What Is a LiPo Battery?

A LiPo battery is a rechargeable lithium battery built as a pouch cell with a polymer electrolyte. The name comes from lithium polymer, and the pouch construction is what makes LiPo different from the cylindrical and prismatic lithium cells most people know. A LiPo cell is a flat, sealed pouch containing the electrodes and electrolyte, rather than a metal can. Because the pouch is not rigid, it can be made in shapes and thicknesses that a metal can cannot match: ultra-thin, curved, round, mini and small cells, all the way down to the 7 mAh ultra-small cell listed on this site.

How a LiPo cell is built

A LiPo cell contains a positive electrode, a negative electrode, a separator and a polymer electrolyte, sealed in a laminated pouch. The pouch is lighter than a metal can for the same capacity, which is one reason LiPo appears in weight-sensitive devices. The cell is supplied with a protection circuit module, a small board that prevents overcharge, over-discharge and short circuit. The boards stated for the polymer range on this site are DW01 plus 8205 and G3J plus 8205 combinations. Cells can be supplied with leads or a connector, so the device maker can connect the cell directly to the device's circuit.

What the numbers mean

A LiPo cell's headline figures are voltage, capacity and size. Voltage for the verified cells on this site is 3.7V nominal. Capacity is measured in milliamp-hours: the 401030 cell is 90 mAh, the 402933 is 320 mAh, the 200911 is 7 mAh and the 201034 is 40 mAh. Size is given as thickness, width and length: the 401030 is 4.0 by 10 by 32 mm, the 402933 is 4.0 by 29 by 35 mm, and the 201034 is 2.0 by 10 by 36 mm. The model numbers themselves encode the size in many cases: 401030 means roughly 4.0 mm thick, 10 mm wide and 30 mm long, which matches the stated size closely. Cycle life for the verified cells is stated at 600 cycles or more, and lead time is 7 days.

Where LiPo batteries are used

LiPo cells appear wherever a small, light, shaped battery is needed. Wearables use thin cells that fit a watch or band. Small consumer electronics use mini and small cells that fit compact housings. Medical devices use small cells that fit handheld instruments. Compact sensors and instruments use ultra-small cells that fit spaces no larger cell can enter. The special polymer page on this site describes the shape families, ultra-thin, mini, curved, round, high-rate and button-shaped, at category level, because those families are listed without a verified specification for each individual size.

Custom LiPo cells

Custom work is a large part of the LiPo business, because devices rarely share a standard size. Custom polymer cells can be made in voltages from 3.7V upward, in a wide capacity range, and in custom sizes and shapes. Battery management systems from 10A to 100A and above can be added, along with charge display, USB charging, communication and balancing functions. A custom quote needs the maximum dimensions, the voltage, the capacity, the discharge current, the connector type and the quantity. Minimum order quantity for polymer cells is confirmed on inquiry, and lead time for listed cells is 7 days.

Handling and limits

LiPo cells need care that larger iron phosphate packs need less of. They should not be punctured, bent sharply, overcharged or charged below freezing, and a damaged pouch should not be used. The protection circuit module handles routine protection, but physical damage is outside what any board can prevent. Storage should be in a cool, dry place, and cells that will sit unused for months should be stored at partial charge. These are general handling notes for the chemistry, not specifications for a particular cell; the specification for a particular cell is on its product page.

Related Pages

• LiPo Battery
• Special LiPo Battery
• Custom LiPo Battery
• LiPo Battery Safety

Reading a LiPo size code

LiPo size codes usually encode thickness, width and length. The 401030 cell is stated at 4.0 mm thick, 10 mm wide and 32 mm long, which matches the code's pattern of thickness, width and length in tenths of a millimetre, with the last digits rounded. The 402933 cell is stated at 4.0 mm thick, 29 mm wide and 35 mm long, again matching the code pattern. The 201034 cell is stated at 2.0 mm thick, 10 mm wide and 36 mm long. Knowing this pattern helps a buyer check a size code against the stated dimensions: if the code and the stated size disagree sharply, the discrepancy should be clarified before ordering, as it is for the 200911 cell on this site, whose size is deliberately not stated because the source gives two conflicting figures.

Capacity does not follow directly from size code. Two cells with similar footprints can differ in capacity because of electrode thickness and chemistry details, and two cells with the same capacity can differ in size. The stated capacities on this site, 90 mAh for the 401030, 320 mAh for the 402933, 7 mAh for the 200911 and 40 mAh for the 201034, are the figures to use for runtime calculations, not estimates derived from size. Runtime follows from capacity divided by average current draw, adjusted for the device's duty cycle and for the protection board's cutoff behavior.

LiPo in series and parallel

Single LiPo cells are 3.7V nominal. Devices that need higher voltage connect cells in series: two cells give 7.4V nominal, three give 11.1V. Devices that need more capacity connect cells in parallel: two 320 mAh cells in parallel give 640 mAh at the same voltage. Packs combine both, as in the ebike line's 13S5P structure, thirteen in series and five in parallel. Series connections need balancing, because cells in series drift apart over cycles and the weakest cell limits the pack; the battery management options stated for custom polymer work, including communication and balancing, exist for this reason. A buyer specifying a multi-cell pack should state the series and parallel arrangement, or the voltage and capacity, and let the pack design follow from that.

Buying checklist for a LiPo cell

A buyer ordering a LiPo cell should confirm six things before paying. The nominal voltage, 3.7V for the cells on this site. The capacity in milliamp-hours against the device's runtime need. The dimensions against the compartment, including the protection board thickness where stated. The lead or connector type against the device's connection. The cycle life and lead time, 600 cycles or more and 7 days for the verified cells here. And the protection board type, DW01 plus 8205 or G3J plus 8205 for this range. A quote that confirms all six is a complete quote; a quote that confirms only capacity leaves the fit and connection to chance.