Words of wisdom: “Bulk charging requires current. Absorption requires relatively high voltage and time at that voltage.” - secessus
Alternator + solar charging has considerable benefits over alternator charging alone, and over solar alone. The alternator can help a small solar install punch well above its weight, acting like a much bigger install in areas of poor insolation.1) Solar can hold the bank at a higher state of charge, thereby reducing current demands on the alternator and allowing the use of smaller DC-DC chargers.
Solar works anytime there is available sunlight, not just when the vehicle is running.
Alt charging alone is unlikely to get lead-acid chemistries fully charged since Absorption can take hours and alternators often cannot hold the required voltage. Incomplete charge (Partial State of Charge, or PSOC) over time will result in battery murder.
How to avoid (or attenuate) battery murder with this scenario:
The reason for this is related to how lead-acid banks charge; they can take a ton of current in the first ~80% of charging (Bulk) then they hit a kind of wall. At that point you could drive 100mph or hook the bank up to the Hoover Dam's electrical output and it wouldn't finish charging significantly faster.
From ~80% to 100% (Absorption stage) charging requires little current but a long time holding at a relatively high voltage. So in our alt+solar scenario the alternator does the heavy lifting in the beginning then a small solar install does the light (but lengthy) work at the end.3)
Having a bit of solar may mean you can us a simpler/cheaper isolator rather than a more $$ DC-DC charger.
Solar power is relatively weak and expensive4) compared to alternator charging. Solar is most challenged in the morning when the bank is deeply discharged. The sun is weak then but the bank needs a large supply of current (early Bulk charging).
PWM controllers are especially hamstrung in this scenario; since they run the panels at battery voltage (Vbatt) they produce the least power exactly when the most power is required. By the time Valt is reached PWM controllers are able to produce meaningful power.
Alt charging shovels current into the bank up to the point that alternator voltage (Valt) is reached. Solar can take it from there. A good metaphor might be a two-stage rocket: the alternator does the heavy lift getting the rocket off the ground then the solar charging takes care of the high altitude, high speed duties.
Use a generator or alternator to get to 85% SOC or so early in the AM, then let Solar spend all day polishing off the last 15% because that takes time more than amps. - a64pilot5)
Boosting solar charging with alternator, genny, etc, works best during bulk stage. This is true for alternators because their terminal voltage is rather low, and for both generators and alternators because Absorption stage takes little current and a great deal of time.6)
With generators, most efficient use will be when banks are deeply discharged, as the genny will run closer to its rated power. Generator charging would cease when Absorption voltage is attained and current has fallen off to a point solar can handle it. Alternator charging would cease when house battery voltage reaches alternator voltage; once solar takes the voltage higher than the alternator puts out the alternator is no longer contributing.
Solar + alternator charging lithium is a bit simpler, and is centered on state of charge.7).
When the chassis and house batteries are connected, deep cycle charging voltages8) from solar or other sources will be passed to the chassis. Since charging voltages differ9) this could be an issue if house batteries were flooded; flooded starter batteries would require more frequent watering. “Maintenance-free” sealed flooded batteries might be expected to fail earlier due to water loss. AGM starter batteries might be overcharged by flooded charging voltages, though voltage drop across the relay and inter-battery wiring may serve to attenuate that possibility.
If overvoltage is a concern, one can use one of these workarounds:
Voltage sensing relays can get "stuck connected" if the alternator and solar were charging the same time. This situation would exist until house voltage dropped below the VSR's disconnection setpoint.11)
It is effectively the same issue as the issue above. Workarounds: