Hi Magnet, and sorry this is so long but here is a bit more info on using unregulated solar panels to charge and maintain batteries with safety and how it works.
There is a lot more to it but I will try to keep this in an understandable form.
All batteries have an internal resistance and this resistance is the reason for the amount of current a battery draws, while charging.
With all forms of resistance, as you increase the voltage difference across the resistance, there is a corresponding increase in the amount of current travelling through that resistance.
This current travelling through a battery is commonly termed as the current load, and from this point on, to keep this as basic as possible, I will not use resistance and only refer to the current or current load.
To charge a battery you need both a voltage higher than the battery's State of Charge voltage ( SoC ) and some amount of current.
If you are charging from an alternator and you have a very low auxiliary battery ( in most D4s, this will be an Optima 55Ah AGM ), then there is going to be quite a high current being drawn by the auxiliary battery, and this current draw can initially be as much as 100+ amps.
In the above case, your auxiliary battery can only draw such a high current because the D4 alternator is capable of producing those sorts of currents.
With your 20w solar panel setup, the VERY maximum current available current, and this will only be for a few hours a day, is around 1.25A.
Over an 8 hour period of good sunlight, you would be very lucky to average as much as 1A. With no load applied to a "12v" solar panel ( called open circuit ), the panel can generate up to 24v
The reason for the difference between the rated wattage and the actual wattage you end up with is because of two related factors.
The first is that to try to get as high an output from a solar panel all day long, you need to have the solar panel track the sun. This means, the solar panel must continually be repositions so that it is facing directly at the sun, as the sun moves across the sky, from horizon to horizon.
Your setup will, like the vast majority of solar panel setups, will have the panel facing directly at the sun for no more than a few hours.
And as this wasn't enough to reduce the average amount of power coming from the panel, at the point where the panel is facing directly at the sun, the sun's energy will heat the panel and as a solar panel's temperature rises, it's total wattage output reduces.
So at the very time you think you are getting the maximum POTENTIAL ( 20w ) output from your solar panel, it will actually be down by as much as 30% ( down to around 14w ) .
OK so that covers the very basics of battery charging and solar energy production. Now to put the two together.
As above, a 20w solar panel has a maximum current output of just 1.25A and this will be for a small portion of a sunny day.
So at best, you have a charge current of 1.25A, and if you battery is low, it will be trying to draw all the current it can get.
Now if you apply a high current draw to a solar panel, the panel will produce all the current ( watts ) it can but the much higher current draw of the battery will pull the solar panel voltage down to pretty close to the battery's internal voltage.
So if the battery voltage is at say 11.0v, the solar panel voltage will be around 11.2v at the battery's terminals.
With this sort of very low current charge, it will take quite a few days before the battery gets anywhere near a fully charged state. But once fully charged, the battery will now draw very little current, so the solar panel voltage will rise at a greater rate than when it was being used to charge the low battery.
With a fully charged battery ( around 12.7v ), even a small current applied will raise the battery's internal voltage considerably.
So at the peak of the day, when your solar panel is putting out it's maximum current ( wattage ) the voltage at the battery will now rise substantially.
But as the solar voltage at the battery rises above the battery's internal voltage, the higher voltage difference will cause the battery starts to try to draw more current, and this higher current draw only has 1.25A available ( the maximum output the 20w solar panel can ever produce ) so the solar panel voltage can only rise until it reaches the solar panel's maximum current ( wattage ) output.
The batteries literally become their own voltage regulator.
Magnet, with your setup, the maximum likely voltage, at the high point of the day, is not likely be much over 14.0v
NOTE, if you use a larger unregulated solar panel, while the batteries will still act as their own voltage regulator, the high current ( wattage ) available will mean the voltage at the batteries will rise.
For instance, I use an 80w unregulated solar panel to recover badly sulphated batteries and a 100Ah battery will rise to 17.5v using an unregulated 80w solar panel. PS, do not carry out this type of charging on a good battery.
Your batteries can tolerate 14.7v for a number of hours each day, with little effect on the batteries. For example, your D4 alternator can run at 14.7v for long periods of time, depending on the state of charge of your cranking battery, and this high voltage has no effect on other batteries connected to the alternator, even if they are already fully charged.
Solar charging in some ways is very like charging from an alternator. When charging from an alternator, you have to be driving and few people drive long enough for even 14.7v to effect battery longevity.
With solar, the similarity is that it only charges the batteries while the sun is shining and even then, it only charges at the solar panel's maximum possible current ( wattage ) for a short period of time.
As Dave posted, test your battery/solar voltage at the high point of the day, and you will then know for sure that you will never over charge your batteries, no matter how many weeks/months you leave them in this type of setup.
