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This article describes a small, cheap, simple, space-efficient solar configuration that may be of use to car-dwellers or minimalist vandwellers.
Instead of deep-cycling like someone might in an RV, the idea here is the starter battery can be used but never dragged below 100% state of charge)).
It does require a bit of discipline and forethought to run bigger loads during the daytime when excess power is available.
Non-cycling is the use of solar power only when the battery is fully charged. The starter battery is even more fully charged than in normal vehicles.
Once you have the car, the rest of the parts will be about $100 if you watch sales.
Simple solar charge controllers like these make different amounts of power depending on a bunch of factors. One of them you can control is battery voltage. This is the max output of the 100w HQST at various battery voltages:
The trick here is to keep battery voltage high enough to allow for good solar harvest while not hurting the battery.
The LVR is the point at which the controller turns the LOADs back on. If running small loads (charging phones, LED lights) the setpoint may be set fairly low (like 13.0v). This will allow loads to be run as soon as possible.
If running larger loads (fans, laptop chargers) the setpoint should be set higher (like 13.5v - 14.0v). This is because the panel will be able to make more power the higher the battery voltage.
Shallow-cycling is a more aggressive approach, allowing for more power in the daytime and some power for use after sundown. It is most suitable for starter batteries with removable cell caps so you can add distilled water; they will outgas a bit under heavy use.
The charging setpoint is set somewhat higher, ~14.2v. This will result in minor outgassing and a bit more power (76.18W using the panel data above). Battery watering is mandatory.
Starter batteries are designed for about 15% depth of discharge3). We can set the Low Voltage Disconnect (Vlvd) to 12.5-12.6v as a floor. In a typical starter battery this could be 5Ah of power usable at night, enough to run an LED light for hours as well as run a small fan all night. All device charging should happen in daylight when power is relatively plentiful.
If the starter battery (when it eventually dies a natural death) were replaced with the heaviest 12v wally world marine battery that would fit one might be able to cycle to 25% DoD. This would mean an Vlvd of ~12.4v and 8-9Ah of power at night.
No-cycling never discharges the battery below 100% state of charge. Looked at another way, no-cycling keeps your starter battery charged all the time.
Because of this, no reduction in the ~36 month average longevity4) is expected.
Shallow-cycling discharges the battery roughly to the level normally experienced in automotive use. But since:
…again, no reduction in the average ~36 month average longevity is expected.
“most of the “defective” batteries returned to manufacturers during free replacement warranty periods are good”6)
… A charging regime might even extend the perceived longevity of the starter battery.
Anecdote: TreborEnglish runs his 75A flooded 12v wally world deep cycle to 87% SoC regularly and it has lasted 3 years as of this writing. He reports it requires 20-30ml of water/month.7)
Total cost, ~$140.
The chemistry of starter and “hybrid” (“marine”) batteries is somewhat different than deep cycle batteries. Speciically, starter batteries typically have calcium added to the positive grid to minimize self-discharge and outgassing.
Problem: calcium-enhanced grids are more susceptible to “positive grid corrosion” from sustained higher voltages.
The question is this: what voltage can starter batteries be held at without damage? One answer might be be “alternator voltage” since that's already happening when we drive vehicles, and they are not damaged by long journeys.
CTEK's starter battery charger designed to be left on charges thusly:10)
So a charge controller with that configuration should be gentle enough on the battery.
This is the tricky part. Shunts (or controllers set with Vabs == Vfloat) have one charging voltage. What should we use?