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It should work if the open circuit voltage of your solar panel is high enough. A fully charged 12V battery will require some over 13V to maintain the charge. I have a solar panel specifically designed to charge a car battery and I think it's open circuit voltage is about 14-15V.
I'm not sure what you mean by initial charging current being high. It will just be whatever the maximum current output of the solar panel is for the given light input. For your 18W panel that would probably be about 1.5A at direct noon sunlight.
1.5A from 11:30 to 12:30 each day
1.0A for each hour from 10:00 to 11:30 and from 12:30 to 2:00 each day.
0.5A for each hour from 6:00 to 10:00 and from 2:00 to 6:00 each day.
The total is 8.5Ah per sunny day in the middle of summer. Maybe less.
It will take nearly 1 week to charge your little battery. Maybe longer.
To maintain the charge, it shall be ok as I think.
But if the 40AH battery is only left 10~20% charge, if pump in >14.2V 0.5A, will it charge? it will charge very slow (> week), or it will never charge at all?
If a car battery is run down then it gets sulphated and won't charge properly anymore. A more expensive "deep discharge" battery can survive deep discharges. But a car battery can supply the very high current for a moment for a car's starter motor. I think a deep discharge battery doesn't have enough current to start a car.
audioguru- Could I ask where you got this data-
1.5A from 11:30 to 12:30 each day
1.0A for each hour from 10:00 to 11:30 and from 12:30 to 2:00 each day.
0.5A for each hour from 6:00 to 10:00 and from 2:00 to 6:00 each day.
Also, VictorPS- is the solar panel fixed, or does it track the sun?
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