Quick Answer
Voltage sag, puffing, and brownouts almost always trace back to internal resistance rising as a LiPo battery wears out or gets damaged. Check the resting voltage, look for puffing, and test the pack under load. Retire any pack that puffs, smells odd, or sags hard, even if it still holds a charge.
What Voltage Sag Actually Is
Every LiPo battery has some internal resistance, even brand new. When you pull current for the motors, that resistance causes the voltage to drop, or "sag," below the resting voltage. A little sag under full throttle is normal.
Sag becomes a problem when it is severe: your quad feels weak, the video system resets, or the flight controller beeps a low voltage warning within seconds of takeoff. This usually means the internal resistance has grown too high for the pack to supply current smoothly.
Internal resistance rises with:
- Age and total number of charge cycles
- Deep discharges below the safe voltage limit
- Physical damage from a hard crash
- Cold weather, which temporarily raises resistance until the pack warms up
Reading the Warning Signs
A few simple checks tell you if a pack is still safe to fly.
- Resting voltage per cell. A healthy 4S or 6S LiPo should read close to its nominal voltage per cell (around 3.7 to 3.85V per cell at rest) after a normal charge and short rest. A pack that reads unusually low without being flown recently may have a bad cell.
- Cell balance. Check each cell's voltage with a balance charger. A gap of more than about 0.05V between cells on a rested pack is a sign of an aging or damaged cell.
- Puffing. Gently press the pack. A firm, flat pack is healthy. A soft, swollen, or uneven shape means gas has built up inside from breakdown of the chemicals, and the pack should be retired.
- Smell and heat. A sharp chemical smell, or a pack that gets warm just sitting on the shelf, is not normal. Stop using it right away.
Why Packs Puff
Puffing happens when the chemical reactions inside the cells break down and release gas. Common causes include:
- Over-discharging the pack below its safe cutoff voltage, often from flying too long or ignoring low-voltage warnings
- Charging or discharging at a rate higher than the pack is rated for
- Storing a pack fully charged for long periods, especially in heat
- Physical damage from a crash or a puncture
A small amount of puffing at the end of a pack's life is common and mostly harmless if flat and mild. A pack that becomes visibly rounded, hard to fit in its case, or hot without use is a fire risk and should be retired and disposed of safely.
What Causes Brownouts
A brownout is when the flight controller or video system loses power for a moment and resets in the air. It is one of the more serious LiPo-related problems, since it can cause a loss of control.
Common causes:
- A pack with high internal resistance that cannot supply a sudden current spike, such as a hard throttle punch
- A loose or corroded battery connector that cannot carry current reliably
- A missing or failed capacitor on the power distribution board
- Wiring that is too thin for the current the motors demand
If you get repeated brownouts with several different batteries, the wiring or capacitor is the more likely cause, not the packs. If only one pack causes it, retire that pack.
Testing a Suspect Pack Safely
To test whether a pack is the cause of a problem:
- Fully charge the pack and let it rest for a few minutes.
- Check resting voltage and cell balance with a balance charger.
- Fly it in a familiar location with a spotter present and land early if anything feels off.
- Compare how much the voltage sags under full throttle against a known good pack of the same size.
- If the pack sags much harder, feels warm afterward, or looks puffed, remove it from use.
A current sensor that reports accurate voltage and current to your flight controller makes this kind of testing much easier, since you get real numbers instead of guessing.
Safe Charging and Storage Habits
Good charging habits extend battery life and reduce the chance of these problems:
- Always use a balance charger designed for LiPo batteries and matched to the pack's cell count.
- Charge at the rate the manufacturer recommends, not the fastest rate the charger allows.
- Store packs at storage voltage (roughly 3.8V per cell) if you will not fly for more than a few days.
- Never charge or store LiPo packs on a flammable surface, and use a fireproof bag or box.
- Let a pack cool to room temperature after flying before you charge it.
For more on picking a good charger, see this LiPo charger guide.
Safe Disposal
A damaged or puffed LiPo battery cannot go in normal household trash. Fully discharge it first using a method meant for LiPo disposal, such as a saltwater bath in a safe container away from anything flammable, over many hours. After the pack is fully discharged and safe to handle, take it to a battery recycling point. Rules and available services vary by location, so check what is available near you.