I have a 100 watt 'flexible' Sunpower solar panel which can fit behind my windshield, without any shadows from the wipers.
On top of the windshield near noon I measured 87 watts into my depleted battery through a MPPT solar controller.
Immediately after, At the same exact angle behind the windshield, I measured 43 watts and this tapered to 37 watts as the panel heated up 15 minutes later.
This is actual data, not opinion.
Any solar panel charger/maintainer behind the windshield needs to reduce the potential expected wattage by ~50% over its claimed rating, and this is at noon at 32 north. Lesser sun angles will be worse.
ALL lead acid batteries take a lot of time to complete a true full charge. The closer the battery is to full charge, the slower it charges.
Getting an 80% charged battery to 100% charged, takes no LESS than 3.5 hours, and this assumes the battery is held in the mid 14 volt range for that 3.5 hours. It also assumes the battery is still relatively healthy. Less than healthy batteries take longer, and in some cases significantly longer before they reach full charge. The lesser the voltage( electrical pressure) they are held at the longer it takes to fully charge them, and less than healthy batteries might never be able to reach full charge at voltages less than the mid 14 range.
Your vehicle's voltage regulator , wherever it is located, whether inside ECM or internal to alternator or on firewall( rare nowadays) is NOT going to hold the battery at this voltage for that long, even if you were to drive that long, so saying the best way to achieve a fully charged battery is a long drive, is incorrect, and helps contribute to batterycide, and collective ignorance of what a lead acid battery needs to remain healthy for as long as possible.
Determining a true full charge is easy with an Ammeter or a hydrometer or both, but no one ever does it, instead assuming that a battery which can still start an engine is fully charged or nearly so. This collective ignorance drives up battery prices for all of us, and wastes resources. Ideally the lead acid battery is always fully charged and kept cool. Anything less than this ideal is detrimental to the battery. The lower the average state of charge, and the higher the average battery temperature, the faster it degrades/ loses capacity and cranking amperage ability.
An AGM can only be positively determined to be truly fully charged, when the amperage it accepts at a higher voltage, tapers to a prescribed level. when a 100 amp hour capacity AGM battery accepts 0.5 amps or less, when held at 14.4 to 14.7v range at 77f battery temperature, only then can it be considered truly fully charged.
Maintenance free flooded batteries are similar but prefer voltages upto 15 and amperages to taper to nearly zero.
Regular flooded starting, marine/dual purpose, and true deep cycle flooded batteries are more like 1 to 3% of their amp/hour capacity, where the AGM is 0.5%.
An AGM cannot be determined to be fully charged via an ammeter when held at lesser voltages.
Green 'full charge' indicators on Smart chargers do NOT indicate a battery is fully charged, only that the charging source has decided to drop to 'float/maintenance' voltage. They usually drop to float around 92 to 95% charged when the battery is newer and healthy and as it ages or it would be lucky to achieve this level. How much charging still occurs after it is held at float voltage depends on the specific battery, its health and temperature and specific float voltage requirements and how long it is held at float voltage. An Ammeter is extremely revealing.
Most AGMS say the proper maintenance/float voltage is in the 13.6v range, where as wet/flooded batteries say 13.1 to 13.2v range. Holding an AGM battery at too little a float maintenance voltage for long periods, is not doing it any favors. Odyssey AGM says outright if you cannot float it at 13.6v at 77f, do not float it but disconnect it from any charging source and recharge it on a regular basis instead.
Any charging of a less than fully charged battery is better than leaving it at a lower state of charge, but achieving a true full charge is the best way to insure best possible longevity and performance during that life, and achieving a true full charge is time consuming and requires the battery be held at a proper voltage for a proper amount of time.
Anybody choosing to confirm a true full charge with observations of actual tools to do so, will be disappointed with their charging source's ability to achieve this especially on older batteries or those cycled deeper, intentionally or otherwise. But ignorance is bliss, and loves company.
Ciggy plugs and receptacles, 12v power ports are horrible unreliable electrical connectors, even if one has a receptacle that remains live after the key is removed from ignition.
The SAE 12v connectors are better, but can wear out quickly and become intermittent or highly resistive. They also have to be either input or output and plugging two inputs together, or two outputs together, results in a direct short and perhaps the magic smoke escapes.
I make use of Anderson Powerpoles, and while these are not perfect, and not inexpensive, and require some crimping skills with regular crimping tools for the 15 and 30 amp versions, or special crimpers for the 45 amp versions, they are much more reliable and can handle much more current and many more connection and disconnection cycles before they wear out, they claim 10,000 under load.
XT60 connectors are also reliable low resistance electrical connectors and much less expensive, but require good soldering skills.
IT is entirely possible that a small solar panel can overvolt / overcharge any given battery. There is a lot of bad information online regarding the requirement of a solar controller when the solar panel is tiny. There are many variables to this, and many uninformed inexperienced opinions stated as absolute fact.
A small panel behind a windshield on a vehicle with a considerable parasitic draw is unlikely to overcharge, but it is also unlikely to ever achieve a true full charge or be able to maintain it there.https://www.youtube.com/watch?v=z5RtZe9AW2E&list=UUoPqTkOluQsuu3RpGnxVwFw&index=9
Lots of solar product 'kits' designed to be put on one's dashboard are extremely overpriced and underperform badly. One can often buy much more capable components individually, for less, but some DIY skills are needed and likely some dedicated fused wires need to pass through firewall to the battery.
It can also be taken to ridiculous extremes in the attempt to achieve perfection.
'Good enough' is subjective, but achieving a true full charge, is not.
The Lead acid battery, whether sealed or AGM or Gel or flooded, all want to be truly fully charged and kept cool, always, and will live an exceptional lifespan if they achieve this ideal.