I am thinking of going to a smaller battery, but I dont' drive my 7 week to 2 weeks at a time, so I have to think of a way to trickle charge the battery, I've found a solar panel charger that does
1.8 watts rated power at 15 volts operating voltage.
is that enough to keep the battery fully charged? Or would it drain more than it can charge? What would be a perfect amount?

 

if you're going with an odyssey battery get a battery tender to trickle charge it. www.odysseyfactory.com and www.batterytender.com have details about it.

 

Well, the diodes in the circuitry should prevent any drainage. As for power consumption, it should just be a few milliamps draw, power for the constant-power items like stereo memory, perhaps the rest of the car's systems draw a few milliamps total.....

If I remember right, watts divided by volts equals amps, so.....1.8 divided by 15 = 120 milliamps that your solar charger will provide.