Deleted member 12051
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Thank you so very, very, much h.The voltage you see on the battery is only an indication of its actual state of charge after it has rested for several hours, with no charge current or discharge current.
The voltage you see when it is being charged is really what the charger has decided it should be.
As the battery is charged, a good charger takes it through three or four charge stages: at first (or second for some chargers, that start very slowly), it pushes power in as fast as it can, and the voltage will go from about 12.0v slowly rising through the 13v range, then when the battery voltage reaches about 14.54v, it controls the charging at that voltage for a while. Some chargers have a fixed time for this phase, some measure the current needed to maintain the voltage at that.
Either way, when the charger is satisfied that the battery is full, it then backs off the voltage to about 13.7v. Some even have a further "back off" phase, dropping to about 12.8v after a few days
So even if all that made your eyes glaze over, the takeaway is that the battery is fully charged when its charger lets the voltage reach 13.7v, but only the SECOND time it reaches that voltage, on the way DOWN from 14.54v.
After the battery is full, if you disconnect the charger and the load, the battery should sit there at around 12.8v for several weeks or months. Unless it is faulty, or unless something is taking power from it, the battery should be 75% charged after six months doing absolutely nothing.
The starter battery normally powers the radio in the dash, the alarm (and maybe the ECU if it is a newish van), so it could possibly run down after a month or two, even if nothing is actually switched on.
In general, the old Zig chargers used to cook batteries if left on all the time, but modern hookup chargers or modern solar charge controllers shouldn't. If the battery is being left at 13.7v or maybe 13.8v when the solar has filled it up, that's fine and I'd leave the solar panel connected.
If you join the two batteries together with your magic fuse, the solar controller will look after both of them.
A smiley face on reading the above this morning and logged in my file. I have taken all of this on board with your easy to follow info. Also, the fuses arrived and voila!!! you were right. Once the 2x 25amp fuses placed back in fuse panel the onboard monitor is now displaying the battery percentage.