Lithium-ion polymer limitations and extended life

A li-polymer battery is a type of lithium-ion battery. the li-polymer battery is usually composed of several identical parallel sub-cells (secondary cells) to increase the discharge current. Or several packs of cells are connected in series to increase the available voltage.

Here is the content:

• Limitations

• Extending the life of multi-cell batteries

Limitations

All Li-ion batteries have a high state of charge (SOC), which can lead to problems with layer separation, reduced life, and reduced efficiency. In rigid batteries, the hard case prevents pole layer separation. But the soft-packaged li-polymer battery pack itself has no such stress. To maintain performance, the cells themselves need the shell to retain their original shape.

Overheating of lithium-poly batteries may lead to expansion or fire.

When the load is discharged, the load power should be stopped immediately when any cell (in series) falls below 3.0 volts. Otherwise, the li-polymer battery will not be able to return to a fully charged state. Or cause a significant voltage drop (internal resistance increase) when the load is powered later. This problem can be prevented by a chip in series with the li-polymer battery to prevent battery overcharge and over-discharge.

Compared with lithium-ion batteries, the charge and discharge cycle life of li-polymer batteries are less competitive. To prevent explosion and fire, when charging lithium-ion batteries need to use a charger designed for lithium-ion.

If the battery is directly short-circuited or passed through a very high current for a short period, it may also cause an explosion. Especially in the case of RC modelers with large power needs, careful attention is paid to the connection points and insulation. The battery may also catch fire when it is perforated. A special charger is used to charge each sub-cell evenly. This also leads to increased costs.

Extending the life of multi-cell batteries

There are two types of mismatches in li-polymer battery packs: the more common state of charge (SOC) mismatch and the capacity/energy (C/E) mismatch. Both of these can cause the capacity (mah) of the pack to be limited by the weakest cell. In the case of cells connected in series or parallel, the front analog end (AFE) can eliminate the cell mismatch and significantly improve cell efficiency and overall capacity. The potential for cell mismatch rises with the number of cells and the load current.

We call a cell balanced when the li-polymer batteries in the pack meet the following two conditions. If all the cells have the same capacity, when they have the same relative state of charge (SOC), it is said to be balanced. Open circuit voltage (OCV) is a good indicator of SOC in this case. If all the cells in an unbalanced pack are charged separately to a fully charged state (at which point they are balanced), the next charge/discharge cycle follows. Then the next charge/discharge cycle will return to normal without additional adjustment.

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