Battery Monitors, Part 3—Calibration and Use

In Part 1 we chose the best type of monitor for each of us (voltage measuring or amp hour counting) and in Part 2 we selected an amp hour counting monitor from Victron for us control-freak nerds. Now, in Part 3, I’m going to take a deep dive into how to calibrate and use an amp hour counting monitor.

We Got Work To Do

But, wait, these are pretty sophisticated devices. Surely we can just install them and read the state of charge of our battery right off the dial? Sadly, not. In fact, the biggest bitch I hear about amp hour counting monitors is:
The state of charge is never even close to right.
That's bad enough, but I'm also betting there are a lot of cruisers out there managing their batteries based on false readings from their battery monitor because they are not even aware that there's a problem in the first place. So what can we do about that? Well, I have good news and better news:
  • The good: the monitor isn't broken.
  • The better: we can achieve acceptable accuracy fairly easily.
And, even better yet, in so doing we will often improve the efficiency of our electrical system and extend the life of our batteries, too—talk about a win, win, win. Let’s dig in:

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 18 years, as co-editor/publisher of AAC.

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