The Offshore Voyaging Reference Site

Battery Monitors, Part 2—Recommended Unit

Update December 2020

If you don’t already have an amp counting monitor, see my update at the end of the last chapter before reading on since there is now a better way to get this done without going through the next two chapters of aggravation. That said, I have left said chapters up for those who already have amp counters and want to know how to make them work properly.

In the last chapter I looked at the two types of battery monitors—volt measuring and amp counting—and concluded that which type we choose is all about how anal retentive and technically nerdy each of us is.

Now here’s a test for you: Since I chose an amp hour counter, which type of monitor is right for anal retentive nerds? (If you fail this test you have not been reading my stuff for long!)

Yup, this post is for card-carrying nerds with a side interest in being a control freak. The rest of you can stop reading now, go buy a volt measuring monitor, and live happily ever after…OK, I can’t guarantee that last part.

Still with me? OK, put on your propeller hat and let’s dig into which amp counting monitor to buy and why.

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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—BMS Requirements
  31. Lithium Batteries Buyer’s Guide—Balancing and Monitoring
  32. Lithium Batteries Buyer’s Guide—Current (Amps) Requirements and Optimal Voltage
  33. Lithium Battery Buyer’s Guide—Fusing
  34. Lithium Buyer’s Guide—Budget: High End System
  35. Lithium Buyer’s Guide—Budget: Economy Options
  36. 10 Reasons Why Hybrid Lithium Lead-Acid Systems are a Bad Idea
  37. 11 Steps To Better Lead Acid Battery Life
  38. How Hard Can We Charge Our Lead-Acid Batteries?
  39. How Lead Acid Batteries Get Wrecked and What To Do About It
  40. Equalizing Batteries, The Reality
  41. Renewable Power
  42. Wind Generators
  43. Solar Power
  44. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  45. Battery Monitors, Part 1—Which Type Is Right For You?
  46. Battery Monitors, Part 2—Recommended Unit
  47. Battery Monitors, Part 3—Calibration and Use
  48. Battery Containment—Part 1
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Dick Stevenson

Hi John,
Nice job and appreciate the parsing out of “necessary” information to do the job. You wrote, “So if you have, as we do (and recommend), a house bank split into two parts with a 1-2-Both switch,”. Do you have a location where you make the rational for a split house bank (with a starter battery) vs one house bank and a starter battery?
Thanks, Dick Stevenson, s/v Alchemy

Gary Warner

Hi John – We installed a Philippi PSM system (German) three years ago that we are quite pleased with. While this system is similar to the Victron in that it will only count AH for one battery and measure voltage for a second battery, it does allow one to install additional shunts in source and load circuits and build a simple diagram of your power system and that will display voltage, current flows, etc. We monitor all charging sources to our house battery including alternator, solar, and AC charger, so we get a nice visual graphic of flows to and from the battery. We do not monitor loads as it’s easy to compute by looking at the visual system representation, although it would be easy to do with another shunt. The PSM system does allow for setting the Peukert constant for the specific battery type. The system will also monitor tank levels and control heavy duty switches from the PSM display if you want that type of integrated capability. We purchased our system components from Bruce Schwab in Bath, ME, who specialized in marine power systems. He is also now selling the Victron units.

That said, I totally agree with your recommendation of the new Victron 712 battery monitor system. We use a Victron Blue Solar 100/50 controller with the remote display on our boat and are very impressed with the quality of their products, the regular firmware updates, and responsiveness of their customer service representatives located in their US offices in Thomaston, ME. We also purchased the bluetooth dongle and plug that in occassionally. The Victron bluetooth is flawless and provides all the information that even a techy like yourself would want. The new Victron units with the built-in bluetooth eliminate the need for the dongle and are a teriffic value. We would upgrade our solar controller to the new integrated unit, but its only three years old and working fine. Another feature we like with the Victron products is the full setting capability that is very easily accessed via bluetooth, and the firmware updates that are easily done with your iphone via the bluetooth connection.

Love your web site and recommend it to all my boating friends, whether sail or power, as the information is invaluable to any type of boater. Keep up the excellent work.


Fred Taute

Hi John,
Nice post, I also have the Victron 702 and am very happy with it. But I have to confess that I bought it simply because the prior model which had died was a Victron, and there was already a nice round hole in my wood panel to accept it. So perhaps another factor when looking at comparable monitors.
Now, on the bluetooth side, I have to wax eloquent about my Victron solar MPPT controller. Watching the action on that from a phone or iPad in the cockpit can get very exciting indeed. .
All the best


I think you should add power consumption to your list.
All the Victron units are pretty good.
702 <4mA
712 <1mA


Hi John,
If it is important depends on the use and technology of your battery bank. The Victron unit for example is sometimes used also as low voltage protection for LiFePo4. In those applications you want to reduce the consumption after disconnect to a minium in order to protect the batteries. For the same reason the 712 have the advantage to controll a bistable relay (instead of a standard relay 702).

I have seen other brands that have as much as 90 mA when all the features are activated. Then it can make a difference even for lead/acid. (67 Ah in a month compared to 0.75 Ah for a 712)

Svein Lamark

I have a Victron battery monitor type 6025. It does not help much as I use old big batteries. They are big, but free to get when 10 years old at the ship wrecker. Normal lifetime is 25 years on such batteries. Most modern yachts are not able to carry and put in such batteries because they are big and heavey. In a traditional yacht they are good. The voltage will not correspond with the power left in the battery. At 23,5 volt they can still have most of the power left. This technology is of course old and probably from the 1. WW., but it works and it does not cost anything. I have always enough power in the battery and I start my diesel by air pressure. So I am not much impressed by modern battery technology. I sometimes think the modern sailors are incompetent and lured by smart business men. I would like to have a better battery, but they are not invented yet.

Stein Varjord

Hi Svein.
I think you have some very good points, even though I’m completely fanatic about saving weight and gaining speed. The battery type you mention has some amazing properties that I’d love to have, but since they’re even way heavier than normal lead acid batteries, (for a given capacity) they’re impossible for me on a decently light catamaran (4,5 tonnes 40 foot) and even more so for the next boat, which will be extremely light.

I think some older technologies have been forgotten too early, and I think many cruising monohulls could like your battery type, if the boat was designed for it. There needs to be space low, so the batteries work as ballast. The boat type that comes to mind is the aluminium lifting keel boats, like Boreal, Ovni and Garcia. They have internal ballast…. For modern boats, one would also need some circuits to stabilize the voltage, since modern electronics are not happy with the voltage change these batteries have.

Marc Dacey

A very good point. I had an opportunity recently to explain how, using a handheld compass and a watch, to determine distance off a coast using a paper chart. It’s basic pilotage, of course: I learned it in 1999 along with sextant usage. But in the intervening years, GPS and related gadgets have so dominated the minds of sailors newer than me…and I came to it late…that the response to my suggestion is as if I were some sort of wizard. I have about 340 kilos of batteries in the middle of the boat, below and just aft of the mast under some stairs. They were put there as ballast…the boat is less tender now.


We love our Victron 702 that we installed this year before our 2 month summer cruise. My wife only bothers with the % mode indicator.
In parallel, I installed a Victron MPPT controller for my 2x 100w solar panel. Monitoring info from both the 702 and the controller help me figured out a few things about my batteries and our power usage, and confirm what my amp clamp meter told me before.
Over our 2 month cruise, we never plugged-in in marinas, staying at anchor. We never got below 70% discharge, thanks to the sun and some occasional help from our basic alternator, the batteries topped up to 100% almost every day. Our main power drain is the fridge, at about 5 amp/h, and I think the monitor does a fairly good job in integrating the Peukert’s effect.
I think I also need to add the optional Bluetooth interface in order to get access to all features on both unit. As an example, without Bluetooth, you cannot activate the equalization mode on the solar controller.

Getting the battery monitor calibrated properly is my next step. Right now, with 3x Trojan 105 amp/h group 27 liquid acid, I did set the total capacity to 270 amps on purpose. And I know it is not right, as I have seen the solar controller (programmed with the right values) in some occasion still being in bulk mode while the monitor said the batteries where at 100%, even with auto sync mode was set to OFF. So I can’t wait for your next chapter…


Good day John

I too ended up choosing the Victron 702.
I’m hoping you can help me with an install question and that this would be the right place to bring that up. Here is a link to a picture of my battery setup as it came from the factory:

As you can see, all of the negatives are wired across all batteries to the negative bus (through the hole and into the cabinet you can’t see in the picture)

My question is this: If I’m understanding how the monitor works correctly, I need to break those out otherwise, the monitor will be counting the amps used across all batteries, not just the “house batteries” correct?

Thanks in advance for any help.

S/V Rocinante


Thank you John
I was hoping I was wrong!

Following up on your recommendation; that box is ~2ft x 2ft. If I take your statement to the extreme and make them exactly the same size, I would end up using ~6’ leads off each battery to said negative bus bar (based on current position of starter battery) thereby creating substantially more more voltage drop, albeit at a consistent level across all batteries. I’m assuming that is preferable to my current setup, but simply want to confirm.

Also, I have a second house bank that is ~18’ away. Talk about voltage drop!
How much damage am I doing to those batteries? Thoughts on how to rectify without moving them?

Thanks again!



Thanks again John.

We’re live-aboard full-time coastal voyaging.
That said we anchor out almost exclusively; at the moment, we’ve been away from shore power for exactly 4 months as of yesterday. So while we do have easier access to power than someone that is off-shore voyaging, it’s still a critical issue for us… guess I have a lot of work ahead of me!

Thanks again,,
S/V Rocinante

Rich Morpurgo

I see no real reason for two house banks. With a good monitor and reasonable maintenance, I have had no trouble with one house bank and a start battery. This has worked for 20 years.


This may not be the right forum for this question, but I have to start somewhere. I have just bought my first boat. It is a former Coast Guard vessel with an impressive looking electrical panel that has an integrated HZ meter (analog) mounted at the very top. What pearls of wisdom can i expect to glean from the information coming from this device? It is located just above the shore power to generator transfer switch. I always thought frequency for AC was pretty much consistent and really didn’t require monitoring… Hopefully someone can help me reduce my ignorance in this very specific area.

Bruce Bayne

I’ve been doing some research (read: google searching) on the new Balmar SG200 Battery Monitor. From what I’ve read, it seems like a new, possibly better, approach to monitoring batteries using their SOH (state of health) algorithm. Any comments or thoughts about this new unit? Possibly a review in the near future?


Bruce Bayne


Thanks for the quick reply. Understood, but I thought it did more of a auto calibration or “learning” over a few discharge/recharges. An excerpt from the manual:

“The SG200 is a self learning product. It continuously monitors the battery bank, and self adjusts over time to improve the accuracy of the two most important parameters, State of Health (SoH) and State of Charge (SoC). On new battery systems, both the State of Health and the State of Charge values may be very close to accurate after the first full charge and a brief relaxation period. This is because the stored characteristics of each battery chemistry is made with healthy batteries. If the batteries are older, it will take a few cycles before these values can be relied upon, and you may see significant changes in these values during this time.”

Stein Varjord

Hi John,
I have no experience with the SG200, but according to what I read, absolutely no calibration is needed, ever. At installation, the battery type and size is chosen. After a few cycles it knows the charge level very accurately, and also the health level, expressed in a percentage. It never gets out of sync. It gets gradually more precise. This also works with lithium.

Craig Harmon

John, am a new subscriber and loving it. Thank you.

I have two banks of two batteries each, which are ordinarily paralleled together so the four battery collection is available for all (12v) house loads other than the bowthruster. When the bow thruster is engaged, a solenoid puts the two banks in series, giving me the required 24v for the thrusters. All 12v house loads are of course connected to one (two battery) bank, which (I think — none of this is my forte) means that they draw unequally from the two banks only during the momentary operation of the thrusters. Forgetting about the separate start battery, do you think I need one monitor or two? Thanks much for any insight. And I recognize that this topic might be old enough that you are no longer answering questions on it. Craig Harmon