The Offshore Voyaging Reference Site

Battery Monitors, Part 1—Which Type Is Right For You?

A couple of years ago, our much-loved Link 2000R battery monitor system bit the big one after some 20 years of faithful service, thereby pushing me into selecting a new monitor.

In the next three chapters I’m going to share:

  • What I learned while investigating the many battery monitor options available.
  • Which we selected.
  • How that choice has worked out over the last couple of seasons.
  • A bunch of tips to make battery monitoring more effective.

Why Bother?

But, first off, why do we even need a battery monitor? Well, if you own a boat that spends most of its life plugged into a marina with only short day trips in-between, you probably don’t, but pretty much every other boat owner will have endless problems and frustrations with their DC electrical system if they can’t and/or don’t monitor it.

Given that, let’s dive into which monitor we should buy:

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

John: Can you recommend a book or two if one wants to delve into the subject a little deeper, especially for a “dummie” when it comes to electricity on a boat?

Dick Stevenson

Hi John,
Yours is a really nice analysis of what a cruising sailor benefits from knowing about his/her electrical system and what instruments contribute to this knowledge: and, perhaps more importantly, you do not mention stuff that is unnecessary to know.
My best, Dick Stevenson, s/v Alchemy

Dick Stevenson

Hi Joseph,
I believe Charlie Wing’s “Boat Owner’s Illustrated Electrical Handbook” to have been a particularly accessible book in this genre, but I do not remember (and do not have mine with me) to what extent it specifically discusses charging systems.
My best, Dick Stevenson, s/v Alchemy

Marc Dacey

Dick, my copy’s on the boat, which suggests it’s very useful, but as I’m at the new apartment, I can’t confirm that Wing covers this off, either.

Dick Stevenson

Hi John,
You mention RC Collins who produces the Marine How To web site and I want to say, in my opinion, that his site is one of the jewels of the marine world. He does not cover everything and seems to pick and choose according to his interests and the work that comes his way, but those projects that he examines are impeccably researched and executed: and then written up with pictures. And they are very helpful to those of us who are doing jobs for the first time or who want to do them correctly the second time around.
I have also been impressed, over the years, by how few people seem to know of his site. At least a few times a season, someone will say: “I am going to replace a thru-hull” or “I will put on a new PSS Shaft Seal” and I will comment on the articles that RC has done on the subject. I can’t remember anyone who has said they have referenced his site before.
So, in that way, it is like AAC, a valuable maritime resource that deserves more attention.
My best, Dick Stevenson, s/v Alchemy

Marc Dacey

I’ve praised “MaineSail” many times in my blog and some of my purchasing decisions, as well as my installation techniques, have been directly influenced by his work. An example would be his method of crimping power lugs with a long-handled FTZ crimper (about $250 but worth every penny, as I will shortly describe). He also recommends finishing the crimp with adhesive heat-shrink tubing. So I did about 40 of these installing my entire DC system. That number of crimps, done at a shop, would have cost me more than the crimper, and saved me hours of transport and annoyance, as I could change wire lengths immediately if I miscalculated.

So I had a cracked pipe nipple last summer and took on some water. My starter was submerged and I had to get it serviced. I thought I would have to replace the 2/0 ga. cable leading to it, but out of curiosity, I clipped off the power lug, trimmed away the heat shrink and found absolutely bright tinned copper with zero evidence of water intrusion. I trimmed the end, crimped on a new lug and bolted it on to the repaired starter. The technique worked and I resumed operations. His is a compendium of “best practices” and I will be patronizing his “shop” for other items in the future.

Luc Bulterys

Hi John,
I am one of those ‘non-technical’ readers of your blog, and I am very happy with this new subject.
As I don’t know yet if you will also compare other systems then Victron and Smart Gauge, I would like to bring this to your attention too:
I would really appreciate to know your opinion about this ‘for people like me’ very comprehensible graphical way of showing what is going on exactly. Personally, I don’t like the way Mastervolt view, Victron, and others are showing things, if they show what we are interested in. But maybe I am too naive, expecting that finally this Simarine Pico is the kind of display and info I was looking forward to for a long time ?
Can’t wait to read what will follow ! Thanks.
Best regards,


Hi John, while I certainly see your point there are some specifics I find quite appealing in the Pico (if it is as described, and I understood it right), mainly the possibility to have not only one but up to 4 shunts measured, so one might measure Input from the alternator separate from the solar array, separate from the windgen. And one might measure the main consumers separately. This is a feature I’d find really nice when I will eventually have my own boat…
OTOH there is the issue with the proprietory bus system which would really push me away, not being a bus-friend on boats anyway. But nowadays you effecitvely can’t escape them 😉 so I’d again prefer a more-or-less proven bus system like the NMEAs.


Hi John,
in Snowball I have the Energy Monitor of Ample Power Company, Seattle. It works with shunts and counts amp hours – among other things. Up to now it performs very well. Other Ample Power products that I have in my boat are a Smart Alternator Regulator (SAR), a Smart Multi-Source Regulator (MSR) and an Eliminator, the latter being a battery-to-battery charger for charging the starter battery from the house bank. The whole stuff is well over 20 years old meanwhile. The MSR quit service 2 years ago, the Eliminator gave up the ghost this year. Ample Power Company went out of business last year, so there won’t be just a replacement new for old. While I replaced the MSR with a simple solar regulator (a MPPT regulator is on the list) I wonder what I should do regarding the Eliminator. Is it still a good way to charge the starter battery from the house bank or is there something better around meanwhile to keep the starter battery topped up.
Sorry if this is not strictly on topic, but I couldn’t find anything about this on your site.

Dick Stevenson

Hi Hans, As someone who owns an a 15-20 year old Eliminator, was there any warning to its demise? And did its demise cause problems (like faulty voltage control) or did it just stop doing its job? Thanks, Dick


Hi John and Dick,
thanks for your prompt answer. John, as I understand it, the Eliminator connects to the house bank as soon as it (the house bank) reaches a voltage threshold of 13,2 (?) volts. It does not connect directly to the charging circuit. It also limits the current it passes to the starter battery so that high starting loads can’t be pulled from the house bank as would be the case with a diode device. I’m well aware of the problems associated with diodes. The Eliminator is called as it is because it eliminates the necessity of using diodes. So I think the Eliminator is in effect a sophisticated battery-to-battery charger. And these are available as I have found.
About the Blue Systems ACR in the link that you provided I’m not sure. What is a “Group 27″(working limit for the ACR) battery ? If it is 100amp hours at most, then it wouldn’t work with my 220 amp hours gel cell house bank.
Dick, I only found out about the Eliminator’s demise because the starter motor was unusually sluggish to start the engine and the next time it quit altogether, voltage on the starter battery being down to 11,5 V, if memory serves. No problem, I could switch to the house bank for starting and next time I had shore power I charged the starter battery directly. I tested the Eliminator by charging up the house bank to a full charge and then observed the voltage on the starter battery: it didn’t rise one bit, so conclusion was death of the Eliminator. Of course I checked wires, fuses and connections first. So, yes, it just stopped doing its job.


Hi John,
I agree completely with everything you write. What I wrote is that the Eliminator doesn’t connect “directly” to the charging circuit, because I see the house bank as a buffer of sorts. But I may be under a misconception here. And of course it doesn’t matter for the purpose here.

Dan P

Hi John
I would like to say I love reading your posts and the comments and find your site an invaluable reference when looking for a right way to do something. (At least I know when I’ve made a compromise and what it is).
We’ve been using a smart gauge for the last 14 months of full time live aboard cruising and find it does exactly what it says it should.
We chose after reading the marine how to review you linked to above, that persuaded us it was better than just marketing smoke and mirrors. Very easy install, and it just works. Mainly it is used to let us know when our solar isn’t quite keeping up with our usage and we need to run the engine for a boost.
An amp counter would sometimes be very nice to have but we use a DC clamp meter if we need to measure current anywhere.

Dick Stevenson

Hi Dan & John,
I personally, at this point with no experience with the Smart Gauge, would not want to be without an amp meter that told me what was going on with my battery bank: just too much important information (especially coupled with an accurate voltmeter) at a glance to give up. Amp clamps are great for diagnostics and forensics, but too often you are separating wires and in awkward places to get a reading.
My thoughts, Dick Stevenson, s/v Alchemy


Hans, there is this device from Balmar
that may do what your Eliminator did.
Also, devices exist (but I don’t have a link) that can charge a battery directly from another battery using a DC-DC converter (so could in theory charge by discharging the source battery). Such devices would be needed when charging a 12V start battery from a 24V house battery but, in theory, they could also charge a 24V battery from a 12V battery.
But I would consider them only in these special cases or when the battery types don’t match (e.g. a Li-Ion house bank and a wet lead acid start battery).
Otherwise I would use a combiner relay as John recommends.
When doing that, try and get a type based on a latching relay that takes significant power only for the split second where it switches on or off, not all the time it is engaged.
I have recently replaced two Victron Cyrix 400A battery combiner relays (not latching) with these latching types:
When I removed the Victron Cyrix I found them almost too hot to touch. They must have had a consumption upward of 1 amp at 12V while engaged. When charging with shore power or an engine alternator, that’s not really a problem but with a nominal 8 amp solar panel, you will waste probably 25% of the effective output of the panel just on one relay (I have two for start and bow batteries, so 50% of one 8A panel).
I am happy with the Blue Sea Systems ACR as, in addition to latching, they also have a remote override switch, an on/off indicator light at the switch and even a manual override that allows to operate without power. They are built around their 500A remote battery switch, one of which I have owned for many years for the bow battery. At the memorable occasion when I flooded the bow locker with sea water, that switch was submerged in sea water for on hour. Even though it doesn’t claim to be waterproof, the relay is none the worse for it even 5 years later.


Hi Henning,
thanks for your input. I considered the Balmar digital duo charge as suitable, now there comes John’s (and your) recommendation of the Blue Sea Systems ACRs and then there is this product of Sterling So there are some devices to choose from. John’s suggestion of using just a manual switch is appealing too, but I’d forget to open the switch some day, that’s for sure (don’t have a blower that would remind me) What I didn’t think about is the consumption of the device itself, so thank you for pointing me to it.


Wouldn’t it be possible to connect the relay from option 1 to the alternator in such a way that it closes (connects the starter battery to the charger) whenever the alternator is humming, and drops when the engine stops?


Hi John,
you clearly found out that I’m not really on top of these electrical matters. Thank you for taking the time to give a clear overview. The house and starter batteries on Snowball are not of the same type. House bank is gel, 220 ah, starter battery is wet lead acid, 88ah. That makes it clear that option three is the one for me. Your manually operated solution also appeals very much to me, but I’m almost certain to forget pushing the button after stopping the engine as I have no engine room blower to remind me.
I seem to have transformed your thread about battery monitors into something else, sorry for that. And thanks again for the advice.


John, I have a Xantrex current monitoring system that is wonderful for all the reasons you enumerate: It shows me what is going in/out. But it gets lost on charge state ftequently. Fortunately, I love redundancy in a boat. Is there any downside (ie crosstalk issues) to installing a Smartgauge as well ?

Rick Gleason

Balmar has a new Battery Monitering Gauge SG200, supposedly developed internally, that is more accurate than the SmartGauge.

Rick Gleason

I’ve purchased it and have been using it this winter to monitor and charge (2) T-105 batteries. Over time it becomes more accurate, however out of the box, after setting just a few selections it worked very well.

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