Inserting Single SIM Card into a Dual SIM Phone I understand phones having Dual SIM card slots use more battery. However, if we use only one SIM in a dual SIM Phone, will the battery still be drained out quickly? What If i choose not to use/insert the other SIM at all?

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You can insert only one SIM and work fine . See this In a dual sim phone, must both sims be used?
By just inserting second SIM battery won't drain extra. If at all there is any battery drain, it would be because of second SIM being active and checking for signal or registering itself on the network.