01 Smart Gauge

The Balmar Smart Gauge

This article is quite in-depth but much less so than it could have been. I have given a very brief overview of my actual testing procedures but enough to explain the methodology. Other than the EnerSys white paper this is the only other independent testing of this product that I know of.

Unlike EnerSys I tested new, used and even well-used marine batteries. If the product had failed my testing, this article would not be here. I don’t believe in writing articles to simply bash a product, unless of course it is a safety hazard.. The Smart Gauge really surprised me and I am not easily satisfied. To be 100% honest I went into this testing with a slight bias that it would not or could not work. I was proven wrong.

Ah / Coulomb Counters:

For many years I have been a big proponent of Ah or Coulomb counting battery monitors and still am, for the right owner. These devices calculate and keep track of the current flowing into and out of a battery bank so you can attempt to keep track of SOC & Ah’s consumed, or at least get a rough approximation.

Coulomb counters give you lots of useful information; voltage, current, Ah’s consumed and SOC to name a few. Some can even give you historical data. These are nice features all packed into one compact unit. They are however not the easiest devices to program, not the easiest to wire and lose accuracy at an alarming rate, if not kept on top of.

Ah counters can be so problematic to use correctly that I had to write an entire article on installing and wiring them properly as well as a sister article to that one on proper programming. Despite both of these articles people still email me because they are confused. The Smart Gauge is a simple two or three wire hook up!

Despite their complexity Ah counters have led almost every one of my customers to longer battery life, when properly used & wired. However, in some cases they have become so out of synch that they have led to erroneous readings that are simply meaningless.

Last summer I had a link 2000 reading -1100 Ah’s on a 65 Ah starting battery… (smirk) That can’t really happen now can it….? (wink)

Up until recently, this type of battery monitor was the best we had. The only other option was an ROCV reading (resting open circuit voltage) or SG measurement (specific gravity) neither of which lend themselves to prudent practical use or accuracy when actually using the vessel.

The problem with traditional Ah or Coulomb counters is keeping them accurate. As batteries age their capacity changes, the charge efficiency changes as does the Peukert’s constant. A battery is an ever moving target, so the 100Ah battery you bought three years ago may now only be a 75Ah battery.

If your battery monitor is still programmed for a 100 Ah capacity, and you are drawing 50% of the assumed Ah capacity out of it, based on this 100Ah’s, you are really drawing the bank to just 25% SOC, rather than the well accepted safe discharge level of 50%.

Follow me on this one.

  • You had 100Ah programmed into the Ah counter for a 100Ah battery
  • Your battery, due to age and use, is now 75Ah’s not 100Ah’s
  • You now draw an assumed 50% of 100Ah’s out of the battery
  • Instead of being at 50% SOC you wind up at 25% SOC
75Ah - 50Ah = 25Ah remaining or approx 25% SOC.

Now if you threw Peukert’s exponent into the mix you may actually be lower or higher depending upon the actual load at which it was drawn.

Holy cow Ah counters are confusing???? (head bonk)

This is but one example where an Ah or Coulomb counter easily and regularly become inaccurate.

A trick many of us in the industry might use is to start with a lower programmed Ah capacity than the bank is rated for. The best option is to physically test the batteries for 20 hour capacity, but this is expensive and time consuming.

For the 100Ah battery I might initially program it at 95Ah’s so the owner is never actually drawing to 50% SOC that first year (self protective feature). The next year I might remove another 3% – 5% off the capacity etc. etc.. These are just rough guesstimates, but they are never perfect. I then also count on the fact the battery bank is being drawn at average currents that are below the 20 hour rated load. This can lead to slightly more usable bank capacity, but again this gets CONFUSING for the average boater to understand any time we bring our good friend Mr. Peukert into the equation.

The only way to accurately know the actual battery capacity is to perform a physical 20 hour load test. This is complicated, time consuming, and very few boaters are willing to do this.

To be honest I don’t know of a single boater who actually has conducted an accurate 20 hour load test. Ah / Coulomb counters rely on the actual 20 hour capacity figures being accurate, to actually remain accurate, over time. No accurate 20 hour capacity figure, no reasonable accuracy in the Ah counter, only a “close enough” range. This may not be half-bad but is a a long way from accurate.

Unfortunately the scenario I laid out for a Coulomb/Ah counter is just one of the many ways these devices can become tripped up and lead to inaccurate readings. There are many more “gotcha” scenarios that can rear their ugly head, including shunt wiring mistakes, battery temperature, false re-synchs caused by solar or wind, incorrect programing etc. etc.. For the last 20+ years however, these are all we’ve had, and they are certainly better than nothing at all.

I suspect the big reason they are, and have been better, is because they make owners more aware of their bank. More awareness of your bank and charge source performance is important, and can play a larger role than we may otherwise assume.

For years I have been trouble shooting and helping owners try to use these devices in a smarter and more accurate manner. When owners understand it they can be very useful and as I said most all of my customers have had longer battery life as a result.

  • FACT: Traditional Ah / Coulomb counters become LESS ACCURATE as time goes on when related to SOC.
  • FACT: The Balmar Smart Gauge gets MORE ACCURATE as time goes on for SOC

The Balmar Smart Gauge is a major paradigm shift in battery SOC monitoring! Read on to find out why..

Traditional Ah Counter

This is what I refer to as a traditional Ah/Coulomb counter. In order to display the SOC correctly, as seen here, requires proper programming, battery temperature (some Ah counters offer this & some don’t), a known accurate 20 hour Ah capacity, a charge efficiency compensation and shunt wiring with no sneaker wires bypassing it.

Can Ah counters be programmed accurately? Yes they can, but certainly not to the tenth of a percent. If they are accurate today they will not be accurate three months or a year from now unless you physically program them for that.

What I am getting at is that Coulomb / Ah counters are only as accurate as you the owner make them. They are not plug and play and they do require human intervention.

BULLET POINT: Ah counters, pretty much all of them, can Coulomb count extremely accurately this is very, very simple stuff to do. Where they miss the mark is that this Ah counting rarely if ever matches your battery due to Peukert, temperature, rate of discharge etc. etc. etc..

Using The Wrong Screen For Data:

Looking at a -Ah’s screen that says -50Ah on a 100Ah rated battery tells you little to nothing about the actual SOC of the battery because;

  • The discharge current at which that -50Ah’s was drawn changes the SOC outcome
  • The battery temperature changes the SOC outcome
  • The actual capacity of the battery changes the SOC outcome

Coulomb counting and looking at only the -Ah counted/consumed screen is not the best and most accurate way to use a Coulomb counter. Programming for actual capacity, Peukert and temp or using a temp sensor, will get you the most accurate SOC readings with an Ah counter.

02 Smart Gauge

03 Smart Gauge

Peukert Effect & Ah Counting

Deep cycle lead acid batteries generally don’t like to deal with high discharge loads such as inverters, windlass motors water makers or electric winches. When you apply a discharge load above than the 20 hour Ah rating the actual usable capacity of the bank shrinks. Conversely if you consistently draw the capacity from the bank at below the 20 hour rating you will get slightly more capacity from the bank. Click the image to make it larger and see what I mean.

Deep cycle lead acid batteries, in the US, are rated at a 20 hour rating. This means a 100Ah battery can supply a 5A load for 20 hours, at 77-80F, before hitting a terminal voltage of 10.5V.

A 400Ah bank can supply a 20A load for 20 hours before hitting 10.5V. Any loads applied that are above the 20 hour rating, diminish the capacity of the bank and loads below the rated load result in slightly more usable capacity.

The 20 hour discharge rate is determined by; Ah rating ÷ by 20.

100Ah battery ÷ 20 = 5A
125Ah battery ÷ 20 = 6.25
225Ah battery÷ 20 = 11.25A

From this it is easy to see why simply looking at the Ah consumed screen of a Coulomb counter can be misleading at best. This is why an Ah counter that can correct for temp, Peukert, charge efficiency etc. will be the most accurate, when properly programmed, and the SOC screen is used.

Unless you have a consistent load that precisely matches the 20 hour rating, of your bank, and the battery is at 77F, then the Ah screen is simply not giving you an accurate representation of SOC.


All lead acid batteries have different Peukert constants. Some AGM batteries are as low as 1.11 and some flooded deep cycles as high as 1.50+.

Lets assume you have an Ah counter and a 100Ah bank and all you look at is the -Ah consumed screen.

The Peukert effect on two 100Ah banks at the same average 9A load:

* Bank 1 100Ah – Peukert 1.11, 9A load, 77F = Capacity at 9A Load = 94 Ah
* Bank 2 100Ah – Peukert 1.35, 9A load, 77F = Capacity at 9A Load = 81.5 Ah

If you only used the -Ah consumed screen on the 1.35 Peukert bank you would have:

* Non-Reality: -50Ah’s = “assumed” 50% SOC (100Ah – 50Ah = 50% SOC)

* Reality: -50Ah’s = 31.5% SOC not 50% SOC. (81.5Ah (9A load) – 50Ah = 31.5% SOC)

Of course your discharge load would never be a steady 9A continuously, and you would never use the entire capacity of the bank, so the numbers and examples are not precise, just as boat use related to Peukert, temp etc. is not precise. They do however give you a good idea of why proper programming, calibration and using the right screen can be the best way to use an Ah counter.

Here is another more simplistic look at it:

100 Ah Battery – Peukert 1.25:

100Ah Battery @ 80 Load = 50 Ah Capacity
100Ah Battery @ 50A Load = 56.23 Ah Capacity
100Ah Battery @ 40A Load =59.5 Ah Capacity
100Ah Battery @ 30A Load = 63.9 Ah Capacity
100Ah Battery @ 20A Load = 70.7 Ah Capacity
100Ah Battery @ 10A Load = 84 Ah Capacity
100Ah Battery @ 5A Load =100 Ah Capacity (20 Hour Discharge Rate)
100Ah Battery @ 3A Load = 113.6 Ah Capacity
100Ah Battery @ 1A Load = 149.5 Ah Capacity

I highlighted the 5A load because that is exactly what Ah capacity ÷ 20 gets you to, and where the battery is “rated”.

Peukert’s Effect =

* Discharge loads above the 20 hour rate result in less usable capacity.

* Discharge loads below the 20 hour rate result in slightly more usable capacity.

This is exactly why using the -Ah consumed screen is simply not an accurate representation of SOC.

Using the SOC screen, which has been properly programed, will result in the most accurate use of an Ah/Coulomb counter.

Are Ah counters complicated? You bet they are…. (wink)

Holy Freak Show..!!!!!

Seriously, welcome to my world. This look like someone spilled spaghetti in the battery compartment… Ouch…..

Believe it or not there is a shunt for a traditional Ah/Coulomb counter at the bottom of this picture. It’s smack dab in the middle with the two brass squares. This bank was so grossly mis-wired there was no chance in hell it could ever be close to accurate.

Traditional Ah counters rely on shunts to measure amperage flowing into or out of the bank. In reality a shunt does not measure amperage, it measures voltage drop at the mV level. The Ah counter transposes this into displayed amperage or calculates -Ah’s consumed or Ah’s returned.

The shunt in this image is a typical 500A X 50mV shunt that comes with many Ah counters. This nomenclature simply means that at 500A of current there will be a 50mV drop between the first brass square and the second brass square. A sense wire on each of the brass squares measures the mV drop or difference across the shunt. Every point in between 0A and 500A has a known calibrated value the battery monitor transposes to current.

Think of a shunt as the electric meter for your house. If you climbed the pole and ran an extension cord directly to the live wires, and then began running your fridge on it, the electric company could not charge you for the fridge use because their meter could not see this use.

Any time a wire sneaks in front of a shunt, a sneaker wire, it essentially does the same thing, it bypasses the battery monitor. With sneaker wires bypassing the Ah counter it can’t see it, so it can’t record it. Wiring shunts is not difficult but I see about 70% +/- of them wired incorrectly, even by pro’s..

With an improperly wired shunt nothing you do to program the Ah counter will make work correctly.

For simple SOC predictions you may want to consider a product like the Balmar Smartgauge. The Smartgauge uses no shunt because and is a shunt-less design. This means there are no large gauge battery lugs to crimp, no large gauge jumper wires to make up, and there is no complicated programing beyond selecting the battery type. This makes the Balmar Smartgauge a DIY’s dream battery monitor. Its easy, simple, never needs programing and stays accurate despite temperature, battery age/condition etc.. It actually gets more accurate the longer it stays connected to the bank with its learning algorithm. The weakness of the Smartgauge is its lack of Ah’s consumed or current data. For most boaters though SOC & voltage is often enough.

04 Smart Gauge

05 Smart Gauge

The Smart Gauge – Simple & Effective

Watch out boys & girls there is a new Sheriff in town and his aim is better than the old Sheriff..

Seriously, the Smart Gauge is a pretty amazing tool. I am a die hard skeptic, go figure, so when new toys like this come along I need to see, touch, poke, prod, test and put them through the paces. I need to see things for myself not what the marketing guy intended for me to see.

As a result of my skepticism I just finished multiple MONTHS of testing the Smart Gauge. I tested it on AGM, FLA (flooded lead acid), GEL and LiFePO4 batteries.

The Smart Gauge does exactly what it says it says it does with AGM, GEL and FLA batteries.

It falls flat on its face with LiFePO4 batteries, but this was to be expected because LiFePO4 has such a flat voltage curve as to be apparently unlearn-able. I suspect if an algorithm was created specifically for LFP batteries it may eventually learn the bank, but I doubt Smart Gauge will do that for such a small niche market.

PHOTO: The Smart Gauge is plain & simple, it displays battery SOC for the house bank and battery voltage for an AUX bank.

Here is is showing the bank at 92% SOC. In the mindset of keep it simple voltage is displayed in 0.05V increments and SOC, displayed as “C”, from 0-100%. That is all it does, see, simple.

The difference is the Smart Gauge does this quite accurately and tracks SOC regardless of temperature, battery age etc.. The longer you use the Smart Gauge, and leave it connected to the bank, the more accurate it becomes.

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When Balmar introduced the UK developed Smart Gauge to the US market I was pretty excited. I had tried to buy one two years ago but my emails went unanswered so I filed it under the scam & snake oil folder in my mental filing cabinet.

I had literally forgotten about the Smart Gauge until Rick Jones of Balmar approached me at the Annapolis show to tell me they had added it to their product line and were now the US distributors. Even though I implicitly trust Rick and the guys at Balmar, the claims still seemed too good to be true. Why had no one been able to do this before?

To make a long story short I tracked down an inside contact at EnerSys (name withheld) to see if I could get my hands on the white paper so often referenced by Smart Gauge.

EnerSys are the makers/inventors of TPPL AGM technology sold under the Odyssey & Die Hard Platinum brand for marine use and they are the inventors of the Optima spiral wound batteries (which has been sold off). EnerSys however is much larger than their presence in the marine market and much of their business is in large standby/UPS systems and military use. Because EnerSys has no financial ties to Smart Gauge I found their white paper to be a breath of fresh air in a credible independent test data manner. I don’t believe EnerSys allows Smart Gauge to use that white paper, and I was asked not to reproduce it, so have only taken excerpts from it.

“But RC how does it work?”

You’ve got me..? I have no idea how it works, at least at the programing/algorithm level (proprietary stuff), but it is reportedly designed to track voltage. Many internet posters have assumed, posited and suggested, that it checks internal resistance and pulses across the battery etc.. It may, but I have not seen evidence of this on the power / volt sensing wires, even with an Oscilloscope. If I had a tracking o-scope I may have seen something but I don’t.

As near as I can tell it simply tracks voltage, up to 1500 times per second, to detect trends and compare it to internally programed data models. I do know it uses computer modeling of actual batteries in real world situations and then feeds this data into an algorithm that can “learn” the bank as time goes on. This modeling must have been time intensive to get to this level of accuracy  and this sort of programing minutia is way to complex for a guy like me who can hardly figure out an iPhone.

The Smart Gauge may pulse the battery every so often, and I just missed it..? All I can say is that over time it seems to adapt to learn your bank and give significantly more accurate SOC readings than an Ah/Coulomb counter can, as programmed and used by the average installer or boat owner.

How it actually does this is as closely a guarded a secret as the Frosted Flakes recipe that Tony the Tiger protects.

The best overview I can give, on the Smart Gauge level of accuracy, is the executive summary of findings by EnerSys.

QUOTE = EnerSys White Paper


SmartGauge® is a Battery Monitoring Unit (BMU) that is intended to be fitted within military vehicles and to provide crucial information to vehicle commanders, such as State of Health (SoH), State of Charge (SoC) and the time remaining they have available to continue operation until battery power runs out.

The working partnership between EnerSys and SmartGauge® has resulted in EnerSys testing the SmartGauge® BMU whilst connected to a Thin Plate Pure Lead (TPPL) battery type within its electrical laboratory at Newport South Wales, to evaluate the performance and accuracy of its data.

The SmartGauge® BMU was tested using a 12V 100Ah TPPL battery which was subjected to a 100% depth of discharge test, followed by a full 12 hour recharge. This cycle was repeated continuously until the battery reached 80% of its rated capacity, the SmartGauge® BMU and EnerSys laboratory data logging equipment (Digatron) continuously monitored the voltage, current, time and from which the State of Charge and State of Health was calculated.

The correlation of State of Charge (SoC) and State of Health (SoH) between the BMU data and Digatron Data was excellent with insignificant variance between the two readings based upon resolution increments of 1%.”

06 Smart Gauge

07 Smart Gauge

The Testing Station

I don’t own a six figure Digatron like EnerSys does so my test station had to suffice for the best accuracy I could attain.

Test Bench:

  • Charging – Mastech 3050EX & BK Precision 1900
  • Charging – 80W Solar Panel – Rogue MPPT 3048
  • Ah Counting – Victron BMV-602, Array DC Load Center, PentaMetric Data Logger
  • Discharging – Array 3721A 40A DC Electronic Load & 400W Inverter
  • Data Logging – PentaMetric Multi-Input Ah/Coulomb Counter & USB Interface
  • Misc. – Fluke 179 DVM (NIST Calibrated)


#1   Determine actual 20 hour capacity for each of the batteries tested through 20 hour discharge capacity testing. Both used and new batteries were tested. New batteries do not deliver full rated capacity, until broken in, so as-is condition for 20 hour Ah capacity had to be determined.

#2   Program Ah counters with the new 20 hour capacity and run load tests side by side with the Smart Gauge.

#3   Test Smart Gauge for accuracy during charging & discharging events, including solar, inverter loads and DC loads. Batteries were determined full when charge current fell to less than .5% of Ah capacity at target absorption voltage. (absorption charging voltages varied by battery type).

#4  Control room temperature to 75F to remove the temperature equations from the test calculations. (77F was just too damn hot). Part way through I added a temp controlled water bath to more accurately maintain battery temp, and keep my room a bit cooler.

#5   Re-test batteries for physical Ah capacity at the end of testing to note if changes were noted or the batteries declined in capacity during testing. Two batteries actually increased Ah capacity by approx 4% (new flooded batteries not yet “broken in”) and the rest were under 1% changes or well within my range of error resolution to even calculate.

#6   All used batteries were equalized and serviced before being put into testing.

NOTE: This testing took approximately four months to complete. This is not quick or easy work if you want to get accuracy as close as you can for the equipment you have on hand. It was a real eye opener as to how inaccurate traditional Ah counters can be, in regards to SOC.

Test Procedure

In order to determine the if the SOC of the Smart Gauge was correct I first had to accurately determine the Ah capacity of the batteries I was testing. I tested both new and used batteries to see how the Smart Gauge would adapt to being inserted into a system with used batteries.. One flooded battery was seven years old.

The 20 Hour Capacity Test:

When conducting these tests the discharge load needed to be constant over the duration of each test. This is difficult if you don’t have the proper equipment because the voltage decays or decreases, as SOC declines. Due to this the discharge current changes due to Ohm’s law. As a result of this testing I now have a beautiful lab grade DC constant-load tester that will hold current precisely where you set it and then disconnect the bank when it hits 10.5V. This made this testing much easier, and more accurate.

Determining Ah Capacity On Used Batteries:

In order to compare the Smart Gauge, to the two Ah counters, I had to first determine the batteries physical, at this point in time, 20 hour rating. This sometimes involved three complete discharge capacity tests all the way to 10.5V.

* First Capacity Test:
Load applied at labeled 20 hour rate and Ah’s delivered were recorded. If the battery did not have the labeled capacity I noted the Ah’s it supplied transferred that Ah capacity to the second test.

* Second Capacity Test:
For simplicity’s sake lets assume we had a 100Ah battery that only delivered 77Ah’s. The first test was at 5A/77F until the bank hit 10.5V. But, if the bank hit 10.5V at only 77Ah’s delivered, I then recalculated the test based on 80Ah’s. This is a small fudge factor I learned while performing these tests. What I wanted to do was come up with a new 20 hour rate for the battery in its current condition. I needed to identify a discharge rate that would allow the battery to run for 20 hours at 77F. If I figured I had an 80Ah bank then 80Ah’s ÷ 20 = 4A load. The second test was then run at 4A and the Ah’s delivered were recorded to see if it ran for 20 hours. If it matched, and it ran for 20 hours,  then no third test was needed.

* Third Capacity Test:
If Ah capacity did not match on the second test I then recalculated and performed a third test. I never had to go beyond a third test and was usually within .5 – 1 Ah. Close enough for this testing and far more accurate than any boater would ever program for on-board with a traditional Ah counter, unless they got very, very lucky.

After each capacity test the battery was immediately, and slowly, recharged at the new 20 hour rate. This recharging was done at constant current until voltage was at 16.0V (not for the GEL or AGM). This was constant-current (CC) only charging with no voltage limit (well technically 17V). It required my attention near the top end of charge and added many days to these tests because to recharge after the 20 hour capacity test took over 20 hours. This is over 40 hours of testing for each capacity test completed.

This type of discharge/recharge is often referred to as a reforming charge. It can tend to put some capacity back into the bank and can help minimize the abuse of taking the battery to 10.5V to find actual capacity. Tedious and time consuming though.

BULLET POINT: It should be noted that only one lead acid battery I tested produced the rated Ah capacity, a new AGM. None of the others did, not a single one.. (Head-Bang) New batteries take many cycles to fully break in and deliver rated capacity and used batteries lose capacity over time. During testing some of the new batteries slightly increased capacity, and some used batteries lost a 1% or so. Ah capacity on lead acid is an ever moving target. If you think your Ah counter is giving you accurate SOC data consider this bullet point and what it took me to find the actual, at this point in time, Ah capacity of these batteries. The only batteries tested, that delivered their ratings, were the LiFePO4 and one AGM..

PHOTO: This photo just represents the screen I have on the PentaMetric data logger. I can change the screen display to add up to three shunts and three voltage sources. I normally use this tool to track solar performance and do A/B comparisons but it is great for data like this too…

Ah’s were tracked simultaneously with both the PentaMetric and the Victron BMV-602 as well as the Array 3721 for discharging.

08 Smart Gauge

09 Smart Gauge

Smart Gauge Wiring

Remember when I said easy? This is it. You run a duplex 14GA wire to the house bank cross-connecting it as shown. Place the negative on one end of the bank and and the fused positive on the other end of the bank. For the start battery voltage a single 14GA wire goes to the B2 terminal.

IMPORTANT: Do not wire the Smart Gauge to any place other than the actual physical battery terminals!. In order for the Smart Gauge to work properly the positive & negative leads from the Smart Gauge need to attach directly to the physical battery terminals, not a positive or negative bus, or anywhere other than the actual battery terminals.

Even if you have a traditional battery monitor you need to bypass the shunt and wire the neg to the neg battery terminal. The consumption draw/load of the Smart Gauge is so small a traditional Ah counter shunt can’t even accurately see it or count it. The Smart Gauge will however track its own miniscule consumption, over time, even when a traditional Ah counter can not.

The Smart Gauge self consumption is less than 5mA (0.005A) when the display is asleep and just 15mA (0.015A)when the display is lit.

In 24 hours the Smart Gauge consumes only 0.12Ah. In an entire week it consumes just .84Ah.

Bullet Point: A traditional battery monitor, using a 500A/50mV shunt, does not have the resolution to track the consumption of the Smart Gauge.

Because the Smart Gauge tracks voltage, not Ah’s, it can actually track its own self consumption. The self consumption of the Smart Gauge can actually be less than a flooded batteries own self discharge in warm weather.

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Smart Gauge Terminals


GND = House Bank Negative Terminal
B1+ = House Bank Positive Terminal (FUSED within 7″ of battery bank)
B2+ = Start/AUX Battery Positive Terminal

Relay / Alarm Connections:

The Smart Gauge also has the ability to drive external relays, alarms etc.. There are many uses for the relay ports and it offers many programing choices for the relay driving terminals.

NO = Normally Open Relay Terminal
COM = Neg Relay Terminal
NC = Normally Closed Relay Terminal

IMPORTANT NOTE: The max permissible load on any of the relay terminals is 500mA or 0.5A. If you need to drive more current, an external relay/contactor, with coil loads under 500mA, needs to be used. The maximum voltage across any of the alarm terminals is 48V…

10 Smart Gauge

11 Smart Gauge

The Results

In this image I am testing the Smart Gauge with a 400Ah LiFeP04 battery bank. This is the only type of battery it failed to track accurately. Not a big deal as it was never designed for Li batteries and very few boat owners use Li.

I found it quite interesting that while trying to find the actual capacity of some of the used lead acid batteries the Smart Gauge was already accurate by the second cycle and I was on my third complete discharge capacity test before finding an accurate new 20 hour rating for the battery to test it at.

How accurate? It is tough to say precisely because I really don’t know how much I trust the Ah counters. Suffice it to say it was most likely as accurate as the Ah counters and probably below a 3% variation in SOC. I don’t have the test equipment resolution to make claims of 1% like EnerSys does, but even if under 5% this is simply outstanding.

The Smart Gauge was lining up with the Ah counters, once the banks were well calibrated to the Ah counters (arghh what a process), to under a 2% – 3% variance. In many cases the Smart Gauge beat me to the actual SOC. The Smart Gauge found the SOC of the used batteries faster than I could by conducting actual physical 20 hour capacity tests. This is truly amazing.

The Smart Gauge seems to work as advertised on GEL, AGM and FLA batteries in discharge mode.

What does that mean?

It means that I did see the Smart Gauge get a bit confused when the bank was being charged. It can’t really track the capacity of a battery charger now can it…? However we are only talking about 10-12% variation from the Ah counters during charging, and not a huge deal when you consider how simple this battery monitoring unit is. Another issue with tracking SOC during charging is charge efficiency variations, so it was much easier to do this testing on the discharge side of the equation.

As soon as the charge source was discontinued, the Smart Gauge fairly quickly identified the accurate SOC of the bank, and was back within approx 2% – 3% of the two painstakingly calibrated Ah counters.


  1. It tracks the voltage of the battery bank up to 1500 times per second and over time learns bank behavior, WITH NO HUMAN INTERVENTION or COMPLICATED PROGRAMING! As time goes on it gets smarter and smarter and more and more accurate. This is GOOD!
  2. It needs a good three to four cycles for it to hone in on SOC and the longer it remains connected the smarter and more accurate it gets. I attempted 5-8 cycles on each bank beyond the capacity tests.
  3.  It provides voltage of the HOUSE and START/AUX banks in 0.05V increments.
  4.  It identifies SOC of the HOUSE bank irrespective of age or condition.
  5.  It requires no programing beyond selecting the battery type and wiring it directly to the battery positive and negative terminals of the HOUSE bank.
  6.  It removes the guess work and tediousness of programing a traditional Ah counter.
  7.  It is the easiest to use battery monitoring unit I have ever used or installed.
  8.  It works with either 12V or 24V banks and automatically detects this when connected.
  9.  It offers low and high voltage alarm relay trigger ports. These ports can even be used to start a generator, if your vessel is so equipped.
  10.  It requires no shunts or heavy gauge wiring and installs with simple 14GA wire. No battery lugs to crimp.
  11.  It tells you all you really want or need to know in order to maximize your battery banks cycle life, the state of charge. It really does not matter what your capacity is just that what ever it is, you are not constantly pulling the bank below 50% SOC.

Pro’s & Cons


  • Easiest to use of any battery monitoring device
  • Easiest to wire of any battery monitoring device
  • Lifetime accuracy with no reprogramming
  • Simple – SOC is really all you need to know
  • Accurate – More accurate than an Ah counter
  • Can track its own very small self consumption
  • Offers alarm or gen start relay ports in both normally open and normally closed
  • No human intervention beyond selecting the battery type
  • Very low self consumption
  • Truly a plug & play battery monitoring unit
  • Can track miniscule parasitic leaks / loads that shunt based devices can miss


  • Non standard hole cut out / display size
     Face Plate = 4 3/8" X 3"
     Cut Out = 3 3/4" X 2 1/2"
     Rear Clearance = 1" plus wires
  • No amperage display (some owners like this)
  • Not as pin point accurate during charging as it is when discharging
  • Price – More money than some Ah counters but less than many others

MarineHowTo.com Overall Rating = TWO THUMBS UP!!!

If I had three thumbs this product would get all three! The sheer simplicity and accuracy of this product are outstanding and I really did doubt it, I was proven wrong…
What matters most to your batteries is your depth of discharge or state of charge. The Balmar Smart Gauge does this accurately and simply! Hands down the Smart Gauge is the easiest SOC meter made.

Good luck & happy boating!!

12 Smart Gauge

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