LiPo against lithium ion
Lithium polymer and lithium ion are both lithium rechargeable chemistries, so the comparison is not lithium against LiPo but one lithium construction against another: LiPo is a soft foil pouch cell built to give up very high current for its weight, and lithium ion is a rigid cylindrical cell built to store more energy for its weight and last more cycles. Both charge to about 4.2 volts per cell. Almost every quadcopter a UK hobby flyer meets runs the pouch; the cylinder turns up where endurance matters more than punch.
The confusion is worth clearing because one of the two words is a category. Lithium is the family; lithium polymer, lithium ion and lithium iron phosphate are members of it, and a charger lists all three as separate modes because they are not interchangeable. A pack labelled lithium polymer is a lithium battery, and so is one labelled lithium ion.
What actually separates the pouch from the cylinder
What separates them is the case and the electrolyte, and everything else follows. A lithium ion cell is wound inside a metal can with a liquid electrolyte and a vent designed into the end. A lithium polymer cell is stacked flat inside a foil laminate pouch with a gel electrolyte, no can and no engineered vent, which is why a LiPo swells when it fails while a lithium ion cell vents instead.
That construction buys the pouch two things a flying machine wants: it is lighter for a given current capability, and it can be made in whatever flat shape the airframe has room for. It costs a shorter working life in cycles, and no protection beyond a skin of foil. Racing quadcopters, small camera drones and radio controlled cars all run lithium polymer, because they ask for large bursts from a light pack. Long endurance and larger fixed wing models use lithium ion, because a slow steady draw is what the cylinder is good at.
The figures the two chemistries share, and the ones they do not
The shared figures are the voltages, which is why the mistake is easy to make. Both are described by the same three numbers, and packs of both by cells in series.
| Attribute | Lithium polymer | Lithium ion |
|---|---|---|
| Cell voltage, full | 4.2V, higher on high voltage packs | 4.2V on the common types |
| Cell voltage, nominal | 3.7V | About 3.6 to 3.7V by cell type |
| Series notation | 3S is 11.1V nominal, 12.6V full | The same arithmetic |
| Discharge rate | High, stated as a C figure. 2.2Ah at 25C is 55A | Modest, stated in amps per cell |
| Energy for its weight | Lower | Higher |
| Cycle life | Shorter, and falls faster the harder it is flown | Longer for the same treatment |
| Shape | Flat pouch, made to fit | Fixed cylinder, packed into blocks |
| Cold weather | Poor. Voltage sags hard | Poor, and recovers more slowly |
The figure that decides most aircraft designs is the discharge rate. A quadcopter hovering draws a steady current and then demands several times that for a fast climb, and a pack that cannot deliver the peak drops its voltage until the flight controller sees a low battery that is not there. That is what the C figure describes, and why racing machines never moved to the cylinder. Drone batteries decodes the rest of the label.
Identifying your pack properly, in four steps
To identify the chemistry properly, look at the pack before looking anything up: four clues settle it in under a minute.
- Feel the shape. A flat block with slightly soft faces is a pouch. Round bulges under the shrink wrap mean cylindrical cells.
- Look for a balance lead. A small multi pin plug beside the main connector, with one more pin than the pack has cells, is the signature of a bare hobby pack, and on a drone it means lithium polymer.
- Read the printing. Hobby packs state chemistry, cell count, capacity and C rating on the shrink wrap.
- Check whether it is sealed. A moulded case with a contact block and no visible leads is a manufacturer pack, and its documentation confirms the chemistry rather than its shape.
Getting this right matters above all for the chemistry mode on a charger, which set wrong charges to the wrong per cell voltage. That is a hazard rather than an inconvenience. Charging sets out the sequence, in which chemistry is selected first.
Caution
Never guess the chemistry mode. If the pack does not state it and the documentation is not to hand, do not charge it. A lithium polymer pack charged as lithium iron phosphate is undercharged and useless; a lower voltage chemistry charged as lithium polymer is over-charged, and over-charging is how these cells fail.
What goes wrong with each chemistry
Each chemistry goes wrong in its own recognisable way, and the pouch gives more warning. A lithium polymer pack being mistreated swells, and swelling is visible, tactile and unambiguous. The causes are consistent: over-charging, over-discharging in flight, heat, damage to the foil, and age. A pack that no longer lies flat is finished, and is not flattened, flown or charged again.
The other problems are quieter. Internal resistance rises with cycles, so the pack sags under throttle and comes back warm. Cells drift apart, so one is over-worked at both ends of every flight. Charging a cold pack plates lithium inside it permanently. Lithium ion packs fail less often and with less notice, because the metal can hides a damaged cell a pouch would have advertised. Both share the same end state and the same response: get people away, get burnable things away, call 999, and take the remains to a battery collection point. Safety and storage covers the bag, the box and the disposal route.
What each chemistry costs, in the shop and afterwards
Bare lithium polymer packs are cheap to buy and expensive to own, and lithium ion is the reverse. A pouch pack for a small quadcopter sits in the under twenty pounds band in the United Kingdom, a larger 4S or 6S pack in the mid tens of pounds, and a sealed manufacturer pack is usually a substantial fraction of the aircraft. Cycle life is short and shortens with hard flying, so packs are a recurring cost.
Lithium ion costs more per pack of the same capacity and gives it back in cycles, which is where the cylinder plainly wins: an aircraft flying long gentle sorties, or a fixed wing model covering distance, runs cheaper on lithium ion over a season and carries the same energy for less weight. It loses the moment the aircraft needs a burst, and again on shape, since a block of cylinders wastes space a pouch would fill. One cost is easy to miss on either chemistry: postal restrictions on lithium cells mean a pack bought online arrives more slowly and dearly than the listing suggests.
Is the pack in your drone a lithium battery?
Yes. If the pack came with a hobby drone bought in the United Kingdom in the last decade it is a lithium battery, and the only question is which construction. A pouch with a balance lead is lithium polymer. A brick of cylinders is lithium ion. A sealed case is one or the other, stated in its documentation, usually the pouch on small consumer aircraft.
Yes, and it matters, because everything the pack demands follows from that answer rather than from the aircraft. Both chemistries want to be charged by something that can see individual cells and stop. Both want to be kept part charged rather than full. Both dislike heat, dislike cold and dislike being flown flat. Both are retired on evidence, not a hunch. The only drone battery this does not describe is the non rechargeable cell in a controller, which is not what wears out on a flying machine.