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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. Also see Deciding on a Power Mix.

Solar and DC-DC alternator charging are going to be the best practices for maintaining charge in one's battery bank. It only requires a little math, a little discipline to stick with the math, and the right expectations about solar intake. – CMDR_Schrodinger1)

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.2) 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.3)

how solar helps

Solar works anytime there is available sunlight, not just when the vehicle is running.

Solar can meet both the voltage and time requirements necessary for complete charging; this is critical with lead-acid chemistries which will otherwise suffer damage from partial states of charge.

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 ←- what this article is about

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.4)

Having a bit of solar may mean you can use a simpler/cheaper combiner rather than a more $$ DC-DC charger.

how alternator charging helps

Alternator charging is extremely powerful and cheap compared to typical solar installs. It works best when solar works worst: when the batteries are deeply discharged in the morning. 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 alternator voltage (Valt) is reached PWM controllers are able to produce meaningful power.

Alt charging shovels current into the bank until Valt is reached, at which point it begins to taper off. 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)

The alternator can help meet lead bank minimum charging current requirements.

optimal alternator contribution

Broadly speaking, alternator charging is most effective in the morning and solar charging is most effective at local solar noon. This pattern is most evident when using isolators/relays and/or lead chemistry batteries, and less evident when using DC-DC chargers and/or lithium chemistries.

There are several factors in play here.

charging stage

During Bulk charging stage the bank will accept maximum current.6) For most 'dwellers alternator charging current will outstrip solar charging current, making alternator charging most helpful during bulk. Bulk typically coincides with early morning.

voltage delta

Current varies proportionally with the difference (“delta”) between the charging voltage and the battery bank voltage (Vbatt). So lower Vbatt and/or higher charger voltage means higher current,7), which means more Ah/Wh replaced in a given amount of time.

battery voltage

Vbatt is most likely to be lowest in the morning, when the bank is most depleted.

charger voltage

The device used for alternator charging affects the charging voltage side of the equation:

  • DC-DC chargers8) typically have the highest charging voltages. Example: 14.4v
  • Isolators/solenoids pass through alternator voltage. Example: 14.2v
  • FET-based isolators typically pass 0.1v less than alternator voltage. Example: 14.1v
  • Diode-based isolators typically pass 0.5v less than alternator voltage. Example: 13.7v

If we stipulate our discharged bank is sitting at 12.2v then the voltage delta could be anywhere from 2.2v to 1.5v. It doesn't sound like much, but all other things being equal +2.2v charging will result in ~47% more current than +1.5v charging.9)

Since the output of solar charge controllers is relatively high (typically the same setpoints as the DC-DC charger), the ability of an isolator to contribute current is reduced as solar harvest comes into play.

PWM solar charge controllers

Getting bank voltage up earlier will also keep PWM solar charge controllers from getting hamstrung due to low Vbatt. MPPT controllers are not affected by Vbatt.

putting it all together

Alternator10) charging is most helpful/efficient11) in the morning when the bank voltage and state of charge are lowest. This may mean shifting driving errands to mornings or rainy days.

If an engine is running solely to charge the bank (idling, or running a generator) optimal use may mean running them through Bulk and early Absorption. Engine charging would cease when Absorption voltage is attained and current has fallen off to a point solar can handle it.

augmented charging with lithium

Solar + alternator charging lithium is a bit simpler, and is centered on state of charge.12). It is still most effective to charge with alternator in the morning, but the importance is lessened because:

  1. the Li voltage curve is very flat in the middle, so the voltage delta will be stable through the “middle 80%”
  2. for average users, Li will not have an Absorption stage in the lead-chemistry sense.13)
  3. Lithium is often charged with DC-DC, which charges at the ~same voltage as solar


general issues

When the chassis and house batteries are connected, deep cycle charging voltages14) from solar or other sources will be passed to the chassis. Since charging voltages differ15) 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.17)

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.
by definition
assuming the charging source can provide it and bank accept it
and solar charge controllers and converter/chargers run off generator, see below
or generator
Ah replaced for a given amount of engine runtime
with lead batts it's both about state of charge and state of health
advanced users may find exceptions to this general statement
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: 2024/03/31 17:05 by frater_secessus