Suspect panel: At what voltage should a 12V panel be charging a battery?

[Sunking]

A1. Nothing of any value

A2. Depends on your location and time of year. You have already discovered a off-grid solar battery is only 50% efficient. The formula is:

Watts x Sun Hours x .5.

So this time of year if you are getting a full 3 Sun Hours then 10 watts x 3 hours x .5 = 15 watt hours.

[Simon99]

I took another set of readings today which threw out my earlier theory. So I plugged the battery into a discharger at a rate of 400mA for 4 hours before putting it back onto the panel when the sun had already begun to obscure it at 3pm. The charge controller pulled a low voltage disconnect on the load and the panel was only charging at 0.2W and 12.45V. My latest test is therefore to see when the panel jumps up in voltage, starts charging a partially discharged battery (and at what rate) under load.

There are a lot of variables to consider. So far, the 10W panel has been 100% reliable out in the field for over 3 months. But I'm working on gathering empirical evidence for why. The first prototype was part engineering, part luck. I'd like to eliminate the latter, especially since I have a bigger design in progress.

[Sunking]

The key is knowing how many watt hours you need in a day, loaction, and time of year use. Once you know that the design is made and then the "LUCK" part does not exist.