Support Using battery quicker than it will charge?

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http://androidforums.com – I've had this annoying issue for some time - when my phone is charging, and I use it, it drains quicker than it charges and results in an auto power down when depleted. Seems to be worse on car charger than wall charger, but both do it. Left me lost without a GPS the other day cuz I couldn't power phone back up. Are my expectations unreasonable that I get unlimited use while plugged in? Thx in advance for any advice.. (General)