The battery of my recently acquired '96 C280 Sport does not seem to hold charge even I drive it for thirty minutes or morer at a time. If the car sits in the garage for a couple of days, the battery will start to weaken to the point that I need a jump. How can I tell if the battery is at its last leg? I don't mind getting a new one but I just want to make sure it is not caused by other problem. Can an autmotive shop such as Sears or the dealer do some checking on the battery before I pay for a new one? It is now on a trickle charger. Any help will be appreciated.

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95 R129
04 Infiniti G35.5 BS
10 X204