Sunday, May 08, 2022 at 14:15
I understand your difficulty out in the bush. Definitely presents some challenges at times.
1. Connecting a solar panel to the Anderson connector on your caravan may or may not work. If, as you say, there is a DC-DC charger in that circuit, it may not even operate at all. A DC-DC charger is looking for an input voltage below 12V from a smart alternator. Modern vehicles incorporate a smart alternator that will charge the engine start battery up to full capacity, then shut down to around 9V. This reduces the parasitic load (via the alternator drive belt) on the engine thereby reducing fuel consumption. The DC-DC charger will start charging when it senses the voltage from the alternator drop and then take the low volts from the alternator and boost its output via an internal inverter up to the required voltage to charge your auxilliary batteries.
I assume the 300W solar panel has a charge regulator attached to it. I suggest you try connecting the solar panel regulator output directly to the battery, even if it was just a temporary connection to see if it works - making sure you get the polarity right!!
2. Batteries have an optimal charge rate based on their AH capacity but will take anything you can give them for a short period. Battery temperature is the limiting factor in most instances when over-charging. So no, batteries do not, in practical terms, only take charge at a certain rate. Your 300W solar panel will, in all probability, not deliver 300W output. What will limit the output of your solar charging system is the efficiency of the panel; regulators capability; ambient temperature (affects the solar panel); amount of solar radiation (from the sun) actually hitting the panel i.e angle and direction of the panel in relation to the suns position and your location (if your are in
Launceston the suns rays travel further through the atmosphere compared to you being in
Darwin). Clouds etc make a big difference so it is my philosophy that you cannot have too much solar!
3. If you had the battery bank get to 14.4V and then changed to the absorb rate with battery volts at around 13V - that sounds pretty normal to me. What it does not indicate, however, is what capacity your batteries have. The fact that hey have been into deep discharge (if that is true) then the probability is you will have lost some capacity. You will not correct any loss of capacity by adding a 4th battery - that would be throwing good money after bad.
It appears that you have the batteries up to their maximum capacity if you can get up to 14.4V at boost charge and maintain around 13V on absorb or float charge. That is as good as it will get for you.
Hope this helps.
FYI, I use 2nd hand household solar panels on my vehicle roof (1 off) and camper trailer (2 off) to charge the auxilliary battery in the back of the ute and house batteries in the camper. These panels have a much higher output voltage (around 42-44 volts) so require a higher voltage regulator such as Victron Energy 75/15. This will take up to 75 volts in and deliver 15 amps out. Cheap and simple. The household panels are strong (withstand hailstones) and very cheap on Gumtree. The big advantage is that you can have long extension leads so that you can place the panels out in the sun away from shade and not suffer the same voltage drop issues that the low voltage auto panels suffer.
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