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Compensating Battery Fuel Gauge Capacity for Self Discharge

IP.com Disclosure Number: IPCOM000215490D
Publication Date: 2012-Mar-01
Document File: 2 page(s) / 27K

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The IP.com Prior Art Database

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Conventional fuel gauges on portable electronic devices (such as smart phones, navigation devices, tablet computers and laptop computers, as well as other battery-powered devices, especially devices with processors) are used to estimate the batteries capacity. A typical fuel gauge uses coulomb counting to monitor the current in and out of the battery to help keep track of the capacity. Colloquially speaking, the general function of a fuel gauge is to inform a user how much battery power is left in the battery.

One problem with estimating remaining battery power by coulomb-counting is battery self-discharge. If a battery sits idle for a significant amount of time, it loses capacity that is not measureable by coulomb-counting. As a result, a fuel gauge that uses coulomb-counting might be supply an inaccurate estimate of actual battery capacity.  The fuel gauge may, for example, report too high, leading the user to believe there is more power remaining than there actually is.  The result could be a premature (from the user’s point of view) depletion of power, which may cause the portable electronic device to turn off or otherwise cause an unpleasant or unacceptable user experience.

The proposed approach is to use non-volatile memory in the battery.  This memory can store calendar date information, and can be accessed by the processor of the portable electronic device. (In many cases, the battery, not the portable electronic device, keeps track of when the battery was last fully charged; although the portable electronic device may store information about charging on the non-volatile memory in the battery.) The portable electronic device processor can then read the last time the device was used and make an estimate of capacity lost based on the time duration and the typical self-discharge characteristic for that cell. Example of some measured self-discharge characteristics Are below. It can be seen that a typical self-discharge characteristic for a Li-ion cell is 2-3% per month. So (for example) we can use or assume a 3%/month discharge rate for a specific battery. Hence, if the battery were stored for 3 months, you could calculate that it lost 3 months at about 3%/month or about 9% capacity due to self discharge. With this estimated amount of loss, the fuel gauge can compensate to reflect the assumed loss.  In other words, using the example of a 9% estimated loss, the fuel gauge capacity indication...