Car-battery hooked to portable speakers

Hi people.

I'm brand new on this forum. Already looking forward to surfing around Here's my first question.

I own a "Logitech Pure-Fi Anywhere 2" (the improved version that has radio-shielding so it won't stutter when my iPhone recieves data).
the festival I'm planning on going to this year lasts for roughly ten days. Thus I expect the battery to run out long before that.

I'm thinking if it's possible to directly hook up a 12V car battery to my little baby. When she's pugged in with the regular charger she receives 12.9V at 1.39 amperes. Which corresponds to rouhly 17-18 watts if I'm not mistaken.

My questions are these.
1. Would it work?
2. Should I build a protective system of some kind?
3. Some years temeprature in the shade reaches 30 degrees celsius. Issue?
4. Can I do rough calcuations on how long a car battery would last? I honestly don't know how many miliamperes the speaker battery is

The speakers are tiny so I don't think their current is high. Logitech's spec's for it are completely useless.

You say it charges its battery at 1.39A and Logitech says a charge plays it for 10 hours so a 70Ah car battery should power it for 504 hours which is 2 months at 8 hours per day.

Is it that simple? That would be terrific.

Of course the battery drain depends on how loud the speakers are played and by what type of music or noise you play.
Some acid rock is continuous noise full blast all the time. Most music has the average power at 1/10th the peak power.