Do you use a good 3-4 stage charger at home between trips?
The reason I ask is that if you fully charge at home, then drive with the battery connected to alternator (Via starter battery positive terminal) by the time you arrive you will have lost some charge (up to 35 % depending how long the trip was)
Here is a quote from an engineer (Collyn Rivers) as to why an alternator is a poor charger
an alternator's field is generated by a current flowing through internally rotating wire coils. The stronger that current, the stronger the field, and the greater the alternator's output.
The strength of that field, and hence the alternator's voltage output, is controlled by the voltage regulator. That regulator attempts to maintain the alternator's output at a constant 14.2 14.4 volts regardless of battery charge, or whether or not windscreen wipers, big audio systems, air-conditioning etc., are on or of.
The starter battery has an integral and vital role in this system of control. Whilst that battery does not normally supply current whilst the engine is running, the voltage across it reflects whatever is happening in the electrical system. If, for example, the rear window demister is turned on, the increased load causes battery voltage to fall.
The regulator continuously monitors this voltage, either across the battery or from within the alternator (whilst battery and alternator are connected directly by heavy cable there can be a voltage difference between the two due to losses along that connecting cable).
Knowing. that the system should operate at 220.127.116.11 volts, the regulator adjusts the alternator's output voltage by switching its field on and off at very high speed (Fig. 4.1). Smoothed by the battery, this output appears to the system as a constant voltage.
As noted in Chapter 2, 18.104.22.168 volts output is a compromise. It's high enough to spin the starter motor, but also low enough to prevent overcharging unless the vehicle is driven non-stop for days on end. But that 22.214.171.124 volts output is too low to charge any conventional lead-acid battery beyond 70.75% in any practicable length of time.
Experts in this field are increasingly quoting 65% as a more realistic norm.
That explains why a standard alternator is a poor charger, why the vehicle system will discharge the previously fully charged battery is a little more complex, but briefly if a charged battery is attached to a discharged battery then they try to equalise, so the leisure battery you charge at home is brought down to the level of the starter battery.
One day I'll have to get all this down in one place for easy reference.....