Note: this information should be regarded as experimental. It is being tested by secessus.
This article describes a small, simple, space-efficient, and inexpensive solar configuration that may be of use to cardwellers or minimalist vandwellers.
Most of the RV world is familiar with deep-cycle 12v systems; these are intended to store power for use overnight. If most power is consumed in the daytime when the system has excess power then deep-cycle batteries may not be required.
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.
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 closer to Vmp. Consider the popular Renogy 100w panel that puts out about 5.29A. At the lower LVR the panel4) will make 68.77W. At the higher LVRs the panel will make 71.42-74.06W
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.
The charging setpoint is set somewhat higher, ~14.4v. 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 discharge5). 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 longevity6) 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”8)
… 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.9)
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:12)
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?