battery monitor accuracy question

[Ross]

Waly, Two things to consider. First how large do you think the effect of the HF noise will be in its effect on the measurements and second without regard to the frequency of the source voltage once it has passed the rectifiers it will be direct current that the battery can absorb and releace as pure DC with no AC component. The battery is the great equalizer in this equation.

 

[Maine Sail]

Im not sure what its bandwidth is and also not sure what it would do for white noise (a broad spectrum of sine waves - which is what Maine Sail had playing on the stereo when he tood the measurement).

Over the top - no doubt..

Walt,

The bottom line is that battery meters are the most accurate form of measurement we have that installs on a boat and can be read at a glance. Perhaps they are not 100% accurate, it's doubtful they are, but they are probably very close.

Really all we're worried about is roughly how much we took out, don't want to go below 50%, but a little +/- either way is not going to hurt, and how much we put back in. If we take out 50% and we only motor long enough to get back to 80% of charge then we know we have only 30% of the banks original capacity left usable before we are back at roughly 50% again and need a charge.

What you are asking is "what makes it tick" and that is a question the engineers who designed the software and device would have to answer. My guess is it is somewhat proprietary information and you may not get all the details on specific filtering and algorithms you want.

As others have said the battery is acting like a huge filter and is probably why the shunt resides on the neg pole of the battery.

 

[tsmwebb]

As long as the all the current that is being used is passing through this shunt resistor it will work because it is direct in-line. This is how a current meter works.
Your concern about current fluctuations in this environment are not measured in the K-hertz range.
They are sudden changes occuring infrequently as load are induced or removed from the circuit.

It seems to me there are two points here:

1) The measurement is inline so if we were sampling continuously (and w/ 0 error) we should expect a perfect analogue record. The question of DC or AC does not really play into this at the logical level. The power company doesn't care if you rectify all your power right after the meter or if you run lights or switching power supplies. They (and we) care about watts. Yes?

2) But, and I think this is where the confusion is, the battery meter is a digital micro controller so it does not continuously sample. Rather it takes samples at some finite rate. Now, if that rate is slow and you have a load or source that changes quickly the micro will very likely have sampling errors. If the micro only sampled every 5 seconds and you had a strobe light blinking every 5 seconds it is probable that the micro would miss the load completely (but possible that it would see it as a constant load). In either case, your amps readings in particular are going to be garbage. And, if you're attempting to track the state of the battery your Peukert numbers are going to be high order garbage. If the load were perfectly digital then you would need to sample the shunt twice as often as the load can switch to be assured of a correct reading. Now, I don't know either what the sampling rate of the monitor is nor the switching rate or magnitude of the loads and sources... In practice, I've never seen a monitor that didn't accumulate errors and need re-setting periodically (some do this automatically) but they all seem to work pretty well short term. Also, they tend to be quite good and responsive digital amp/volt meters. So, my feeling is that IN PRACTICE you don't have to worry about any of this... That's what you paid the engineers for.

--Tom.

 

[walt]

Welll, I admit Im being ANAL to even think about this.. and I would agree that the Linklite seems to work just fine - not completely necessary but it helps tremendously in understanding your power "budget" in a boat.

I think Tom sumarized one issue - if Im trying to get the true RMS of a changing waveform (in this case, current), and Im using a sampling method (my guess at they way it actually works- and this woud be a good thing), I have to sample the signal a lot faster than the signal is changing or I lose information. So if the method the meter uses is sampling, how fast of a changing signal can the meter handle.

Second issue relates somewhat to the measurement method. Take for example a DC to AC inverter under a moderately light load. I have not looked at the waveform of one of these but likely there is a short relitively high amplitude current spike which occurs 120 times per second. If I were to use a typical "inexpensive" hand held DVM to measure this, it would pick the peak amplitude of the current spike and assume that the current was a full sine wave. The RMS (ie, average) current in the short duration current spike could be much less than the RMS current in a full sine wave - even though the peak amplitudes were the same. The inexpensive current meter could have a large error in this case. I doubt the linklite works this way but could be wrong.

Some example of things on a boat which could have periodic current waveforms that are not sinusoidal:

DC to AC inverter
12 volt TV which have DC to DC switchers
Alternators (however, these current waveforms will be somewhat sinusoidal)
Stereo playing music. The current waveform will to some extent follow the low frequency content of the music.

DC current measurements are no problem

Note that the battery does filter voltage. But it does not filter current fluctations in or out of the battery and that is what the shunt is measuring.

Yes, very anal...

 

[tsmwebb]

RMS?

if Im trying to get the true RMS of a changing waveform (in this case, current)

Since we're being ... Pedantic ... I think you want mean not root mean squared when looking at DC current.

--Tom.

 

[Stu Jackson]

Perhaps you are missing the point

Some example of things on a boat which could have periodic current waveforms that are not sinusoidal:
DC to AC inverter
12 volt TV which have DC to DC switchers
Alternators (however, these current waveforms will be somewhat sinusoidal)
Stereo playing music. The current waveform will to some extent follow the low frequency content of the music.
DC current measurements are no problem

Note that the battery does filter voltage. But it does not filter current fluctations in or out of the battery and that is what the shunt is measuring.

Yes, very anal...

1. You still have NOT explained WHY this is an issue, given the fact that the shunt's on (-) side AND that it's after the battery

2. If "DC current measurement are not problem", what, pray tell, could be a problem?

3. Analysis of your items:

DC to AC inverter - DC load is what is being measured, that's ALL: an inverter TAKES DC power and converts it to AC; while the AC side can "wiggle" all over the place, the load's on the DC side from the battery

12 volt TV which have DC to DC switchers - doesn't matter since it's DC, too

Alternators (however, these current waveforms will be somewhat sinusoidal) - by the time they through the "stuff" "inside" the alternator, AND through the battery to the shunt which is measuring things, who cares? really...

Stereo playing music. The current waveform will to some extent follow the low frequency content of the music. - Nonsense - the DC load is simply the VOLUME, need I turn it up any higher?

C'mon, it's been fun having you pull our legs, but after over ten years of actual use, for the INTENDED PURPOSE - measuring amps in and out, you are really kinda off the wall , while somewhat interesting.

It's always fun to question HOW something works, but if ya can't give some answers to questions you raise (not that Q&A isn't the idea here), how are we to deal with any validity of your approach?

You've come back a few times, but still haven't answered the basic question of why you asked it in the first place if the monitors you are asking about work quite well for what they were designed to do.

You said: "DC current measurements are no problem"

That's the entire and whole purpose these devices.

You wrote: "Note that the battery does filter voltage. But it does not filter current fluctations in or out of the battery and that is what the shunt is measuring."

NO, no, no,NOT, NOPE, that's just completely wrong!!! The shunt is measuring current OUT of the battery, not into it. Unless you want to play the + to (-) or (-) to + game. Or could I be missing a basic fact? Wouldn't be the first time, but c'mon...

If you'd like to explain to us HOW they work, we'd be happy to listen.

Anal, naw, misguided and questionable at best, perhaps, but, really, help us out here, because what you're promoting could well do some damage to the listening community here on this board. People could start to disbelieve their measurement devices. Please explain how we all could be so wrong for so long.

Please...

 

[walt]

1. You still have NOT explained WHY this is an issue, given the fact that the shunt's on (-) side AND that it's after the battery

Some answers are free and some cost $100 per hour...

 

[Ross]

Stu, very well said! Some questions begin to get rather tiresome.

 

[tsmwebb]

what you're promoting could well do some damage to the listening community here on this board. People could start to disbelieve their measurement devices. .

A healthy dose of skepticism can be a good thing I've got an Ample Power energy monitor and it gets cumulative errors and has a two's complement bug. It does provide a pretty good rough and ready battery meter most of the time and it controls my solar panels and alternators. So, I'm not complaining too much. In fact, it is a very useful bit of kit. Still, other folks I know with other brands of energy monitors have noted similar problems. So, if your battery monitor says your battery has negative 6k amp hours left when your battery is showing 12.8v no load the monitor is wrong, not the battery! These devices, though quite useful, could be better.

--Tom.

 

[Stu Jackson]

A healthy dose of skepticism can be a good thing So, if your battery monitor says your battery has negative 6k amp hours left when your battery is showing 12.8v no load the monitor is wrong, not the battery! These devices, though quite useful, could be better.

Tom, I agree that skepticism can be a good thing, but voltage alone in figuring battery state of charge is what we've discussed before as "old school technology."

And, using your example, if I have a simple single bank with 100 ah, those 6 AH would make a heck of a bigger difference than for our 360 ah house bank. It's not appropriate to simply throw only "some" numbers around...

The issue, like all navigational tools, for instance, should all be treated with some "skepticism" to the extent that we do not trust only ONE source of information.

That's why Links and other monitors usually have more than one function, and allow you to check most, if not all, of the parameters. Almost all of them have cumulative errors which is why they provide the user with users manuals. As guys, we rarely read 'em, but we could. I do so every once in awhile, and learn something new all the time, from the manuals and forums like this. They've been very helpful in suggesting that I check the voltage, too, but learn how to use the meters.

We discussed this, here, awhile ago: http://forums.catalina.sailboatowners.com/showthread.php?p=601417 (It starts in the middle of the thread, so start at the top - I remembered using the phrase "old school technology" and used it to find it quickly with the search engine, Thanks, Phil, for that important feature)

Three things kill batteries: undercharging, overcharging and incomplete recharges. Your boat, your $$, but I'd sure like to know the most information I can get when it comes to my investment in lead.

That said, I eagerly await Walt's reply to figure out what his original question actually means and if it has any validity in the real world of amperage measurement on recreational boats.

 

[Witzend]

Touche Stu, well put.
T

 

[tsmwebb]

And, using your example, if I have a simple single bank with 100 ah, those 6 AH would make a heck of a bigger difference than for our 360 ah house bank. It's not appropriate to simply throw only "some" numbers around...

The issue, like all navigational tools, for instance, should all be treated with some "skepticism" to the extent that we do not trust only ONE source of information.

Ok, I agree with all of that --but I'll add something anyway . Just a clarification, actually. When my Ample meter goes into it's overrun-two's complement error it starts treating inputs as drains and drains as inputs and, of course, reports really "large" negative numbers. This is a bug in the software (doubtless because the C compilers for micro controlers generally suck -- but that's another rant). I only brought it up as an example of why it might be good to have some skepticism when you read these things and why it might be a useful to understand how they work (so you could have a feel for what kind of errors there might be).

Sorry to be so longwinded on this as: 1) I think we agree and 2) it I certainly don't want to steer folks away from energy monitors; I just want to suggest that they use them in an educated way. I'm not sure exactly what the OP was after either.

--Tom.

 

[Stu Jackson]

Thanks, Tom

From my understanding, Ample is a quality company. I also understand that many companies update their software. I assume you've contacted them. If so, have you not received adequate attention to the problem? Kinda a separate question from the thread, but I'd be interested to learn about Ample's service if it was NOT up to the quality of their reputation. Thanks, Stu

 

[walt]

I did some measurements this morning with my charge monitor to get some real data.

First, at the boat, I put a 10 ohm load on a "12" volt source which has a return current going thought the battery monitor. The battery monitor read 1.2 amps with a voltage of 11.8 volts (my batteries probably need to be equalized). 11.8 volts / 10 ohms = 1.18 amps so the meter works reasonably well (the meter only displays to 100 mv).

Second, I plugged in an inverter running a 60 watt light bulb. The DC voltage dropped to 11.4 volts and the Linklite meter read 5.7 amps.

I took the inverter and 60 watt light bulb to a different location with a different 12 volt car battery and scope. I put a .0083 ohm shunt in the ground line similar to how the Linklite shunt is wired. The voltage on the battery with the converter and light bulb on (just bought it today, hasn't been charged) showed 11.58 volts. So the two batteries had similar voltages when powering the inverter-light bulb.

I measured the current to the inverter using two different DVM-amp meters. Both showed 5.55 amps.

The attached picture shows the voltage on the shunt. Because the load was fairly high for the inverter, the current is on for most of the time and you can see a periodic pulsing at 120 hz (i.e., 8.33 msec period). This is the same waveform that the linklite will see on the shunt.

In the waveform, we will have to approximate a little, but the current is at 6.168 amps for 6.7msec and its at 2.42 amps for 1.6 msec. The average current per cycle from these numbers is 5.44 amps.

In this example, I would say the Linklite works fairly well. The range of average currents using the three methods was 5.44 amps to 5.7 amps.

If I get the time, I would like to try this same thing with a lighter load on the inverter so that the high current pulse is shorter duration compared to the cycle time.

My apologies for ruffling some feather, I was only asking a question and felt it was important to follow up with this measurement.


Stu, Im not sure why I need to answer any of your questions???

 

[Ross]

Walt , just because it can be measured doesn't mean it is important.
If you feel the need for answers to questions of this sort by all means pursue them if it will help you sleep at night. The fuel gage on your tank has some built in inaccuracies but if it indicates 1/4 on the empty-full scale it is probably enough to tell that before long you need to refill the tank. By the same token if your battery monitor indicates that you have drawn 85 per cent of your capacity down it matters not if it is actually 84 % or 86 %.