I found what I was looking for. I'll answer my own post in case any one else needs the information. This site says I need about a 50 mA charger, if any at all.

Trickle charging or maintaining 12V batteries

 

Steve—

Given that you have AGM batteries, if you have them disconnected, you shouldn't need a charger to keep them topped off, since AGM batteries have a very low self-discharge rate. Just make sure the batteries are fully charged before you disconnect them.

If you're leaving them connected, be careful that you don't have parasitic loads that you're not aware of draining the batteries... bilge pumps, stereo memory, etc., are often such loads that you may not be aware of as they're often attached directly to the batteries.

 

I was leaving my boat for a full year so I needed to do a little charging. I did as SD pointed out and put in a battery switch which disconnected everything boat related from the batteries but left them connected in parallel (2 in series and each 12 V pair in parallel.)

I charged the batteries fully before disconnecting the boat loads and then connected a 15W panel WITH A REGULATOR.

I am not sure if this will work (I'll let you know in April) but my research seemed to suggest it would.

A 15W panel could never bring four AGMs up to full charge from a discharged state BUT it hopefully with keep them from discharging to a state that would cause them damage.

It is actually better to let the batteries deep cycle a bit (not letting them get too discharge - see your operating range)

If I could I would just connect a plug in charger every 3 months and bring the AGMs back to full charge I would.

 

When 110 volt is available, I have used an automotive charger plugged into a lamp timer . I set it to charge for 1 hour every 24.

 

Steve, the batery maker can tell you what the self-discharge rate is on those batteries. Typically 30mAH for a Group 24-27 battery, per day, so perhaps...150-200mAH per day for yours?

AGMs are supposed to be able to tolerate 3-6 months of self-discharge with no harm or damage once they are recharged, but again, check with the maker, and see what you would need for your setup. If your batteries are in parallel, you might not want to leave them joined over the winter, since one bad cell in one battery could then ruin them all. And that could mean separate smaller solar panels to charge them, complicating life still further. As opposed to just letting them sit, and coming by once a month or so to check the boat & top them up. (Which will be a quick job with AGMs.)