This charger does not have a diode in it. Without a diode, when there is no light shinning on the panel, the battery will back feed and discharge the battery. You have two choices, install a diode in line on the positive lead with the banded end towards the plug end or disconnect the charger once there is no sun. The charger works as advertised and is great to keep up a fully charged battery. If you have a newer vehicle, they have a continuous battery discharge from things such as alarms, clocks and other things. This charger may or may not put out enough amperage to produce more charging power than is removed by the vehicle. On an older vehicle, there are far fewer things that draw current from the battery with the vehicle off, so the charger will work to keep your battery charged.
This is not a battery charger, it a maintainer for a fully charged battery, as designed.
If you have a newer vehicle, I'd get a 5 watt or 10 watt panel and a charge controller so the higher output from a larger panel will not overcharge the battery. This panel is also great if camping, ect. for the times you are using a battery to power a devise. It will help the battery keep up it's voltage longer.
Overall, it's a great deal if you install a diode. The diode costs less than one dollar. Go to Radio Shack and let them know what the diode is for and they will get you the right part, otherwise, disconnect the panel at dusk.
Also, don't position the panel so shadows fall on it. It's not as critical with this type of panel as it is with some others.

Planning to leave the country for two years, I put my truck in storage in summer 2018, sitting outdoors in Texas, no sun shade. I had just bought a brand new battery for the truck and thought it would be a shame to let the $90 battery die while sitting there. After reading a lot of reviews of inexpensive solar battery tenders, I bought this, feeling no confidence it would work, much less even survive. I disconnected the battery from the vehicle, connected this tender, and left.

When I came back, life happened, and I didn't get my truck back until late 2023, when I towed it to the driveway to sit until now, Feb. 2024.

I just assumed the battery was dead after five years, tender or not, and was already removing the bracket to trade it in on a new one, but I thought, why not just test it? The multimeter said 12V, but I still didn't quite believe, so I connected the battery back up to the truck. I couldn't believe it actually cranked over, and very strong, too!

However, the battery tender's plastic cover melted under the Texas sun. I don't think that's the tender's fault, though: you can see the cracks in the truck's dash that weren't there before.

So it's liking like this turned out to be a good bet: $30 to save a $90 battery.

Not sure how to rate this first one worked great all summer winter came and it dropped down to 11 volts and light dimmed light dimming let me know it wasn;t working, replaced without a problem

I purchased this unit from Amazon in January of 2019 to keep my ridding lawn tractor's battery charged. While my meter said it was still outputting over 20vdc with no load it wouldn't charge my battery anymore (current battery is less than a year old). I replaced this solar panel with the SP-400 and the Schumacher SPC-7A Charge Controller and the battery immediately started charging.

The Internet says solar cells should be good for 20-25 years so 2.5 years is a little disappointing. Hopefully the replacement unit will last longer. While it was functioning properly I didn't have to hook a wired charger to my tractor's battery and wait for it to charge up during those times in the winter when grass cutting is at a minimum.

These units are used to trickle charge two diesel trucks (with two large-capacity lead-acid batteries in each) and the starter battery for a 20 KWatt emergency generator. All three Schumacher units are outside and exposed to the weather. The two maintainers that are used to charge vehicles sit on the outside of the windshields and are held in place by the windshield wipers (the cable goes directly to the battery terminals). The generator start battery maintainer is mounted on a post. After a typical severe winter (-50 deg. F at nights, 45 deg. F daytime) here in the high desert, high-speed grit blasting by the winds, snow, ice cover, and being exposed to very high levels of ultraviolet radiation, all three units are holding up well. The stick-on brand labels have come unstuck and blown away, but there is no evidence of water intrusion into the cases, and the cable shows only the slightest amount of degradation (very slight chalkiness). There are no cracks and all three are still working. My estimate is that these should survive four or five years of constant outdoor exposure, if you need to use them in that way.