If the panel was tracking the sun and aimed directly at the sun all day, assuming 10 hours of full sunlight. A fixed panel would put out considerably less that that. In fact a fixed panel would likely put out less than half that, so say a maximum of 6-7 amp-hours on a day of full sun. I suspect it would actually be less than that.

A solar panel doesn't need to be regulated if the panel's output is less than the self discharge rate of the battery. In Maine with an average winter temperature of around 32F (0C) - averaged over the entire winter storage period, battery discharge for lead acid batteries should be less than 10%. Assuming a seven month layup period a 225 amp-hour battery bank would self discharge by approximately 22.5 amp-hours (likely less for new batteries). Thus the solar panel needs to be sized to replace that capacity over 210 days (7 months). 22.5 amp-hours/210 = 0.107 amp-hours per day. A 20 watt panel mounted outside the winter cover aimed south at about 40 degrees angle would likely average about 3-5 amp-hours per day on full sun days - say 4 for calculation purposes. Overcast will reduuce that significantly and snow will drop it to essentially zero. If we assume 20 snow days over a typical winter and 10 days a month that are clear with the remaining days at 50% clear. The 20 watt panel could put out about 500 amp-hours over the 7 month layup. That is about 20 times what is needed to keep the batteries fully charged. So your client's solar panel was too big for the battery bank. Furthermore, the situation would be worst at the beginning and ends of the winter when the days are longer and the fixed panel was aimed more directly at the sun. However, if the panels are under a white tarp the panel output will be drastically reduced particularly when you consider that permanently mounted panels on a cabin top are horizontil, not aimed at the sun and likely partly shaded by the ridge pole at all times. That situation would certainly reduce the charge rate of the panels enough so they wouldn't damage the batteries as long as the solar panel capacity wasn't huge. Basically, you can't compare an exterior panel aimed at the sun to a similar horizontil panel under a translucent tarp and opaque frame.

The rule of thumb that I have heard is actually 1% of battery capacity not 10%. How do you get that 20 watts is 9% of a 225 amp-hr battery bank capacity. The 1% rule of thumb that I am familiar with is calculated by taking the panel's output in amps (watts/~13) and dividing that by the amp-hour capacity of the battery. Calculated that way a 20 watt panel is a 20/13 = 1.5 amps (approximately). Dividing 1.5 by 225 gives 0.67%.