The Onderdonk

What a great name for a boat! I stumbled upon the Preece and Onderdonk equations when learning about limits of electrical wires.

I’ve been designing the electrical system for our boat. Currently, we have a solar panel (60W), a lead acid battery (100 Ah), and a handful of navigation and cabin lights. We only use 12V devices on board and don’t have an engine, alternator, or shore power connection.

The battery box: charge controller, battery, fuses, switches all in one place.

The battery box: charge controller, battery, fuses, switches all in one place.

We will be increasing the capacity for new luxuries of a windlass, a laptop, SSB radio, cordless power tools, USB ports, VHF radio, etc.

My method for designing the system:

  1. Loads. Calculate your typical loads (amps) and duration (amp-hours) for a typical day. Not every device on the boat will be on at the same time!
  2. Battery capacity. Aim to have battery capacity of 3x that typical load, so that the batteries are only discharged by 30% per cycle.
  3. Solar panels. Estimate typical solar panel output on a “good” day, and on a typical day.
  4. Charge control. Without a charge controller, solar panels will overcharge and destroy a battery. The controller must be be rated for the charging current.
  5. Wiring. All wire will reduce the voltage and lose a percentage of power. Size the wiring to minimize voltage drop and power loss.
  6. Safety and distribution. Add sufficient fuses and circuit breakers, and an ability to switch circuits on and off.
  7. Iterate. Go back through these steps and make sure it still makes sense. Pricing provides the final decision.

What has been decided?

  • The batteries will be 12V AGM or gel with 160 to 220 Amp-hours capacity.
  • 400 W of solar panels which I estimate to provide 60 Ah/day to the batteries. The panels provide a peak 24 amps.
  • The charge controller will be a Flexcharge NC25A and is capable of handling AGM or gel batteries at a charging current of 25 amps.
  • The wiring will be 2/4 AWG for the main distribution, 10 AWG for the solar panels, and 14 AWG for general instruments.

More updates as we begin ordering and installing the parts!

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