The Offshore Voyaging Reference Site

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:


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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. Lithium Batteries Buyer’s Guide—Part 3, Current (Amps) Requirements and Optimal Voltage
  33. 11 Steps To Better Lead Acid Battery Life
  34. How Hard Can We Charge Our Lead-Acid Batteries?
  35. How Lead Acid Batteries Get Wrecked and What To Do About It
  36. Equalizing Batteries, The Reality
  37. Renewable Power
  38. Wind Generators
  39. Solar Power
  40. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  41. Battery Monitors, Part 1—Which Type Is Right For You?
  42. Battery Monitors, Part 2—Recommended Unit
  43. Battery Monitors, Part 3—Calibration and Use
  44. Battery Containment—Part 1
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Roberto

Hi John,
very useful as usual, thanks. Which values should I use for my 24V system?

Dick Stevenson

Hi John and all,
I have been gearing up to upgrade electrical monitoring recently as my venerable Link 20 has been giving signs that its days working may be coming to an end. For almost 20 years it has been giving me accurate readings of voltage and amps in and out and, early on when I paid attention to it, SOC.
I am pleased that your researches in instrumentation in this area match what I have been leaning toward, and appreciate the added details and the thinking considerations that you include in your reports.
I have gravitated over the years of living aboard to what I think of as a seat-of-the-pants appraisal of SOC. I probably check, takes but a second, the v and amps in/out at least ½ dozen times a day which gives me a picture of the moment as well as a sense of changes over time. For years that was in conjunction with the “amp counter”/SOC capabilities of the Link 20, but gradually I stopped using these capabilities (too fussy and I felt they did not contribute much) and just used v and amps in/out and my multiple checks a day to guide me.
It is my take, subjective as I know it to be, that I my seat-of-the-pants assessment of SOC is not far off while also clear that I have little in the way of base line for this assessment. I will be interested to see, with the new equipment, if they do actually contribute in a substantive way to my battery management.
My best, Dick Stevenson, s/v Alchemy

Dick Stevenson

Hi John,
Agree with the above. I will be curious about what I learn from the new instrumentation and how it changes my patterns.
Thanks, Dick

Bob Arrington

Hi John,

Great series of articles on monitors. We have the original 702 monitor (non-Bluetooth) connected to a Victron Color Controller, I can’t find where to enter the settings you recommend. I understand the picture you show of your settings is on the app on your phone, but can’t I set ours up the same way, just through the device itself or through the controller?

Patrick Genovese

Hi John,

First off thank you for the very informative articles. Putting into practice what I learnt here has probably saved me thousands of € and ended up with a much better and reliable boat in the process.

Out of curiosity what made you opt for a “traditional” amp counter vs the Bamar/Merlin Smartgauge ?

Regards
Patrick

Nick

In part 1 you made a good case for having both types of monitor – I’d come to same conclusion myself and have BMV-700 and SmartGuage. Very simply I use the SmartGuage to tell me how depleted my batteries are, and the BMV to indicate rate of discharge (or charge).
Can I just rely on the SmartGuage to give confidence that I’m charging to 100%?
Nick

Nick

John – thanks for your thoughts… looks like some careful observations around the 99-100% transition on the SmartGuage are needed!
Nick

Phil

I think something is missing: what if the batteries you are using with the brand new installed monitor are not new ? Chances are that they do not have the announced capacity any more. How can we get a good idea of the remaining total amp/h of our battery bank ? This matters, as we do not want to go below 50% of the real capacity too often ?