How Batteries Charge (Multiple Charging Sources Too)

One of three battery chargers, together with two alternator voltage regulators (one hot spare), on "Morgan's Cloud".

In the last chapter I explained Ohm's Law, that wonderfully elegant and simple relationship between three variables that allows us to clearly understand just about any problem we have with our boat's electrical system.

Now let's look at some practical applications: how batteries charge and voltage regulators work—two of the most misunderstood pieces of gear on a voyaging boat.

To make this easy, we are going to use lead-acid batteries in all or our examples. Yes, I know, you want lithium. That's fine, we will get to those in a later chapter and the stuff we learn in this chapter will make understanding lithium systems way easier.

First we need to do one of those algebra trickery things (that we all slept through in school) so Ohm's Law will get us the answer for any one variable as long as we know two others:
ohms-law- That is:

  • Amps equal volts divided by ohms.
  • Ohms equal volts divided by amps.
  • Volts equal amps multiplied by ohms.

Real World

Now let's say:

  • Our system is 12 volt.
  • Our lead-acid battery bank has 400 amp hours capacity and is half discharged.
  • We are sailing along using 20 amps for various loads.
  • We have 200 watts of solar panels and it's sunny so they are putting out 100% of their capacity—unlikely, but it doesn't matter for our purposes.
  • We have a 100 amp alternator on the engine.
  • Both the solar panels and the alternator have regulators.

  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?
Notify of
Inline Feedbacks
View all comments