Regard prior questions re. 15 watt solar batt. maintainer

[dznf0g]

The question of whether a 15 watt solar panel is sufficient for battery maintenance while in storage has come up before and I now have some data to discuss definitively. My answers, in the past were based on remote a storage situation and finding the batt full after weeks at the lot. My trailer is now on my own pad at home, so I can offer some daily observations.

Condition:

1) two group 29 Interstates

2) parasitic drain of 100mA due to Trimetric monitor (~30mA while off, ~60mA while on and checking), Morningstar controller (~60mA) and converter (LED light is on even though 120V is disconnected ~10mA?)

3)Store switch is off

So roughly 2.4Ah are consumed per day. The panel supplies about 700mA at direct overhead sun angle (net is 600mA per Trimetric)

7 hours of usable charging at an average (observed SWAG) 400mA = 2.8Ah of charging per day

In a perfect world (in the desert or very consistently sunny areas) 15 watts is more than enough.

Here is a real world observation though. I had 5 days of very cloudy days. At the end of the 5 days, the batts were at 96% and showed a deficit of about 8Ah.

If the following days had been 100% clear, it should take a little less than 3 days to "break even" and be at 100% again.

I am in day 3 of the "recovery" from cloudy weather (It has been partly to mostly sunny for 3 days) and it appears it will take most of 4 days to fully recover to 100%.

So, is 15 watts enough? Barely, and I'm not sure that floating between 95 and 100% on an ongoing basis is a reason to worry about battery health or freezing.

BUT, if i had it to do over again, I'd look for a 20 or 25 watt panel.

YMMV, of course, depending on your parasitic drain, Latitude, average weather, and solar setup.