I'm using a 15 watt solar panel to maintain a 650 amp deep cycle battery in my old bug out trailer.

The instructions say I need a 7 amp controller.

I didn't think 15 watts would overcharge the battery... right now I've got it charging full time.

Opinions ?

A 15 W panel would put out about 1 amp in full sun.  With a 650 amp-hour battery pack you should be fine.  BUT... you need to see of the panel has a blocking diode to prevent the battery from discharging into it when it's dark out.

15 watts would be about 1 amp (or 1,000 mA) at 15 volts, in the brightest sunlight available.

After your battery reaches a full state of charge, it's only going to require a few dozen mA to keep it fully charged (assuming it's not sulfated or otherwise in poor condition).

Forcing an amp through it for 4-6 hours a day is probably going to overcharge it.

Measure the voltage across the battery, after it's fully charged and has had the panel connected to it for the good part of a day in bright sunlight. If the voltage measures much above 13.5 volts (which is considered to be the ideal long-term "maintenance" voltage), it'll lose water quick, and will die prematurely - in which case you either need to disconnect the panel manually, or install a charge controller.

BTW, since your panel is only capable of supplying about 1 amp, you would only need a controller that's capable of handling at least 1 amp.

I searched and found 7 amp controllers for $16-60.

Given the cost of the panel, and the battery, I think that's cheap insurance for both.

Get it.  

The panel does have a block diode.
The highest panel voltage I've read in direct sunlight is 18V.

Good idea on just monitoring the battery voltage...
It pretty much stays at 12.8V (with the panel charging).

I'm using one of these $15.00 voltage readout monitors from Wally World.    

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Thanks for the replies, I appreciate it