LiPo battery swelling in agriculture drones is a serious safety issue. A visibly swollen pack is finished. It is not repairable, cannot be nursed through the spraying season, and no venting, puncturing, or re-balancing trick can make it airworthy again. Swelling means gas has been generated inside a sealed pouch by chemical decomposition that does not reverse. Ground the pack immediately, isolate it, retire it through a proper recycling route, and replace it.
This article covers what is happening inside the pack, the six causes that account for almost all swelling in Indian agriculture drone fleets, how to inspect a pack before every flight, and how to tell worn-out batteries from damaged ones.
What LiPo swelling physically is
A lithium polymer cell is a soft foil-laminate pouch containing electrode layers and a liquid organic electrolyte. There is no rigid casing and no pressure-relief vent. When the electrolyte breaks down, it produces gas — largely from electrolyte oxidation at the positive electrode, plus gas released as the solid electrolyte interphase (SEI) layer on the negative electrode grows and reforms. The pouch has nowhere to put that gas, so it inflates.
LiPo battery swelling occurs when gas builds up inside the sealed pouch, creating visible expansion and increasing the risk of internal damage. Two consequences matter in the field. The gas is flammable, so a swollen pouch is a pressurised volume of combustible gas sitting against a high-energy electrode stack. The reaction is one-way: nothing at the workbench turns that gas back into electrolyte, and the capacity lost to the decomposition is gone. Internal separator damage frequently accompanies the gassing, which is the mechanism that turns a swollen pack into an internal short and then a fire.
A 6S 22,000 mAh agriculture pack holds roughly 488 Wh. That is a different risk class from the 1,500 mAh hobby packs most articles on this subject are written about.
Is LiPo battery swelling normal?
No. Pouch cells do grow very slightly over hundreds of cycles, but visible, measurable swelling is a defect condition, not a maintenance milestone. If you can see the dome across the room, feel it under your palm, or if the pack now needs pressure to seat in the tray, it has crossed the line.
Make the test dimensional rather than visual. Every reputable pack publishes a thickness with a tolerance. Our Evercell 6S 22.2V 22000mAh 25C LiPo pack is specified at 196 × 85 × 64 ±1 mm. Measure the thickest point with a vernier or steel rule: a pack rated 64 mm that now reads 67 mm is not “a bit puffy”, it is retired. Do the same against whatever brand you fly.
Why agriculture drone packs swell: the six real causes
| Cause | What happens inside the pack | Field signature |
|---|---|---|
| Over-discharge below the per-cell floor | Copper from the anode current collector dissolves and re-plates, damaging the separator; electrolyte decomposes | Pack pulled hard on the last tank of the day; one cell reads well below the rest |
| Heat, in flight and after | Accelerates every parasitic side reaction — as a rule of thumb, reaction rates roughly double per 10°C rise | Pack too hot to hold on landing; swelling appears mid-season rather than gradually |
| Storage at full charge | Holding cells at 4.2 V keeps the electrolyte at its most oxidising potential | Fleet charged “ready for tomorrow”, then the job slips a fortnight |
| Physical damage from hard landings | Internal layer displacement and micro-shorts from compression or impact | Bulge localised to one corner or one cell; usually traceable to an incident |
| Overcharging or a failed balance lead | One cell pushed past 4.2 V while the pack total still looks normal | Balance readings drifting apart by more than about 0.05 V at top of charge |
| Age and cycle count | Cumulative SEI growth and electrolyte consumption — the honest end of life | Even swelling across the whole pack; capacity down before any bulge appears |
Over-discharge is the cause operators create most often
A 6S pack is six cells in series: 22.2 V nominal, 25.2 V fully charged. The Evercell 6S specification sets a discharge cut-off of 20.0 V — 3.33 V per cell. Below that floor the chemistry damages itself permanently, even if the pack comes back up on the next charge.
The trap in spraying is that the last minute of a sortie is the worst minute. The tank is empty and the aircraft is light, but the pack is at its lowest state of charge with its internal resistance at its highest, so a climb or a headwind leg drags voltage down fast under load. A working margin, not a law: land with resting cell voltage no lower than about 3.5 V per cell — roughly 21 V on a 6S pack — and set the flight controller’s low-voltage return trigger to protect that, rather than flying to the manufacturer’s absolute floor.
Heat is the cause Indian operators underestimate
Most guidance on LiPo swelling is written for temperate hobby use. Indian spraying is not that. The pre-monsoon and post-monsoon windows put drones in the air at 38–45°C ambient, over bare or stubble fields that radiate more, often at midday. A pack that has just delivered 150–220 A of continuous discharge is already 15–25°C above ambient when it leaves the tray. What happens next decides whether it swells.
- Stacking hot packs into a closed vehicle or a tin-roofed shed between fields. A parked vehicle interior in an Indian summer runs 20°C or more above ambient. That is well outside the Evercell pack’s stated storage range of 0°C to 35°C.
- Charging a hot pack. The Evercell charge window is 0°C to 45°C. Putting a 50°C pack on a charger holds it in the damaging zone for another hour. Let packs fall to near ambient first — for a 2.55 kg pack in still air that is a real 20–30 minutes, not five.
- Monsoon humidity. Moisture does not swell a pack directly, but condensation on balance plugs and XT90 terminals drives corrosion and contact resistance, producing local heating and false balance readings. Store packs with a desiccant in a sealed case through the monsoon and check connectors for green or white deposits.
Storage at full charge is the quiet killer of fleet packs
Cells age fastest at high state of charge. Leaving a fleet at 25.2 V “so it is ready”, then not flying for three weeks, does more cumulative damage than the flying does. Park cells at storage voltage — about 3.8 V each, roughly 22.8 V on a 6S pack — for any gap longer than two or three days. The Evercell EC2000 and other agriculture drone battery chargers we stock include a dedicated Storage mode that parks cells at approximately 3.8 V for exactly this purpose. Use it as routine, not as an off-season ritual. Our companion article on how to maintain agriculture drone batteries for longer flight time and better performance covers the full charge and storage routine.
The one-minute pre-flight battery inspection
Run this on every pack, before every flight.
- Shape. Lay the pack on a flat surface. Any dome, corner bulge or rocking means swelling. Retire.
- Thickness. Spot-check against the published dimension. Beyond tolerance, retire.
- Fit. If the pack needs force to seat, or the retention strap will not close at its usual setting, retire. Never compress a swollen pack into a tray.
- Skin. Punctures, scuffs through the wrap, impact dents, wetness or an oily film — any breach of the pouch, retire.
- Smell. A sweet, solvent-like odour means electrolyte is escaping. Retire and isolate immediately.
- Connectors. XT90 or AS150 clean and un-melted; JST-XH balance plug intact, no bent or corroded pins.
- Cell balance. All six cells within roughly 0.03–0.05 V of each other. A persistent outlier is the warning that precedes swelling.
- Temperature. Pack at or near ambient. Never launch on a pack still warm from charging.
What to do with a swollen pack
This is a retirement procedure. There is no repair procedure — any source offering one, whether puncturing to vent the gas, slitting and re-sealing the pouch, or “cycling it back into shape”, is describing a way to start a fire.
At the field, immediately:
- Stop. Do not fly it. Do not charge it. Do not balance-charge it “just to see”.
- Isolate it. Move it well away from other packs, fuel, spray chemicals, dry stubble and the vehicle. Put it on bare ground, concrete or sand — never on grass, a tarpaulin or a vehicle seat.
- Do not compress it. No strapping it flat, nothing stacked on it.
- If it is hot, hissing, leaking or the pouch has ruptured, treat it as an active hazard: leave it outdoors on non-combustible ground at a safe distance, keep people clear, and attempt no discharge or handling until it is cold and stable.
Back at base, for a stable but swollen pack:
- Bring it to a low state of charge using a charger with a controlled, monitored discharge function, in an open non-combustible area, with the pack watched throughout. Do not use the saltwater-bath method circulated on hobby forums — it does not reliably discharge a large pack, it corrodes the terminals into an uncertain state, and it can release chlorine gas.
- Store it separately in a fireproof battery bag or a lidded metal or ceramic container, away from the workshop, the drone store and the charging bench. Label it “RETIRED — DO NOT CHARGE” with the date.
- Hand it to an authorised recycler. Never household waste, never the field, never burning.
Disposal in India
Under the Battery Waste Management Rules, 2022, notified by the Ministry of Environment, Forest and Climate Change on 24 August 2022, waste batteries must be collected and sent for recycling or refurbishment, and disposal in landfills and incineration is prohibited (PIB, Government of India). The rules cover portable, automotive, industrial and electric-vehicle batteries and place Extended Producer Responsibility on producers and importers, with recyclers and refurbishers registering on the Central Pollution Control Board’s EPR Battery portal.
Practically: route retired packs to a CPCB-registered battery recycler or an authorised e-waste collection point, and ask your battery supplier what take-back route they operate. Keep a simple disposal record — for a commercial fleet, that record is your evidence the packs were handled lawfully.
"My drone battery is not charging" — is that the same problem?
Often it is the same damage, showing up one step earlier. A charger that refuses a pack, throws a cell-voltage error or stops part-way is usually reporting one of four things:
- A cell driven too low. Many chargers will not start a LiPo programme if any cell sits below roughly 2.5–3.0 V, because charging a deeply over-discharged cell is a genuine fire risk. That is the charger protecting you.
- Cell divergence beyond what the balancing circuit can close.
- A broken balance lead or corroded JST-XH pin — common after a monsoon — making a healthy cell read as absent.
- A pack outside its charge temperature window, too hot off a sortie or too cold on a winter morning.
If the pack is also swollen, stop: it is retired and the charging fault is a symptom. If it is dimensionally sound, check the balance lead and connectors, let it reach ambient temperature and re-test. A pack that reads badly out of balance on every charge, even after a proper balance cycle, is at the end of its service life whether or not it has bulged.
When a pack is simply finished
Not every retirement is a failure. Cells wear out, and a pack that has done its work should be replaced on schedule rather than run to destruction.
Reading cycle count. A standard agriculture LiPo — including the Evercell 6S 22000 mAh, which uses a conventional JST-XH balance plug — has no internal counter. You track cycles yourself: number every pack and increment a label or shed logbook per charge. Smart packs with an integrated battery management system (BMS) report cycle count over their data link, so read it rather than guessing.
| Observation | Verdict |
|---|---|
| Any visible swelling, dimension beyond tolerance, or pouch damage | Retire now — regardless of cycle count |
| Usable capacity below roughly 80% of rated, measured on a full cycle | End of useful spraying life; plan replacement |
| Cells that will not hold balance after a proper balance charge | Replace |
| Approaching rated cycle life (500+ cycles for the Evercell 6S 22000 mAh) with normal capacity | Move to lower-duty use or replace at your maintenance interval |
| Sortie time noticeably shorter at the same tank volume and conditions | Investigate capacity before it becomes an in-flight problem |
A fleet that retires packs on a rule loses a pack. A fleet that retires packs on a failure can lose an aircraft.
What to look for in a replacement
Buy against the failure mode that killed the last pack — heat and sustained load — not against the lowest price per mAh.
- Correct S-count and capacity for the airframe. Do not change voltage to chase flight time; match what the ESCs and flight controller were specified for. Our guide on how to choose a 6S LiPo battery for drone applications covers the sizing logic.
- Continuous discharge rating with real headroom. A 10 L to 20 L spray drone pulls heavy sustained current, not brief bursts. The Evercell 6S 22000 mAh is rated 10C continuous (220 A) with a 25C pulse for up to 3 seconds. Read continuous and pulse separately — a headline “25C” that is pulse-only tells you little about sustained thermal behaviour.
- A published temperature window. −20°C to 60°C discharge, 0°C to 45°C charge and 0°C to 35°C storage on the Evercell pack are the numbers to check your shed and vehicle against.
- Stated cycle life and honest tolerances. A supplier who publishes 196 × 85 × 64 ±1 mm is handing you the measurement you will later use to judge swelling.
- Physical fit and connector match. XT90 or AS150 on the discharge lead, JST-XH balance plug, dimensions that fit the tray without compression.
- Serviceable in India. A pack that must be air-freighted for support is a pack that grounds an aircraft for weeks.
The Evercell Make-in-India battery line is Bharat Skytech’s own, built and supported domestically, with cell chemistry selected to resist the heat-driven swelling described above. The Evercell 6S 22.2V 22000mAh 25C LiPo battery is ₹15,999.00 (Exclusive of GST) and is the direct replacement for the most common 6S 22 Ah agriculture pack. For other configurations, the full range of agriculture drone batteries covers 6S and 12S packs from 16,000 mAh to 22,000 mAh across Evercell, Herewin and Tattu.
Conclusion
Swelling is the pack telling you its electrolyte has broken down and produced gas that cannot escape. It is a one-way chemical event, it carries fire risk, and it has no field repair. Treat it as immediate, permanent grounding: isolate the pack, bring it to a low state of charge under control, store it in a fireproof container away from the workshop, and send it to a registered recycler.
Then work backwards to the cause. In Indian agriculture drone operations it is nearly always heat, over-discharge on the last sortie of the day, or storage at full charge — and all three are controllable with a charger that has a proper Storage mode, a low-voltage trigger set with margin, and a shed that is not 50°C at two in the afternoon. Get those right and packs reach their rated cycle life instead of dying halfway through it.
