The Danger of Voltage Drops From High Current (Amp) Loads

In the last three chapters we figured out what battery bank size we need for normal usage. But what about really big loads like induction cook tops.

How big do our batteries need to be to supply the peak loads we will be taking on without the voltage dropping below acceptable levels, even for an instant?

Loads We Need to Think About

To calculate that we need to be realistic about:

  • The worst case total load if multiple high-load devices are used at the same time.
  • The acceptable lowest voltage.

Not Just Electric Cooking

And further, this is something we need to take into account before installing any of the high-current-draw electrical gadgets becoming prevalent on boats today:

  • Electric cooking
  • Electric winches
  • In-mast and in-boom roller furlers
  • And on it goes.

That said, we probably don't need to take into account loads like:

  • Bow and stern thrusters, since the main engine will be running and supplying amps, at least as long as there is a big reliable alternator installed.
  • AC loads like washing machines and dryers, as long as we are willing to restrict their use to when the generator is running as part of our daily charge cycle.

(Note that I'm not advocating for installing either of the above, just being even-handed.)

Lowest Acceptable Voltage

11.75 volts is the absolute minimum we ever want to see on an offshore cruising boat's battery terminals, even for a moment.

Why that number? If batteries get below 11.75 volts, it's likely that the voltage at the inputs to important gear like radar, autopilots, NMEA 2000 networks, etc., will dip below 10.5 to 11 volts, depending on losses in the feed cabling (more on that in the next article), which is the level where said gear will get flaky, or even shut down completely.

It Matters

To see how important this is, just imagine we are broad reaching along in big tradewind seas while the cook is fixing dinner with two induction rings on—pasta and sauce—and the crew on deck trims the jib with an electric winch.

...The NMEA 2000 network chokes, the autopilot loses heading input from the compass...wipe out...dinner splattered all over the galley...cook is severely pissed off and goes on strike...survive on sandwiches for the rest of the voyage.

And that's a comparatively benign result. If we were running off, there could easily be a crash jibe, and if the preventer parts off—just happened to some friends of mine—horrible injuries or even a fatality are a real possibility.

Max Load

So let's use that scenario to calculate some average and maximum current loads (amperage):

So which number should we use: maximum or average? I guess that depends on how scared we are of crash jibes. Me? Going with maximum every time.

Battery Behaviour

So the next thing we need to think about is how big the battery bank needs to be to stand up to these kinds of loads without the voltage dropping lower than 11.75.

  1. Why Most New-To-Us Boat Electrical Systems Must Be Rebuilt
  2. One Simple Law That Makes Electrical Systems Easy to Understand
  3. How Batteries Charge (Multiple Charging Sources Too)
  4. 5 Safety Tips For Working on Boat DC Electrical Systems
  5. 7 Checks To Stop Our DC Electrical System From Burning Our Boat
  6. Cruising Boat Electrical System Design, Part 1—Loads and Conservation
  7. Cruising Boat Electrical System Design, Part 2—Thinking About Systems
  8. Cruising Boat Electrical System Design, Part 3—Specifying Optimal Battery Bank Size
  9. The Danger of Voltage Drops From High Current (Amp) Loads
  10. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1
  11. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 2
  12. Battery Bank Separation and Cross-Charging Best Practices
  13. Choosing & Installing Battery Switches
  14. Cross-Bank Battery Charging—Splitters and Relays
  15. Cross-Bank Battery Charging—DC/DC Chargers
  16. 10 Tips To Install An Alternator
  17. Stupid Alternator Regulators Get Smarter…Finally
  18. WakeSpeed WS500—Best Alternator Regulator for Lead Acid¹ and Lithium Batteries
  19. Smart Chargers Are Not That Smart
  20. Do You Need A Generator?
  21. Efficient Generator-Based Electrical Systems For Yachts
  22. Battery Bank Size and Generator Run Time, A Case Study
  23. Battery Options, Part 1—Lithium
  24. Battery Options, Part 2—Lead Acid
  25. Why Lithium Battery Load Dumps Matter
  26. 8 Tips To Prevent Lithium Battery Load Dumps
  27. Building a Seamanlike Lithium Battery System
  28. Lithium Ion Batteries Explained
  29. 11 Steps To Better Lead Acid Battery Life
  30. How Hard Can We Charge Our Lead-Acid Batteries?
  31. How Lead Acid Batteries Get Wrecked and What To Do About It
  32. Equalizing Batteries, The Reality
  33. Renewable Power
  34. Wind Generators
  35. Solar Power
  36. Hydro Power
  37. Watt & Sea Hydro Generator Review
  38. Battery Monitors, Part 1—Which Type Is Right For You?
  39. Battery Monitors, Part 2—Recommended Unit
  40. Battery Monitors, Part 3—Calibration and Use
  41. Battery Containment—Part 1
  42. Q&A—Are Battery Desulphators a Good Idea?
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