Battery Monitoring - Amps In v Amps Out?

[Richard10002]

I fitted a NASA BM2 a couple of months ago, and it does exactly what I expected, and wanted... Which is great!

I have a query:

There seems to be a concensus that you need to put more Ah in than you take out, due to inefficiencies, (heat creation, etc.).

So..... If my monitor says I've used, say, 70Ah since last charged, (or zeroed, or whatever), to get back to the same state of charge, I probably have to put in 115Ah, (50% extra for inefficiency).

If this is the case, the zero for Ah in/out doesn't represent a particular state of charge........

Am I understanding the situation correctly, such that there remains a degree of interpretation necessary, even with a monitor.

I also have an observation:

The state of charge percentage seems to present a moveable feast.... it can say 75% one minute, then, after using say 10Ah an hour later, it can say 65%. 10% of my 330Ah bank would be 33Ah. there is nothing consistent about the differences, e.g. I could use 10Ah, and the SoC percentage can be a bit higher.

Thus, the SoC percentage is a guide for trends over a longer period, rather than an actual representation, or so it would seem.

I chose NASA rather than Smartguage because i wanted to see Amps in and out. The Smartguage monitor seems to represent itself as absolutely accurate with respect to State of Charge, but I would guess it is subject to similar vagaries, dependent on the particular equations it uses in its "brain".

 

[nigelmercier]

Does your meter know the capacity of the battery, and its Peukert's Constant?

 

[noelex]

The battery monitors allow for battery inefficiency, generally by only counting a percentage of the amp hours in. The better ones alow this percentage to be adjustable and allow for Peukerts equation. I think the NASA has the later, but not the former feature.

If correctly wired the monitor can keep good track of the AHr count. The better ones zero themselves automatically when the battery return amps get low.

The NASA unit is unique in that it uses a voltage algorithm like the smart guage as well as an amp hour count. This is the reason why you do not see a direct linear relationship with the AHrs. I am not a great fan of this feature, but the unit still works reasonably and is great value.

 

[SK700]

Its Complicated

One of the factors is the discharge rate, if you draw loads of current in a short time the battery will deliver significantly less ah than a slower discharge. I understand that there are heating and chemistry issues, a look at the battery data sheet will confirm the levels involved.

 

[Georgio]

Hi Richard, as SK700 has said it is complicated, especially as the rate of charge and discharge needs to be taken into account.

I have had a Nasa BM for about 4 years now and do rate it highly (now that i have replaced the inline fuse holder that is a bit ****py). the % used figure does need to be treated with a bit of a pinch of salt but with the amp/hr used counter, voltage and amps going in and out you get a very good idea of what is happening in your batteries.

 

[pvb]

One of the factors is the discharge rate, if you draw loads of current in a short time the battery will deliver significantly less ah than a slower discharge.

That's the Peukert's effect. The Nasa monitor takes this into account, but it's not possible to change the value of Peukert's exponent. More expensive monitors allow this.

 

[Marsupial]

its just a theory - well almost anyway

the answer to your question is "its a black art" or a best "a moveable feast" with lots of variables and many unknowns; in practise its very difficult to measure used and stored capacity but the meters and monitors do a reasonable job. You will always put more amps in than you get out but make sure you recharge when volts drops to around 12.20 ish and the batteries wont suffer.

What with sealed batteries and different lead acid technologies I have come to the conclusion over the years the a good voltmeter is all you need.

 

[Trundlebug]

I fitted a NASA BM2 a couple of months ago, and it does exactly what I expected, and wanted... Which is great!

I have a query:

There seems to be a concensus that you need to put more Ah in than you take out, due to inefficiencies, (heat creation, etc.).

So..... If my monitor says I've used, say, 70Ah since last charged, (or zeroed, or whatever), to get back to the same state of charge, I probably have to put in 115Ah, (50% extra for inefficiency).

If this is the case, the zero for Ah in/out doesn't represent a particular state of charge........

Am I understanding the situation correctly, such that there remains a degree of interpretation necessary, even with a monitor.

I also have an observation:

The state of charge percentage seems to present a moveable feast.... it can say 75% one minute, then, after using say 10Ah an hour later, it can say 65%. 10% of my 330Ah bank would be 33Ah. there is nothing consistent about the differences, e.g. I could use 10Ah, and the SoC percentage can be a bit higher.

Thus, the SoC percentage is a guide for trends over a longer period, rather than an actual representation, or so it would seem.

I chose NASA rather than Smartguage because i wanted to see Amps in and out. The Smartguage monitor seems to represent itself as absolutely accurate with respect to State of Charge, but I would guess it is subject to similar vagaries, dependent on the particular equations it uses in its "brain".

Richard, in the same way that your car fuel gauge can go up or down when on a gradient, your BM is, like your fuel gauge, an indication rather than an absolute. Your fuel gauge doesn't show how many gallons to the nearest 0.1g you have left, but it gives you an idea.

Yes, I know it calculates Ah etc and displays everything as numbers down to the last 0.1 but as you can see from the comments already posted, as there are so many variables and complicating factors the simple truth is that it can only be used as a guide.

However to answer your question more directly:-

To get the best out of the guide, when arriving somewhere after a long day's engine use I tend to zero the Ah gauge (which will inevitably show some residual Ah charged) because you know that the batteries are fully charged. What you want to know is, how many Ah have you used whilst moored up?

The bar graph gauge on the V&A screen is again only a very rough guide, as you have already discovered. Generally I don't pay that much attention to it. The "time to discharge (hrs)" figure is quite useful though, helps to focus the mind at times...

 

[wotayottie]

The state of charge percentage seems to present a moveable feast.... it can say 75% one minute, then, after using say 10Ah an hour later, it can say 65%. 10% of my 330Ah bank would be 33Ah.

Its unlikely that your 330aH actually is 330aH. Test it by starting with fully charged batteries and monitoring the number of aH it takes to reduce the battery voltage to 12.1. That when doubled will give you a rough idea of the real battery capacity you have - batteries lose capacity as they age. Bit like us really.

 

[Marsupial]

Its unlikely that your 330aH actually is 330aH. Test it by starting with fully charged batteries and monitoring the number of aH it takes to reduce the battery voltage to 12.1. That when doubled will give you a rough idea of the real battery capacity you have - batteries lose capacity as they age. Bit like us really.

Yes agreed but what is a "fully charged" battery? Run all day with the alternator charging and you might get to 95% full charge with a smart charger - but you would only know if you left the batteries for an hour or two to rest with no load tested the voltage and specific gravity; so in the real world we dont really know what the state charge is, add to that the fact that at 12.1 volts the capacity is half gone and you dont want to draw the other half or you will shorten the life of the battery and things are getting a bit fraught if you need all 165AH you could/should/might have. The reallity is that most of the time "fully charged" 110AH batteries hold about 95AH and of that you have about 40AH to use before the voltage drops below 12.1. SO on a good day when the batteries are new you might have 120AH to use with three on board.

 

[Richard10002]

Does your meter know the capacity of the battery, and its Peukert's Constant?

It knows the capacity, but has no facility to alter the Peukerts constant. The bank is 3x 110Ah, new in March. I know this will be less as time goes by.

 

[cliff]

That's the Peukert's effect. The Nasa monitor takes this into account, but it's not possible to change the value of Peukert's exponent. More expensive monitors allow this.

Such as? - I was looking at fitting a Battery Monitor so am interested in what is available particularily the more upmarket varieties

 

[NickRobinson]

Oh Peuk!

The BEP has an adjustable Peukarts value.

And lots of other user adjustable settings.. I've had trouble stetting mine, a bit of RTFM but the manual is not very intuitive either.

If interested,

http://www.ybw.com/forums/showthread.php?p=3729579

My old boat just had two voltmeters, ALDI, £3 each, one for each bank. Like most, I love the idea of a battery 'fuel gauge' letting me plan usage and charging, over journey and overnight. In reality it's a bit more rough and ready.. for me at least.

i.e. inverter's 'low volts' is sounding... finish emailing..
i.e. yey! engine's started... (100% reliable so far...)

Nick (Rivendell)

 

[pvb]

Such as? - I was looking at fitting a Battery Monitor so am interested in what is available particularily the more upmarket varieties

Most of the better quality monitors allow you to change the value of Peukert's Exponent, for example monitors such as the Xantrex Linklite, Xantrex Linkpro, BEP DCSM, BEP DCM600, Victron BMV-600S, Victron BMV-602S, etc, etc.

 

[nigelmercier]

Such as? - I was looking at fitting a Battery Monitor so am interested in what is available particularily the more upmarket varieties

CruzPro

 

[James Marinero]

SO on a good day when the batteries are new you might have 120AH to use with three on board.

It's great to know that of the 2 x 200 ah batteries I have, I'm lugging the dead weight of one about with me. Why do we need all that weight to give us 50% of what they hold (don't answer that pleeease....).