The Offshore Voyaging Reference Site

How Batteries Charge (Multiple Charging Sources Too)

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.

More Articles From Online Book: Electrical Systems For Cruising Boats:

  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. Balancing Battery Bank and Solar Array Size
  10. The Danger of Voltage Drops From High Current (Amp) Loads
  11. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1
  12. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 2
  13. Battery Bank Separation and Cross-Charging Best Practices
  14. Choosing & Installing Battery Switches
  15. Cross-Bank Battery Charging—Splitters and Relays
  16. Cross-Bank Battery Charging—DC/DC Chargers
  17. 10 Tips To Install An Alternator
  18. Stupid Alternator Regulators Get Smarter…Finally
  19. WakeSpeed WS500—Best Alternator Regulator for Lead Acid¹ and Lithium Batteries
  20. Smart Chargers Are Not That Smart
  21. Replacing Diesel-Generated Electricity With Renewables, Part 1—Loads and Options
  22. Replacing Diesel-Generated Electricity With Renewables, Part 2—Case Studies
  23. Efficient Generator-Based Electrical Systems For Yachts
  24. Battery Bank Size and Generator Run Time, A Case Study
  25. A Simple Way to Decide Between Lithium or Lead-Acid Batteries for a Cruising Boat
  26. Eight Steps to Get Ready For Lithium Batteries
  27. Why Lithium Battery Load Dumps Matter
  28. 8 Tips To Prevent Lithium Battery Black Outs
  29. Building a Seamanlike Lithium Battery System
  30. Lithium Batteries Buyer’s Guide—Part 1, BMS Requirements
  31. Lithium Batteries Buyer’s Guide—Part 2, Balancing and Monitoring
  32. 11 Steps To Better Lead Acid Battery Life
  33. How Hard Can We Charge Our Lead-Acid Batteries?
  34. How Lead Acid Batteries Get Wrecked and What To Do About It
  35. Equalizing Batteries, The Reality
  36. Renewable Power
  37. Wind Generators
  38. Solar Power
  39. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  40. Battery Monitors, Part 1—Which Type Is Right For You?
  41. Battery Monitors, Part 2—Recommended Unit
  42. Battery Monitors, Part 3—Calibration and Use
  43. Battery Containment—Part 1