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Nedre spenningsgrense for litiumbatterier

Dette klippet er hentet fra en artikkel fra Sony om nedre grense for Li-Po-batterier, skaffet til veie fra Gunnar Gangstø, som har lang fartstid i Atmel i å lage intelligente batteripakker.
Den forteller at spenningen på ei Li-Po-celle kan gå ned til 3.0v, med en 'hard end of charge voltage' på 2.8V. Det betyr at et 11,1v Li-Po batteri kan brukes helt ned til 9 volt, og en 22,2v kan brukes helt ned til 18v.


Lithium-ion

The nominal voltage of lithium-ion is 3.60V/cell. Some cell manufacturers mark their Li-ion as 3.70V/cell or higher. This offers a marketing advantage because the higher voltage boosts the watt-hours on paper (voltage multiplied by current equals watts). The 3.70V/cell rating also creates unfamiliar references of 11.1V and 14.8V when connecting three and four cells in series rather than the more familiar 10.80V and 14.40V respectively. Equipment manufacturers adhere to the nominal cell voltage of 3.60V for most Li-ion systems as a power source.

How did this higher voltage creep in? The cell manufacturer plots the voltage of a fully charged cell that measures 4.20V, discharges it at 0.5C to 3.00V and takes the mid-way point. For Li-cobalt the mid-way point is about 3.60V. The same scan done on Li-manganese with a lower internal resistance gives an average voltage of about 3.70V. It should be noted that the higher voltage is often set arbitrarily and does not affect the operation of portable devices or the setting of the chargers. But there are exceptions.

Some Li-ion batteries with LCO architecture feature a surface coating and electrolyte additives that increase the nominal cell voltage and permit higher charge voltages. To get the full capacity, the charge cut-off voltage for these batteries must be set accordingly. Figure 1 shows typical voltage settings.
 

Nominal cell voltage Typical end-of-discharge Max charge voltage Notes
3.6V 2.8–3.0V 4.2V Classic nominal voltage of cobalt-based Li-ion battery
3.7V 2.8–3.0V 4.2V Marketing advantage. Achieved by low internal resistance
3.8V 2.8–3.0V 4.35V Surface coating and electrolyte additives. Charger must have correct full-charge voltage for added capacity
3.85V 2.8–3.0V 4.4V Surface coating and electrolyte additives. Charger must have correct full-charge voltage for added capacity

Figure 1: Voltages of cobalt-based Li-ion batteries. End-of-charge voltage must be set correctly to achieve the capacity gain.


The phosphate-based lithium-ion has a nominal cell voltage of 3.20V and 3.30V; lithium-titanate is 2.40V. This voltage difference makes these chemistries incompatible with regular Li-ion in terms of cell count and charging algorithm.