Basic Battery State Monitoring

[motion30]

The part I most like about the battery monitor is you can see excatly how many amps each device or light uses You then can adjust your usage to save engery

[btrayfors]

"Voltmeters are worthless". But, wait:

"Amp-hour measurements are really worthless".

Strong language, guys. And, as is so often the case, the truth most likely lies somewhere in between.

Can a simple digital voltmeter be helpful in estimating the state-of-charge (SOC) of your batteries? Yes, absolutely. Like Kettlewell, I've been using such a meter on my boat for 22 years (and on previous boats for many more). Used correctly, and with a little knowledge of your boat and of batteries, a voltmeter can be all you need. I haven't bothered to install a modern battery monitor like a Link or a Victron, because I don't need one.

Is this the average case? No, absolutely not. For many boatowners lacking the intimate knowledge of their boat and the technical understanding of battery behavior....and, probably, the patience....a voltmeter-only solution would likely be insufficient.

OK, can a modern battery monitor (amp-hour counter) be helpful in estimating SOC and capacity remaining? Yes, absolutely. While there are some potential pitfalls, as discussed in the referenced link above touting the properties of the "SmartGuage", these have been mostly mitigated or overcome with modern designs. Modern battery monitors like the Victron 600 take full account of Peukert's equation, of inefficiencies in battery charging, of battery aging, of temperature, etc., etc. Used correctly, they are pretty neat and will keep most users out of trouble. They even have some self-adjusting characteristics to take account of reduced battery capacity over time.

Note that voltmeters and battery monitors are measuring very different kinds of things.

Voltmeters measure approximate SOC if left resting after charge for a few hours, with a small drain. They do not measure battery capacity.

Battery monitors such as the Link and the Victron attempt to estimate battery capacity remaining, based on a measurement of net amps in and out, adjusted by their settings and by their internal algorithms. They also measure and display battery voltage and SOC.

Which is best for you?

The answer is...it depends. Most sailors will likely be happier with a battery monitor. Some, like me and Kettlewell, will actually prefer to forego the battery monitor and go with a voltmeter and intimate knowledge of their boat, their batteries, and battery basics.

FWIW,

Bill

[Kettlewell]

As always, Bill has some sage advice and he's right the term "worthless" was too strong. The problem I think is that people are using these amp measuring devices and believing they are more precise and accurate than they really are, and they are spending a lot of time, money, and effort to get this dubious information, whereas a voltmeter really does give you accuracy: the voltage at some point in your system at that particular time. Now I can look at that reading in the morning and know how it compares to the reading I saw the previous morning and the morning before that, and this has been with the batteries at rest for around 12 hours or so. Cheap and simple. Not perfect, but pretty good. This gives me a decent idea of how things are going. Also, my main point is that if funds are tight I think most boaters will be better served by purchasing more battery power, more charging capacity, and/or more energy conservation devices like LED lights instead of monitors. Switching to an LED anchor light makes a big difference in a small boat's energy budget. I cut way down on refrigeration running time on one boat by simply packing the area around the box with as much insulation as I could, and I got most of it for free. Etc.

[Capt.Don]

Wouldn't a simple rule of thumb be to use the battery until voltage reaches 50% capacity, or around 12.2v? The battery manufacturers supply the open circuit voltage. If you look at the discharge voltage over time, it is pretty flat and then rapidly falls off. Using a voltmeter, when voltage starts to drop, you can tell where in the discharge cycle this is and when it is time to recharge.

From a day-to-day usage, you develop an idea of how many amps consumed. I suspect this figure is pretty constant based on your usage. From that you can get pretty close to when you reach the 50% point and need to recharge.

[btrayfors]

Capt.Don,

Yes, that's the general idea.

When you're cruising, you get a pretty good indication of power usage by monitoring battery voltage. On many cruising boats, the usage isn't that heavy overnite: an anchor lite, a frig, and maybe a bit more. So, in the morning you can look at the battery voltage, still under modest load, and get a pretty good idea of the State of Charge (SOC), and whether or not it's time to do some recharging.

I try never to take my house battery bank (675AH capacity) below about 12.2 volts under load. Since the normal load is less than 10A, and most of that is for the frig (about 6A), this means that at 12.2 volts the battery SOC is something above 50%. OK, I see 12.2V and it's time to recharge.

Note that I really don't care about amps used and amps remaining. Why? BECAUSE I CAN'T DO ANYTHING ABOUT BATTERY CAPACITY. Battery capacity is dependent on the health of the battery, not just on its SOC.

If, over the years, I see that 12.2V is being reached earlier than usual -- like by 2AM when I get up to pee -- then I know that the batteries are losing capacity and/or that they weren't adequately charged the night before.

This kind of "monitoring" requires a conscious attempt to keep track of your battery voltage....all day and night. I make it a habit to glance at the voltage display many times a day.

Another "test". What happens when you don't charge the batteries for several days and there is very little load on them? Mine are in that condition right now, because of a power post problem and a dropped neutral at the dock. This is another story for another time....a very dangerous condition. But, my dockside power has been off for 2.5 days now. Battery voltage is 12.6. Near full charge. I don't really care if 2AH or 12AH or 24AH have been consumed over the past 2.5 days. What I care about is the present SOC of the batteries, and how to keep them fully charged. Hope to do that tomorrow, with a new 50A outlet on the power post...properly wired and tested!

Bill

[HopCar]

Newmar makes a nice battery monitor:
These highly versatile and sleekly designed digital instruments provide comprehensive monitoring of all on-board AC and DC electrical systems. They give accurate and up to the moment information of your electrical systems including voltage, power con

[HopCar]

Wow I didn't know all that text would post. It is a link to Newmar's website.

[conachair]

Measuring A Lead Acid Battery State of Charge Photo Gallery by Compass Marine at pbase.com

24 hours for the voltage to stabalise after charging.

[goboatingnow]

"Voltmeters are worthless". But, wait:

"Amp-hour measurements are really worthless".

Strong language, guys. And, as is so often the case, the truth most likely lies somewhere in between.

And all the other stuff. Lets look at the science and lets forget the " its worked for me for 30 years etc etc, its a science after all).

Lead acid Terminal voltage of a battery under charge or discharge does not in any way accurately reflect the SOC. Its the SOC you need to know. It can reflect it to a reasonable accuracy after 12 to 24 hours complete disconnection from any charge or discharges

A voltmeter does not indicate that a battery is fully charged as lead acid terminal voltages rise quickly under charge. Equally by the time you have got a very low reading the battery is ruined. All voltages tell you is the charging voltage. Thats why they used to exist on cars etc and that practice followed into boats, yet the situation is entirely different, especially in sailing boats

"Amp hour meters are wothless". They are not.

This comment is based on the "SmartGuard" web site stuff, which play fast and loose with the science. In practice, SmartGuard and all other amp hour meters, can only measure terminal voltage and battery current. Equally the science of using that information is well known and fixed. WIth a proper implementation, dynamic peukerts exponent calulation, battery aging computation etc, Intelligent Amp-hour meters are the closest thing you can come to judging SOC.

The difficulty for the user is of course determining from the marketing hype what battery monitors ( and all of them are essentially amp hour counters) actually are accurate and which are not. This is why some try the "blind em with science" and others actually say nothing at all.


The best of course is a accurate hydrometer.

The Smartguard article is at worst misleading and full of obfuscatrion. For example their piece on accuracy is especially misleading. They are trying to convince you they have "discovered" some new technique when in fact all it is is a reasonable and intelligent amp-hour counter.

As to the " it worked for me for 30 years" brigade. Most people simply charge and discharge based on experience. Ie they know that they need x hours after a certain time, this is usually found through trial and error. In the process they usually damage their batteries, but of course they have no way of knowing that.

THey tend look at voltmeters and say , "lets recharge", but of course they get away with it, becuase they are cautious and infact the bank isnt probably significantly discharged at all.

Equally , weekend warriors, return to the comfort of shorepower and that recharges the batteries while they slave away at the day job. Again the system works, but almost despite the owners attention.


BUT, to return to the OP. A current meter, ie an ammeter, is much more useful then a volt meter. Install that and so some basic manual amp counting. It also lets you see the efficiency of your charging systems.


Dave

[pteron]

Originally Posted by goboatingnow

And all the other stuff. Lets look at the science and lets forget the " its worked for me for 30 years etc etc, its a science after all).

I wouldn't dismiss peoples' experience so quickly. Humans are incredibly good at detecting patterns of behaviour, and I'd suggest that those who've watched their volt meter response to usage over 30 years have developed an internal model of their system and can thus calculate the approximate state of charge of their batteries.

Lead acid Terminal voltage of a battery under charge or discharge does not in any way accurately reflect the SOC. Its the SOC you need to know. It can reflect it to a reasonable accuracy after 12 to 24 hours complete disconnection from any charge or discharges

But history of voltage change does reflect SOC.

"Amp hour meters are wothless". They are not.

This comment is based on the "SmartGuard" web site stuff, which play fast and loose with the science. In practice, SmartGuard and all other amp hour meters, can only measure terminal voltage and battery current. Equally the science of using that information is well known and fixed. WIth a proper implementation, dynamic peukerts exponent calulation, battery aging computation etc, Intelligent Amp-hour meters are the closest thing you can come to judging SOC.

Smartguard uses a model of the battery system and is a perfectly acceptable way of estimating SOC. The site raises valid concerns about amp hours meters but doesn't use the language I would have chosen!

Modern mobile devices are gradually shifting to using model based calculation of SOC because of the limitations with amp hours counting. An amp hour system needs a reset point otherwise it gradually drifts out of calibration. That reset point can be fully charged or fully discharged, but it ideally needs both occasionally to establish how the battery is ageing. Model based systems rely on characterisation of multiple batteries to produce a baseline which then gets modified based on the actual behaviour of the battery being monitored.

The best of course is a accurate hydrometer.

Only after the battery has stabilised and difficult to read remotely!

The Smartguard article is at worst misleading and full of obfuscatrion. For example their piece on accuracy is especially misleading. They are trying to convince you they have "discovered" some new technique when in fact all it is is a reasonable and intelligent amp-hour counter.

Not sure how you can state this as it clearly isn't an amp hour counter. The site is a little polemical, but it has some very valid points. Perhaps you could give examples of information that is demonstrably wrong as we are discussing the science?

For me, I'd do a model based system that monitors both volts and current. (Actually, my company has done just this but not for lead acid batteries!)

[goboatingnow]

But history of voltage change does reflect SOC.

No it doesnt, in reality it says very little. simply looking at a particular voltage and repeatidly assuming a SOC is very misleading and only works if the charge discharge cycles are repeatable and similar

Smartguard uses a model of the battery system and is a perfectly acceptable way of estimating SOC. The site raises valid concerns about amp hours meters but doesn't use the language I would have chosen!

No it doesnt, it raises theorectical issues. but read say its paragrapgh on accuracy. The underlying assumption being that accuracy is all positive or all negative. Engineers know that accuracy is generally +/- hence many are cancelled out. Thats paragraqh is theorectically true, but in practice misleading

Not sure how you can state this as it clearly isn't an amp hour counter. The site is a little polemical, but it has some very valid points. Perhaps you could give examples of information that is demonstrably wrong as we are discussing the science?

For me, I'd do a model based system that monitors both volts and current. (Actually, my company has done just this but not for lead acid batteries!)

Smartguard is as much a "amp counter" as any others. ALL BMs are amp counters, since that all you can measure. Most intelligent BMs then try and model the bank using Peukerts etc. The implementaions vary and thats why you get good and bad BMs

All intelligent BMs ( are there are other types, I havent seen them) use an internal "model" since thats the only way you can attempt SOC determination. Equally all BMs need a reset point to reinitialise the algorythm. The only real one is "fully charged", as "fully discharged" is not a setpoint that can be used in reality.

I had been heavily involved in LA, NIcd, Nimh charge and discharge systems, I wrote a PC based modellng system myself. To model the battery accurately ( especially if you have no reset point) is very difficult and in practice almost impossible to any degree of accuracy.

Dave

[pteron]

Originally Posted by goboatingnow

No it doesnt, in reality it says very little. simply looking at a particular voltage and repeatidly assuming a SOC is very misleading and only works if the charge discharge cycles are repeatable and similar

That's a bit of a straw man, as I said history of voltage not a particular voltage. For instance, if you measure the voltage curve of a car battery as it is attempting to start the car, you can predict the state of charge of the battery to an impressive degree of accuracy. You also get state of health. Not any one voltage. A series of voltage/time measurements made at a high sample rate over many seconds. (About 25 years ago there was a Dunlop car battery that had one of my chips in it that did just this).

Smartguard is as much a "amp counter" as any others. ALL BMs are amp counters, since that all you can measure. Most intelligent BMs then try and model the bank using Peukerts etc. The implementaions vary and thats why you get good and bad BMs

You claimed it was an amp hours counter. It does not measure amps at all - there is no shunt. It measures voltage history and predicts SOC. If you look at its output it is in percentage SOC.

We supply household names with battery management - to the tune of hundreds of millions of devices per year. You or your friends almost certainly own or have owned a device with a battery management chip in it that my company produced.

[goboatingnow]

You claimed it was an amp hours counter. It does not measure amps at all - there is no shunt. It measures voltage history and predicts SOC. If you look at its output it is in percentage SOC.

Yes you are correct, I read its polemic on amp meters. Its a voltage profiler. I have done that type of thing myself, but I have my views on its accuracy, based on a very extensive set of test we did many years ago. It has its faults like any other modelling system

Of course the primary problem , is that short of a proper series of tests, the user cant verify the claims made and hence we get exposed to the "pro-argent, sensitive teeth toothpaste) style of marketing.

That's a bit of a straw man, as I said history of voltage not a particular voltage. For instance, if you measure the voltage curve of a car battery as it is attempting to start the car, you can predict the state of charge of the battery to an impressive degree of accuracy. You also get state of health. Not any one voltage. A series of voltage/time measurements made at a high sample rate over many seconds

Yes yes I accept that ,I was refering to the use of a common voltmeter, not proper voltage profilling.

[pteron]

Originally Posted by goboatingnow

Yes yes I accept that ,I was refering to the use of a common voltmeter, not proper voltage profilling.

Then it appears we are in violent agreement!

[goboatingnow]

as pennance for decending into such specifics, I shall solve a few Nernst equations.

Dave