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Note: although this page was written with alternator-augmented solar charging in mind, the general principles also apply to solar+shore power charging and solar+generator charging.

Words of wisdom: “Bulk charging requires current. Absorption requires relatively high voltage and time at that voltage.” - secessus

Augmented solar charging

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.

Note that adding alternator charging to the mix does not necessarily require a fancy, expensive isolator; in some ways the simple solenoid provides best bang-for-buck with the fewest drawbacks.2)

how solar helps

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:

  • get access to shore power every couple of days to get the bank fully charged. This will retard the murderification process; or
  • use a different (and $$$) battery alternative like LiFePO4 (no problems with PSOC) or carbon foam (needs full charge every couple of weeks?); or
  • add in a small amount of solar.

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.

how alternator charging helps

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)

when augmented charging works best

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.

augmented charging with lithium

Solar + alternator charging lithium is a bit simpler, and is centered on state of charge.7).


general issues

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

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:

  • any a workaround from the list above; or
  • add a momentary disconnect switch; or
  • use a plain solenoid that would disconnect when the vehicle's key is turned off; or
  • induce a large enough load to lower house voltage temporarily
especially with PWM
see the Gotchas section
The solar will also contribute during Bulk when the sun is shining.
for the amount of power produced
particularly true for lead, although lithium charging voltages <3.5vpc typically require some Absorption time to get to a given SoC
with lead batts it's both about state of charge and stage of health
Vabs, Vequal most significantly
lithium typically the lowest ~14.2v, around ~14.4v, and flooded the highest ~14.8v
ie, sun goes down
electrical/12v/alt_and_solar.txt · Last modified: 2022/07/31 00:19 by frater_secessus