Review on Renogy Solar Battery Maintainer Trickle by Dontra Majmudar

My 4runner's battery drains a bit when not in use, so I bought it to keep it charged. I have it on my dash so the glass might lose some, also it only gets sun a few hours a day and now the winter sun is at a low angle. Knowing I was going to use it that way, I bought a 10 watt panel, thinking it would be enough as many service people only have a few watts. But it doesn't seem to do much. I fully charged my battery with my 110 charger a few days ago and two days later my battery showed 12.43 volts. I don't know if he will keep it there or if he will keep falling. I put my multimeter on the panel itself and saw 21 volts (optimal specs are 18 volts and 0.56 amps, and the short is 23.5 volts, so maybe within spec, although 21 volts seems higher than ideal). But for current I only see 0.12 amps on a nice sunny winter day in Colorado. A total of only 2.5 watts. I pushed the panel out the window to see if that would make a difference and the amps stayed the same so it's not a window tint which I don't think my front window has. I'm surprised people with even smaller panels say they keep their batteries charged, but I may need to upgrade this 10 watt panel to something bigger. It wasn't overly expensive so not a big deal but still a bit disappointing. If anyone else has feedback on their device's performance, I'd be interested in hearing it. The quality looks and feels good except the suction cups don't stay in place. I tried attaching it to the window and it fell off both times. I wouldn't trust them, but you can use wire to secure that or something. It's not something you'd want to leave outside in the winter, so it's really not weatherproof, but I never intended that. I trust the Renogy name and suppose it should be as good as other brands and cheap Chinese ones, but I'm still not sure if it will work to charge my battery. I have no intention of moving the panel to get maximum sun exposure or anything like that. Renogy says there is a non-return valve so the battery shouldn't be drained unless it's in direct sunlight or at night, but 2.5W for a few hours a day might not be enough to help me.