://dothecalculation.com/blog/hobby/lipo-battery-guide). Quote freely with attribution and a link to this page._