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Note: this information should be regarded as experimental. It is being tested by secessus. Beginners may want to start with a gentle intro to solar.

Here is a trimmed-down version of this page.

microsolar without deep-cycle batteries

This article describes a small, cheap, simple, space-efficient solar configuration that may be of use to car-dwellers or minimalist vandwellers.


  • extremely inexpensive
  • provides 12v power and USB power during daylight
  • could run small inverter loads (<100w?), up to the ability of the panels to deliver. Note: inverters impose additional losses (typical 10-20%) which can be significant in microsolar setups. Running loads from native DC power is usually preferable.
  • will never deplete the starter battery so you get stranded
  • actively maintains the starter battery
  • taking power from the controller uses less power than using the car's ACCessory position, since other car electronics are not being energized.

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.

how it works

  1. a simple shunt charge controller holds the starter battery at a medium Float voltage (13.2 - 13.8v, see notes below) when charging, this is below the gassing point
  2. power is taken from the controller's LOAD output; 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.
  3. the controller's LOAD output disconnects when battery voltage falls to 100% 1) - no more power is provided
  4. the controller's LOAD output reconnects when charging and battery voltage has risen to >= ~13.0v2) - power once again flows


Once you have the car, the rest of the parts will be about $100 if you watch sales.

  • vehicle with starter battery (you already have this!)
  • 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; 100W polycrystalline this one would work well. Home Depot and others have sales on 100w panels for as low as $73. If you are using a PWM controller a low Vmp spec is preferable. With MPPT a higher Vmp is preferable.
  • connector of some kind if panel is mounted off-vehicle; cable gland if the panels are mounted on the vehicle and the wire is passed through the
  • wiring from panel to controller and from controller to starter battery
  • usb/12v power outlet wired to the LOAD output of the controller, if desired. An outlet with USB ports in it would allow the controller itself to be mounted discreetly and closer to the battery.

a word about battery voltage and solar power

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:

  • 12.7v (full but resting) == 69.85w
  • 13.2v (mild Float charging) == 72.6w
  • 13.8v (high Float charging) == 75.9w
  • 14.2v (shallow cycle charging) == 78.1v

The trick here is to keep battery voltage high enough to allow for good solar harvest while not hurting the battery.

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 the higher the battery voltage.

Note: running a small MPPT charge controller would be much more $$$ ($40 instead of $10) but solar harvest would no longer be hampered by battery voltage.


Shallow-cycling is a more aggressive approach, allowing for more power in the daytime and some power for use after sundown. It may also allow the use of inverter loads that have startup surges. This approach 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.

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 longevity4) 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 death5); 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”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)

sample build

Total cost, ~$140.

  • 100w poly8) 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 area9) 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:10)

  • 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.

getting more power into the cabin

electrical/solar/shallow_cycling.txt · Last modified: 2023/04/08 13:41 by frater_secessus