Programming And Optimizing Battery Monitor Shunt “fuel Gauge”

I recently installed a DC-DC charger in an effort to regularly obtain a “fully-charged battery”. Subsequent to this installation, I began to understand the fallacy of relying solely upon open circuit voltage (OCV) as a means of determining the trailer battery’s true State of Charge (SOC). This was an eye-opener, because knowing my battery’s actual SOC and not exceeding its draw-down limits, in order for me to get a full life out of the battery, was essential to me and my piece of mind. So I had to find another way to measure and monitor the battery’s "fuel tank".

A more accurate alternative to OCV monitoring, and to determine the battery’s SOC, is the use of a shunt. Which I also just recently installed. Rather than looking solely at the volts, a shunt counts the current (Amp hours) going in and going out of the battery, and creates a virtual “fuel gauge” to read and follow. Except… while I was installing it, I discovered that the shunt has it’s own inherent inaccuracy drawbacks which will rear their heads, sometimes all at the same time, if not well understood, acknowledged, or compensated for. I decided to create this “rule book” for myself to help increase its accuracy and safeguard the battery against unwitting deep discharges and an early death.

Others on this forum have installed shunts. The Victron Smartshunt 500A seems to be the go-to, though there are many others; what follows is generally applicable to those others as well. Further, this mainly applies to those with SLA and AGM batteries; those with Lithium trailer batteries get a pass. Due to the different chemistry, charging and discharging characteristics of lithiums, shunts are far better at predicting and estimating a Li battery’s capacity and SOC, and many of the concerns or workarounds that I regurgitate here don’t necessarily apply to Li’s.

I have paraphrased R.C. on Marine How To.

Making Your Battery Monitor More Accurate - Marine How To and Installing A Battery Monitor - Marine How To

Shunts work by counting the current that goes through it, in either direction. Adding and subtracting. That’s it. It doesn’t have a clue what is on either end of it, using the energy, or providing the energy. It relies heavily upon the user to calculate what the nature, condition and estimated capacity of the battery is, and to tell the shunt so that it can make reasonably educated guesses about the percentage of the battery that you’ve used, what’s left, and when you should put it back.

The vast majority of us C-I owners have one SLA or AGM Deep Cycle battery of a Group 24 size. They have a labeled Ah capacity of somewhere between 70 and 90 Ah, depending upon the make, chemistry type and quality of build. However, the manufacturer’s stated Ah capacity on your battery is very much just a ballpark figure. This caveat applies at any stage of the battery's life or use. Even if was accurate at any one moment in time, it will always be a constantly moving target. Why?

1. Battery Ah ratings and capacity are rarely completely accurate right out of the box.
2. SLA, and in particular AGM batteries, take time lots of cycles to ramp up to their full Ah rating. Some never even get to reach this optimal level before they are prematurely compromised through imperfect use.
3. Once they’ve reached that apex, they start ageing and declining, losing Ah, even if properly cared for.
4. Replaced Ah (or charging), which the shunt uses in its fuel gauge calculations, isn’t an exact science either. 50 Ah which go past the shunt and into the battery never, ever results in 50Ah actually being stored in the battery. This is the (Re)Charging (In)Efficiency Problem.
5. The manufacturer's Ah rating was arrived at under very controlled draw-down circumstances. Exceed those parameters under normal use (which would be 99% of the time) and the Ah end number changes. Such as:
6. Drawing down on the battery when its temperature is not within 75-80F range.
7. Exceeding the tested rate of draw-down (the “20-hour Rate”) which will lower the available Ah, and engage the Peukert Coefficient as a compensating factor. But… PC is only a ballpark figure in and of itself. So it’s a double-whammy of inaccuracy.)

As you can see, getting a proper bead on your battery’s actual (not stated) Ah capacity throughout the day, and throughout its life will take some work.


What is the Ah Rating? Batteries are given Ah rating, usually based upon the “20-hr rate” and performed scientifically as such: Take 5% of the stated (or predicted) capacity, and apply it as a constant Amp load while the battery is at constant 77 temperature. Take the battery down to 10.5V OCV and calculate how much time it took to do so. A 5A load applied to a 100 Ah battery should take 20 hours to reach 10.5V. if it is 100% healthy. Anything outside the 20 hr window will change its rating either up or down. A 4.3A load (which is 5% of an 85 Ah battery) should take 20 hours as well. If it takes just 16 hours to reach 10.5V, then the battery has a maximum capacity of only 80% of its rating (16/20). In other words, this particular 85 Ah battery can store only 68 Ah.

Why is that a concern? On one level, not only do you have fewer Ah at your disposal than you or the shunt thought or bargained for, but if you blindly let the shunt guide you into taking out 50% of the rated Ah (the generally recommended lowest turnaround point) and without making any compensation efforts, you will have removed 42 Ah from the battery. 42 from 68 leaves you with only 26 Ah. This is only 30% SOC. This is when you thought you were being a responsible battery owner and were only taking it down to the bare minimum 50%. Do this often enough (or deep enough) and you will greatly shorten the battery’s natural life.

Given all the possible variables working against you and the battery, listed above in #1-#7, a battery at only 80% of its stated capacity, when you and the shunt thought it was at 100%, is not exactly a stretch of the imagination. This is even more likely if you don’t apply any of the compensating adjustments and actions I touch upon below. On another level, a battery at only 80% maximum capacity, or State of Health, is at the very end if its life; it won’t tolerate many more erroneous “50% draw-downs”, and is “mostly” dead. (No Princess Bride or Monty Python jokes coming down the pipe, BTW)

Of course, knowing what your battery’s actual capacity is would be ideal, and a great starting point. But a properly conducted 20 Hr rate test is about the only way one can do this. Can you do your own 20 Hr rate test? Not really. The proper equipment is expensive (in the thousands of dollars), and maintaining the test parameters of constant draw and battery temperature is likely beyond the modest abilities of most of us who live and work outside a battery testing facility. A partial test in which one somehow maintained a constant battery temp, applied a known constant load, measured the time, took a resting OCV, and then applied some considerable calculations could yield some sort of ballpark figure of some efficacy about its current capacity and State of Health. But I doubt that the effort would be worth it for the vast majority of us Rv’ers.

What to do then, if your battery’s true Ah capacity is essentially, and not practically, knowable? How do you rely on your shunt as your fuel gauge?

There are a number of practices you can easily employ and educated assumptions you can make that will increase the chances that you frequently end up on the safe side of your battery usage:

1. Manually factor into the shunt the battery’s age and naturally declining capacity
2. Enter the battery’s “best” correct Charge Efficiency Factor at it’s given age.
3. Do a Full Recharge before you start using the battery again. i.e. don’t start a recharge if you know you can’t finish it without taking energy out.
4. Enter the best Peukert Coefficient
5. Do a Manual Reset of the Full Capacity rather than relying on an automatic reset.
6. Seasonally adjust downward the battery’s capacity when you know you will be using it at temperatures consistently lower than 77F.

None of these things take a great deal of time, and can be practically implemented, both on the road, and off, and in-between travel seasons. Most of these are topics in the Victron owner’s manual, but are never explained in any great detail, or the reasons, such as why (for example) you would even want to do a Manual Reset, or to do one (gasp!) as your regular routine.

The above points, in greater detail:

Battery Age

Again, the best way to determine a battery’s age or State of Health is to do a 20 Hour Test. In the absence of this onerous task, you can make a WAG and incrementally reduce the overall stated capacity that you input by 1-2% per year. After 5 years you will have adjusted downward its capacity by 5-10%. Is this accurate? This is a hard ‘No’. Where did I get these numbers? I pulled them out of “some place”. But at the very least they’re going in the same direction as the battery’s decline and is better than incorrectly assuming it is staying at its full advertised capacity throughout its entire life. Which we all know the battery definitely is not doing. Better than doing nothing.

Charge Efficiency Factor

This is an entry you can make on the app to tell the shunt how to measure and compensate for the current coming into the shunt and on to the battery. Ah going out to loads are always accurate; those going in to the battery, are less so. It may take up to 130% of the Ah flowing past the shunt for 100% of those Ah to actual stay put in the battery. It’s a chemistry thing. This factor is always a best guesstimate by the manufacturer, at the beginning, and the inefficiency only increases and becomes more unpredictable with age. But for a quality battery, programming for 90% efficiency (or 110% inefficiency… whichever term you prefer) during the first 3-4 years of the battery’s life, and then only 85% during its last 3-4 anticipated years, isn’t an unreasonable approach to take. If you want to live on the wild side, go with 95 and 90.

Try to do a Full Recharge before Discharging Again

The shunt has a problem accurately calculating incoming Ah during different stages of recharging (Bulk, Absorption, Float) and differentiating between the times these stages take place as well. Taking Ah out of the battery while it is charging during any of these stages can confuse the shunt and compound the errors even more. However, showing up at a campsite with a full battery after a full charge on the road will help a lot. (See DC-DC CTEK charger posts). Or fully charging overnight on shore power, with minimal to no battery consumption will get you full as well. The corollary is: hold off on recharging (if you can) unless you know you can complete the task before you must begin depleting the battery again.

The Peukert Coefficient

When you draw down on the battery at a rate greater than the 20-hr rate, you lose rated Ah. When you draw down at a rate less than the 20-hr rate, you gain Ah. Again, it’s chemistry. But shunts generally aren’t able to add Ah back in on that side of its internal ledger. Consider this to be a fortuitous, invisible buffer there to further protect you if your battery use habits are on the mild side. Now ignore it. Heavy users, however, with multiple fridges or a CPAP, who may actually routinely exceed the 20 hr rate of draw, may want to get this one right. Or as right as possible.

The Peukert coefficient, which you program into the shunt, compensates for reduced overall Ah at higher draw rates. For a Lithium, this reduction is next to nothing. For an AGM, it may be anywhere in the 1.15 to 1.30 range. Meaning: a worst case scenario, at consistently high draw rates you could have 15-30% fewer Ah than you thought you had, if you had simply entered “1.00” for the PC. Which would be a big reduction in your overall capacity. The Victron default value is 1.25, which you may wish to change as even Victron cryptically states this to be “the acceptable average”. The Fullriver value is 1.20, apparently. Consult your manufacturer if it isn’t listed on its spec sheet. Mine wasn’t.

To make matters and calculations more troublesome, the PC increases with battery age. The only way to know what it really is up to at any given time, is to do two different hr-rate tests (a 20 and a 5, or something like that) and compare them and do the necessary calculations. Again, not terribly practical for us trailer people if we can’t wrap our heads around doing just one accurately.

Best Guess Adjustment: Start slightly low at first, increase to the manufacture’s value at mid-life, and increase again with an older, later-life battery.

Do a Manual Reset All The Time

Allowing the shunt to automatically recalibrate itself back to 100% capacity could be a mistake, if it is itself working with some bad assumptions at the time. Allowing it to do so multiple times under the same inaccurate circumstances, will create a snowball, compounding error. Reset to 100% manually whenever, and every time, you absolutely know that the battery is fully charged. Its a click of a button on the app screen. This will reduce the frequency of errors and compounding errors. Or… you can turn off the Auto feature permanently as he suggests.

How do you know it if is fully charged? With no loads, on shore charger or DC-DC, at an Absorption level of 14.4V, (NOT Float) with a current going in of 0.5% - 1.5% of the stated Ah capacity, you are fully charged. For an 85 Ah AGM battery, the amps going in at that stage would be 0.4A to 1.3A.

As a further incentive to go all-manual, the greater the number of shared charging sources (shore, solar, alternator) for your battery, the more unlikely it is that an Auto Reset to 100% will be anywhere close to accurate. They don't play well together with the shunt. Become familiar with your charging voltages, amperages, and stages, and reset manually, is his advice.

Seasonal adjustment

My Fullriver Ah capacity is reduced to 80% capacity at freezing and 90% at 50F. Much of our travel is the shoulder seasons. I will lower the stated capacity for the shunt accordingly during these travel times. The Fullriver came with a temp chart; hopefully yours does too. Adjusting for the very marginal increase in Ah at temps above 77F simply isn’t worth the effort. The Victron has the ability to monitor the colder battery temperature and then make the reduced Ah calculations. I don’t have the accessory cord/probe (yet) but it appears that you can customize the % reduction per deg C particular to your battery, and that it’s not some pre-determined generalized adjustment inside the shunt app. In reality, however, reduced capacity due to temperature is a curve drop-off and not a linear relationship. So I don’t know how programmable, or how accurately the temperature accessory and app can account for this. I guess I’ll see.



Some shunts are Smart, can actually learn from how you cycle your battery, knows when it is full, or has aged, and can actually take a few of these tasks off your plate. The Balmar 200 is reputed to do some of this. But it was outside my price-point for monitoring a single battery. I can’t speak as to how effective these added, advertised capabilities really are.

It's worth mentioning that none of the above, nor can the shunt itself, account for the battery's natural self-discharge. These figures, however, to me, would seem to be a non-issue as it relates to the accuracy of the shunt, as you or I would likely not start drawing down on a trailer battery that has been sitting, not charging, for several months, without first fully charging it.


This is all (my) Cole’s Notes of the Marine How To pages. I have come across snippets of the same information (some with the author quoted) scattered elsewhere, or behind paywalls. But none of his experience and writings is generally well distributed or well known. At least it doesn’t jump out during a Google search. Which would explain why he has come across so many compromised batteries in conjunction with the use of shunts. And owners who thought that their shunts were a plug-and-play panacea. Please re-read his articles if you need more detail (or assurances) than what I have provided here. But be fore-warned, he jumps around a lot. Which was the impetus for me to organize and consolidate his thoughts and writing here so that I could easily understand and refer to them in the future. And when I will have (predictably) forgotten the important nuances of what I just wrote. I hope it helps.

Attach a sticky to the battery (or the underside of the bin lid) with battery install date, all your projected threshold dates and adjustment parameters. Just so that 2 years don’t go by and you have forgotten to raise the Peukert, or lower the Efficiency Factor, for example.

As well, there are those of you who are already well-versed in these technical things and have some, or much, experience using shunts and monitors. Please jump in if I have misspoken, misinterpreted, or simply “missed” something.

Steve

 

Thats it. I'm retiring Steve --- you have the con.

 

TY Steve. Really good explanation and highly useful.

 

Kevin: NP.

I should add that what I wrote doesn't come close to covering all aspects of installing and programming the monitor. Only the Doom and Gloom stuff that no one else mentions or tries to deal with. There are lots of other places where you can get the basics and the conventional information on how to install, program it, and use it or the app, if the the owner's manual leaves you scratching your head. This is one and is well-written:

Installing and configuring a Victron battery monitor

Curiously, he enthusiastically acknowledges right up front the great articles in Marine How To from which I almost entirely drew my information. But he fails to even hint at any of the rarely-discussed, baked-in assumptions that can lead you far astray with these devices. Which was the main thrust of the articles. Nor does he incorporate any of Rodd Collins's mitigating strategies into his own article and advice. Weird.