Cell count sets voltage, capacity sets everything else
A lithium polymer pack is a stack of cells in series. Each cell has a nominal voltage of 3.7 V, sits at roughly 4.2 V when full, and has a manufacturer-specified lower limit somewhere around 3.0 to 3.5 V that you must not go below. Multiply by the cell count and you have the pack: a 6S pack is 22.2 V nominal, 25.2 V full, and has a floor in the region of 19.8 V depending on whose cells they are. The S number is the whole story on voltage, and it is why a pack cannot be substituted for one with a different S count without checking what it is plugged into.
Capacity in amp-hours times nominal voltage gives stored energy in watt-hours, and watt-hours is the figure worth carrying in your head, because it is what compares packs across different cell counts. A 5,200 mAh 6S pack is 5.2 x 22.2, about 115 Wh. A 10,000 mAh 3S pack is 10 x 11.1, about 111 Wh. Those are almost the same amount of energy in very different packages.
What C actually means, in both directions
C is a multiplier on capacity expressed in amps. One C is the current that would empty the pack in one hour, so for a 5.2 Ah pack, 1C is 5.2 A. Everything else follows from that single definition.
On the discharge side, a 35C rating on a 5.2 Ah pack claims 182 A continuous. Treat that as a marketing ceiling rather than a working figure. Discharge ratings are the least standardised number on a pack label and there is no shared test behind them; a pack that genuinely holds its rated current without heating or sagging is unusual. Sizing a system to draw well under the rating is the reliable approach, and a pack that gets hot or sags hard in use is telling you the rating was optimistic regardless of what is printed on it.
On the charge side, C is much better behaved because charge current is something you choose. Charge current in amps equals capacity in amp-hours times the charge C rate: a 5.2 Ah pack at 1C charges at 5.2 A. One C is the traditional default and is safe for essentially any modern pack. Higher rates are permitted by some manufacturers and cost cycle life. Never charge above what the manufacturer of that specific pack states, and never assume a rate carries over from another pack because it looks similar.
Why the charge time is longer than the division
A charger runs at constant current until the pack reaches full voltage, then holds that voltage while the current tapers off. The constant-current phase is the part that matches the arithmetic; the tapering phase at the end is not, and it takes real time to squeeze in the last few percent. The tail percentage in this calculator adds that time back. Twenty percent is a reasonable starting figure and it varies with the pack, its age and its temperature.
The other limit is the charger itself. Charge current times the full pack voltage is the power the charger has to deliver at the end of the charge, which is where the demand peaks. A 5.2 A charge on a 6S pack needs 5.2 x 25.2, about 131 W, and a charger rated below that will quietly charge more slowly rather than fail. Enter the charger rating and the calculator will tell you which of the two is really setting your charge time.
Storage is the part people skip
A lithium pack left at full charge degrades measurably faster than one left at around 3.8 V per cell, and a pack left flat can fall below the point where it should be charged again at all. Storage voltage for a 6S pack is about 22.8 V, a little under half charge. Most chargers have a storage mode that takes a pack to that point from either direction, and using it is the single cheapest thing you can do for pack life. The rule of thumb is that if a charged pack is not going to fly within a couple of days, it should be brought down to storage.
| Per cell | 6S pack | What it is |
|---|---|---|
| 4.20 V | 25.2 V | Fully charged. Do not leave it here. |
| 3.80 V | 22.8 V | Storage. Where a pack should sit between jobs. |
| 3.70 V | 22.2 V | Nominal, the figure used for energy calculations |
| Manufacturer cutoff | varies | The floor. Below it is permanent damage. |
Where this connects
The current the pack has to supply comes from the aircraft, so work that out first with the drone flight time calculator and check that the working current sits well inside the discharge rating above. The weight of the pack is part of the thrust to weight budget, and the charge time here is what sets the queue in the drone survey coverage calculator when a job needs more packs than you own. For house-scale banks rather than flight packs, the battery charge time calculator and the battery runtime calculator handle a different chemistry and a different problem.
The two things that actually hurt people
Lithium polymer packs are a real fire hazard, not a theoretical one. A pack that has been punctured, crushed in a crash, swollen into a pillow shape, or run flat below its cutoff can ignite without warning and burns hot enough that smothering it is not a plan. Almost every pack fire that gets written about happened while the pack was charging with nobody in the room. Charge where you can see it, on a surface that does not care, and stop the charge if a pack gets hot or gains thickness. A damaged pack should be moved away from anything that will burn and kept there; for getting rid of it, follow whatever your local hazardous waste program says, because that is a local question and not one a website should answer.
Propellers cut. They are stiff, they turn fast, and a multirotor that has armed on the ground is a running machine. A drone that loses a motor in flight does not glide anywhere; it comes down more or less where it was. Keep people out from under the aircraft and keep your hands away from the arms while the battery is connected.
Questions people ask
What charge current should I use?
Capacity in amp-hours times the charge C rate gives amps, and 1C is the sensible default: a 5,200 mAh pack is 5.2 Ah, so 1C is 5.2 A. That rate is within specification for essentially every modern pack and it is easy on them. Higher rates are permitted by some manufacturers and are always harder on the pack, costing cycle life in exchange for time. The number that governs is whatever the manufacturer of that specific pack states, not a general figure and not what a similar-looking pack allows. If you cannot find a statement, 1C is the conservative choice.
Is the C rating on the label real?
Treat it as a ceiling claimed under conditions nobody publishes. There is no shared industry test behind discharge C ratings, so the numbers are not comparable between manufacturers and are frequently optimistic. What is real is how the pack behaves: a pack held near its claimed rating that stays cool and holds voltage under load is doing well, and one that gets hot or sags noticeably is telling you the label was ambitious. Design to draw comfortably under the rating and use the pack behaviour as the evidence, not the print.
What happens if I over-discharge a pack?
Permanent damage, and possibly a hazard. Below the manufacturer cutoff the cell chemistry changes in ways that do not reverse, so the pack loses capacity and gains internal resistance immediately and keeps losing both. A pack that has been run well below cutoff should be treated as compromised even if it appears to charge again, because a damaged cell can fail later, under load, with heat. This is exactly why the flight time calculation on this site reserves a share of the pack rather than planning to empty it: the reserve is a battery health decision as much as a landing one.
Should I store packs charged or empty?
Neither. Store them at around 3.8 V per cell, which is roughly a little under half charge, in a cool place. Full storage accelerates degradation, and empty storage risks drifting below the point where the pack should be recharged at all. Most chargers have a storage mode that takes a pack there from above or below. The practical habit is to charge packs the day before or the morning of a job and put anything unused back to storage the same evening, rather than leaving a full pack in a bag for a month.
What do I do with a pack that has swollen?
Stop using it, and stop charging it. Swelling means gas has formed inside the cell and the pack is damaged; it will not recover and it is more likely to fail than a healthy pack. Move it somewhere it cannot set anything else alight and leave it there rather than putting it back in a bag with your other packs. For getting rid of it, follow whatever your local hazardous waste program says, which will differ by place and is not something a website should be improvising for you. Do not attempt to repair, puncture or open a lithium pack under any circumstances.