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. Lithium Battery Buyer’s Guide—Part 4, Fusing
  34. Lithium Buyer’s Guide—Budget: High End System
  35. 11 Steps To Better Lead Acid Battery Life
  36. How Hard Can We Charge Our Lead-Acid Batteries?
  37. How Lead Acid Batteries Get Wrecked and What To Do About It
  38. Equalizing Batteries, The Reality
  39. Renewable Power
  40. Wind Generators
  41. Solar Power
  42. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  43. Battery Monitors, Part 1—Which Type Is Right For You?
  44. Battery Monitors, Part 2—Recommended Unit
  45. Battery Monitors, Part 3—Calibration and Use
  46. Battery Containment—Part 1
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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 ?



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%?


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


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 ?

Matt Boney

Hi John I would like to clarify some other reasons as to why battery monitors get more and more inaccurate as the batteries age. This is why I think Mainsail’s manual reset is a must for all BM users.

As you rightly say for a battery monitor to display the Ah capacity properly three battery parameters have to be correctly input to the software on installation, but many people leave the default settings as they are. Nasa do not even allow two very important parameters to be programmed for different batteries – Peukert’s Constant and the Charge Efficiency.

The one value that MUST be input is the battery capacity of the service bank, but every 12 months an estimate of the capacity reduction of maybe 5% to 10% should be guessed and input into the BM.

The other parameters that also change as the battery ages are Peukert’s constant and the Charge Efficiency, but these changes are difficult to determine to try and improve the accuracy of any BM.

An accurate charging Ah count is also affected when the “charge efficiency” changes with the state of charge. At 50% SoC the battery efficiency maybe nearly 95% but at 85% SoC the battery efficiency maybe less than 50%. If a bank is always only discharged to 80% before re-charging then the Ah count will be wrong. (Ref Sandia Laboritories)

One final comment there is also a problem with solar panels and wind generators that under varying conditions do not deliver a high enough current but may meet the Auto Re-sync voltage values. This will repeatedly prematurely re-set the Battery Monitor to 100% too early. You will discover this when you get some solar!

The other major problem with shunt based Ah counting monitors is the installation of the shunt itself. Battery monitors do not work accurately if the shunt needed to measure the current is not measuring all the loads or all the charging sources. Often extra equipment is added at a later date and the installer may well have followed the installations instructions to the letter and installed the new equipment “directly to the battery”, but not via the shunt bus bar. Make sure you tell an electrician fitting new kit that you have a Battery Monitor installed.

These are all the reasons why the Merlin Smartgauge was developed.

SmartGauge works in a completely different way so none of the above effects its performance. It ‘learns’ your system and gets more and more accurate as the batteries age. Yes we would all love to know exactly how it works – but it does. I have been a liveaboard for 13 years and had my SmartGauge for 10 years. I bought it because my BEP battery monitor was getting more and more inaccurate, no matter how I tried to change all the parameters.

SmartGauge proved to be so good that Merlin Equipment in the UK bought the company – and Chris Gibson – and it is now being very successfully used in their hugely expanding Military market. They have recently used the SG technology as the focal point of their Data Cell 11 battery monitoring system in the M1 Abrams tank and the M3 Bradley Fighting Vehicle, claiming an accuracy of around 1%.

Balmar have just licensed the SG technology from Merlin.

Matt Boney

MPPT solar controllers don’t follow Ohm’s Law!!!!!

Solar panels are current generators and the voltage output stays pretty constant. I think your tests when you get solar will prove my point – see MainSail’s comments on this also.

Marc Dacey

This is pretty esoteric stuff, guys, but I like esoteric stuff. Care and feeding of batteries properly never gets old.


Slight drift but a heads up for the geeks – with a raspberry pi running openplotter and a few bucks spent on a couple of sensors you can record very accurate data & plot it easily. An INA219 current sensor piggy backed across your battery shunt will measure current, far more accurate than I thought but seems fine down to a couple of hundred mA, and a ads1115 will record extremely accurate voltage. Both together paint a fascinating picture of what’s going on, better than youtube if you are geeky enough 🙂


Hi John, I have been going through my manuals for charge controllers (solar, Balmar, Mastervolt) but i cannot find a parameter to set the “Charged Voltage to 13.9V”. I can set the bulk, absorption and float voltages (3 stages) but nothing labeled as “Charged Voltage”. From what i understand of your article, the fully charged state could be anywhere between bulk and absorption as the charge controller “controls” the amount of current going into the battery at the voltage that it decides to charge and i wont know when that current is going to reach 2% of the total Ah. Hence I am not sure how to program for the voltage to reach 13.9V AND the charging current to reach 2% of the total Ah for the charge controller to go to float stage. Am I missing something here?


If the Tail Current and Charge Detection Time are a function of variances in alternator regulators, where should these be set if my house bank charging source is only an onboard generator or shore power through a Victron Quattro inverter/charger? We’re aboard a trawler.