Renters Dilemma..... Charging the battery... Need help!

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Hey gents!,

I live in the city and I just bought a 2010 boxster for the weekends when I'm not working. So my car as it stands is just only used on the weekends. So it sits and of course with sitting the battery doesn't have the chance to keep a charge going. I've had Porsche test the battery and it's fine it's just needs a trickle charger. I'm renting and I don't have access to a outlet.

So........ I don't know what to do at this point. It's possible that it may just need a new battery. Since it's the original OEM. But even so the battery is only 3-4yrs old... For an OEM battery that's not too bad. Is there any device anyone can think of like a lithium batter pack that you can charge a battery from or something. I'm desperate at this point since I just don't know what to do. I'm pretty sure the car is grounded well I still have the Porsche Warranty.

Thanks a ton guys for any suggestions.