Patent ID: 7023111

Claim:
An electronic system comprising: a host device including host circuitry and at least one host battery; and an attachment removably engaged with the host device and including an attachment battery, the attachment including at least a first circuit that is always powered by the attachment battery and at least a second circuit that is selectively powered by the host battery or the attachment battery, depending at least in part on a power consumption of at least one designated circuit associated with the attachment, wherein when the power consumption of the designated circuit is above a threshold, the second circuit is powered by the host battery, otherwise the second circuit is powered by the host battery, wherein the determination of which battery powers the second circuit depends at least in part on the inequality Cc >Hc(A+C/2)/(B+C/2), wherein Cc is the remaining charge capacity of the attachment battery, Hc is the remaining charge capacity of the host battery, A is the current in at least the first circuit, B is the current in at least one circuit that is always powered by the host battery, and C is the current in at least the second circuit.