20W 12V Solar Panel Car Battery Charger - Portable Waterproof Solar Trickle Charger & Maintainer for Car, Boat, RV, Automotive - with Cigarette Lighter Plug & Alligator Clips - Perfect for Camping, Road Trips & Off-Grid Power

Here are my observations. First off I park facing a 2015 Ram Dually with the 6.7L Cummins facing due east - this gives me the front and side windows to mount two in during rhe winter months, and the side window alone to mount one in dyring the summer. The solar cell mat sits well in the passenger window by itself. And I lay the other in the front dash. These also stay hooked in full time - never being disconnected (stow them on the floor between the passenger seat and the door at the belt base to keep them upright).Truck has an Aftermarket Car Alarm that tends to work the batteries moreso than a normal truck.Truck stays sitting for a week at a time (during summers up to a month, and maybe once or twice like this during the winter) in Buffalo, NY. Batteries are 2-each set of $350 Odyssey H7 Batteries, Absorbed Glass Mat (AGM) P/N ODP-AGM94RH7.First off, I run one solar charger in the summer and two in the winter. What I found was that during the winter, a single one was OK, but due to the amount of snow buildup (and just the lower amount of sun), that I needed two running together to top off the battery. And, that and the fact that they tend to go bad in the winter and I couldn't risk loosing charge on a 4 week run and flatten $700's worth of batteries. Also, Batteries always tend to go bad in single digit weather in January and February.Observations:Brand new I could get the charge to 12.4V during the summer during the first year. The winter's, it's more like 11.8-12.2V; but it honestly couldn't seem to keep up, finding it after two weeks straight at 11.4V at times with just the single charger.On year two I noticed the batteries couldn't seem to go north of 12.2V in the summer.In November, I bought a replacement solar charger as the first one finally went; but then I also bought a second one due to the likelihood that while sitting for several weeks straight, that this one charger failing could take out both of these batteries. So now with two Solar Chargers installed, with a dual-headed 12V splitter (Power Drive PD9223BL 2-way 12V Adapter), I was able to get to 12.2V, and even 11.8V with the windows covered (the side window less than an inch, but the front burried) - honestly, the solar cells still charge somehow. So during rhe winter of 24-25 (and at this time of writing), this configuratuon has worked flawlessly!!