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Note: this information should be regarded as experimental. It is being tested by secessus.

microsolar without deep-cycle batteries

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.

Instead of deep-cycling, the starter battery is non-cycled1) as described below. It can also be shallow-cycled.2)


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.

how it works

  1. a simple shunt charge controller holds the battery at a mild Absorption voltage (14.0, - 14.2v, see notes below) when charging, just below the significant gassing point
  2. the controller's LOAD output disconnects (LVD) at 100% SoC (~12.7v)
  3. the controller's LOAD output reconnects (LVR) at ~13.0v)
  4. power is only taken from the controller's LOAD output;3) this ensures the starter battery is never discharged below the intended setpoint. Since the load is disconnected below 12.7v the battery cannot be run down by loads.

Low Voltage Reconnect (LVR) setpoint

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

Note: running a small MPPT charge controller would make this immaterial, as the controller would run the panel at max power (Vmp) at 18.9v.


  • vehicle with starter battery
  • simple 10A controller **with configurable setpoints**. The controller should be mounted in a location easy for easy access if the built-in USB ports are going to be used.
  • small solar panel; 25w-100W would be fine.
  • connector of some kind if panel is mounted off-vehicle
  • wiring from panel to controller and from controller to starter battery
  • usb/12v power outlet. An outlet with USB ports in it would allow the controller itself to be mounted discreetly and closer to the battery.


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.

effect on starter battery

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:

  1. low water level is a major cause of auto starter battery premature death7); and
  2. battery watering is mandatory in this scenario

…again, no reduction in the average ~36 month average longevity is expected.

Further, since

“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)

sample build

Total cost, ~$140.

  • 100w poly10) panel on the roof, either a rack-mounted framed panel that fits rack dimensions or a flex panel adhered to the surface.
  • cable gland for getting wires through roof
  • panel wires runs down the vehicle A-pillar inside the passenger area11) and into the PANEL terminals of a $10 shunt solar charge controller mounted in the footwell or elsewhere
  • wire run from controller's BATTERY terminals to the starter battery.
  • wire run from controller's LOAD terminals to 12v/usb outlet, power bus, etc

charging voltages and starter batteries

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.

staged charging

CTEK's starter battery charger designed to be left on charges thusly:12)

  • 14.4v Absorption
  • 13.6v Float

So a charge controller with that configuration should be gentle enough on the battery.

single voltage charging

This is the tricky part. Shunts (or controllers set with Vabs == Vfloat) have one charging voltage. What should we use?

  • folks who shallow cycle might charge to 14.4v to help counteract sulfation from overnight cycling.
  • folks who non-cycle might charge to 13.6v as if the battery is simply being maintained. This leaves some power on the table (~8.5%) for non-MPPT controllers but may be easier on the battery.
electrical/solar/shallow_cycling.txt · Last modified: 2020/10/11 19:48 (external edit)