Online Book: Battery Installation & Maintenance, Chapter 4 of 18

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. But 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 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.

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Meet the Author


John was born and brought up in Bermuda and started sailing as a child, racing locally and offshore before turning to cruising. He has sailed over 100,000 miles, most of it on his McCurdy & Rhodes 56, Morgan's Cloud, including eight ocean races to Bermuda, culminating in winning his class twice in the Newport Bermuda Race. He has skippered a series of voyages in the North Atlantic, the majority of which have been to the high latitudes. John has been helping others go voyaging by sharing his experience for twenty years, first in yachting magazines and, for the last 12 years, as co-editor/publisher of AAC.

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