ghostlymoron
Well-Known Member
I'm not convinced of the value of a battery monitor. Long term they are just too inaccurate and IMO wrong information is worse than no information.
Voltage meter on House batts? necessary?
- Thread starter symondo
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oldvarnish
Well-Known Member
I've got one of these
http://www.smartgauge.co.uk/smartgauge.html
Don't understand how it works, but it seems more sophisticated in its operation than NASA, but at a price.
http://www.smartgauge.co.uk/smartgauge.html
Don't understand how it works, but it seems more sophisticated in its operation than NASA, but at a price.
VicS
Well-Known Member
It monitors only the voltage and then works out the rest by "mathematical jiggery-pokery"
The Nasa one measures the current going in and out, on the face of it the more obvious thing to do, but still has to use mathematical gymnastics to deliver meaningful data
vyv_cox
Well-Known Member
We have never had a battery monitor, nor really felt the need for one. We typically cruise for half the year, almost never connect to shore power and have never had any failed battery incidents in 30 years of cruising. We have a volt/amp digital meter that is very useful indeed and a dual battery bank solar panel regulator that also displays volts and amps. As has been said, monitoring the starter battery is pretty much unnecessary but we watch the domestic bank voltage keenly.
sailinglegend420
Well-Known Member
I know exactly what he means. If not all the load AND charging sources are connected via the shunt then the Ah count will be wrong. It might have been installed correctly, but later someone has come along and installed something else and not realised there was a battery monitor and a shunt!!!!!
There are three battery parameters that must be input correctly into a good battery monitor. It's a great shame that the cheapo Nasa doesn't allow you to programme two of these essential parameters!
Peukert's equation gives you a very precise understanding of what you batteries will deliver under different current discharges. Read about it and then adjust your OPINION afterwards.
Peukert’s law says that the apparent Ah size of a bank changes depending on the current draw.
A bank is designed to deliver a capacity with a current discharge that will flatten the battery in 20 hours. (The 20 hour rate)
So with a 100Ah battery, a 5A load will flatten the battery to 10.5v in 20 hours.
When drawing currents higher than 5 amps the "actual" bank size will be much smaller, so the bank will not last as long before it needs re-charging. Conversely when using much less than 5 amps the bank size will be larger and will deliver more Ah.
If a 100 Ah that battery has a Peukert value of 1.25, then higher or lower loads than 5 amps will change the actual capacity of the battery by the following amounts.
With a 10A load for 20 hours there are only 84Ah's in the bank.
With a 1A load for 20 hours there are 150Ah's in the bank.
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GrahamM376
Well-Known Member
I've found the NASAs resonably accurate over a few years and better than some more expensive ones which don't even display current being drawn or input - in my miind one of the most useful functions of the NASA.
GHA
Well-Known Member
But they are a useful source of both voltage and current in/out. So easy to see how well your solar./wind are doing when the fridge is running etc. And maybe more importantly, can give a good indication of when the batts are close to fully charged, if voltage is in the low fourteens and just few amps going in then it's looking good. Mine resets to 100% with definable volt/amp settings so if you are being nice to your up batteries and getting them fully charged every day should never drift too far.
Amulet
Well-Known Member
Try a bit harder - you could surely manage a bit more patronising. I'm sure you're right about everything your say. I'm aware of how little I know, and was largely plagiarising on the views of a battery researcher I know. Comfortably confident that I understand my ignorance, I have spent long enough in scientific research to gain self-awareness in that regard. All of your message when though my shunt - which caused it to heat up enough to affect the precision of my response.
Amulet
Well-Known Member
Gosh. Which particular one caused you to form that opinion? Even my cheapo NASA's calibrate pretty accurately against more sophisticated equipment - I check them all the time with almost any meters I can get my hands on. I wouldn't be without them. I used to check the batteries directly with digital multimeters before I had the monitors. Even if the monitors are pretty inaccurate, which is not what I measure, the ability to check at a glance that nothing really silly is happening is very reassuring. I am well aware of the budget-limited frailties of my setup, but despite my obsessive double checking on it, it hasn't ever seemed to seriously mislead. I don't depend on the % charge reading, but the monitors are worth it to me just for voltage and current. That doesn't mean that I know the % charge to be wrong, but since I avoid the bottom half, I'll probably never know.
sailinglegend420
Well-Known Member
You are referring to the SmartGauge which I bought when my even more expensive BEP Battery Monitor became very inaccurate. This happens as the batteries age and all the parameters that were entered when installed have changed.
ghostlymoron was right about the longterm accuracy - but they do have an accurate Voltage and hopefully Current reading which are very useful.
With all these inaccuracy problems the military finally demanded more accurate battery monitors - so enter the SmartGuage.
This is a very sophisticated monitor that uses clever mathematical algorithms to work out the % depth of discharge of the batteries just by measuring the voltage, and it does this over one thousand times a second. It learns the battery status and becomes more accurate the more it "learns", whereas a shunt based Battery Monitor becomes more and more inaccurate over time. Yes the SmartGauge doesn't have Ah readings, but these current meters are fairly cheap and should be installed with a SmartGauge.
Because it doesn’t use a shunt, it is cheaper than most Battery Monitors and is very much easier to install as only two wires need to be connected. The only information it needs is the type of battery being monitored. It does have a huge range of alarms and a very long and detailed user manual for those who want to get the most out of it. A second battery can also be connected to monitor battery voltage only. It has been proved by Enersys, who make Optima batteries, to be much more accurate than a shunt based Battery Monitor.
Amulet
Well-Known Member
Eh? The shunt has got b ugg er all to do with reading the voltage. So the fact that a monitor is "shunt based" does not stop it doing the calculation based on voltage as you say, and that's what NASA say their monitor is doing. I guess they must be lying. The NASA monitor ALSO integrates amps in and out in order to learn the battery parameters by comparing it with the Peukert calculation. At least that's what they say, but I guess they could be lying about that too. The integrative calculation can wander a bit from reality (they say). If the monitor is switched off, restarted, and then left under a light load for a few hours the instrument will recalibrate itself - they claim.
halcyon
Well-Known Member
Agree to a point, a voltmeter and ammeter allow you to see the point when further charge will gain little capacity, whatever the capacity may be it's your full value, be it 90 or 95% but rarely 100%.
Problems monitoring current is that all the variables, shunt tolerance, connecting cables, connections, battery etc, all add up over time, coupled to what is a 100% to start your reading at. With voltage, what voltage to read ? battery voltage varies with so many factors, including load applied, it's a bit like measuring a piece of elastic.
Best method is a voltmeter and ammeter plus the human brain, will allow most people to maintain batteries at a except able level to give a good battery life.
Brian
Amulet
Well-Known Member
Yes I have done that dozens of times with different meters under different loads. It has never given a different result to the battery monitor. Why should it? The voltmeter contains a shunt - that's how they work - and the test leads are equivalent to the leads from the monitor. The monitor is (among other things) a built-in volt and ammeter, and I cannot understand why anyone would expect it's being built in to make any difference to the reading. This is not university physics, it's not even A level physics. The voltage and current readings from my cheap monitor are perfectly good, and the ability to glance at it at any moment means you can see instantly what effect it has when, say, someone charges their iPad. The percentage capacity left indicated on a monitor can only ever be an estimate except for ones with a built in time-machine. How much you get out will depend on the load and also quite heavily on temperature.
sailinglegend420
Well-Known Member
Eh? I'm not quite sure what points your are trying to make in your last post!
For clarity to others who might also be confused - as far as I read from the same manual that you have read it would seem Nasa use the battery voltage to continuously correct the Ah capacity reading. Most other shunt based Battery Monitors only use the voltage to correct the Ah reading when they think the battery is at or near fully charged and then they reset their Ah reading to 100% charged. Their default setting for this might be that when the voltage is over 13.6 volts for 10 minutes and the current is less than 2% of the battery capacity then it Auto-Syncs the Ah capacity to FULL. These values can be programmed in - but many manufacturers default value are set way to low. A battery is actually FULLY charged when the current going in is at 0.5% of battery capacity at a voltage near its absorption voltage - about 14.4 volts - for a 400Ah bank this will be 2 amps. There will be a big difference in the current going into the battery at different voltages. You can test this yourself. As soon as the battery charger goes into float the charging current may drop to 2 amps - turn the charger off and back on to force it back into Absorption voltage when the current may well rise to 10 amps at 14.4 volts indicating the battery is NOT fully charged. This is "The Charging Gotcha" - a battery is not fully charged when the charger drops to Float mode, because a charger that doesn't actually measure the amps going into a battery it will never know when it is charged - they all stop charging after a timed period. The are designed to charge batteries - not overcharge them. After 12 hours on Float your battery may then be fully charged.
There's another "Charging Gotcha" with these "auto-resync" features that makes them useless if you have solar panels. Under cloudy conditions or shade the solar panels may be putting out only 13.6 volts at 2 amps for 10 minutes, so even if the batteries are 75% charged the Battery Monitor will reset to 100%. It is best to disable this feature by simply setting the re-sync voltage to 16+ volts which it will never reach. Re-Sync to 100% when you know the batteries are 100% charged.
+1
Always use the volts and amps to check on the rough accuracy of the Ah count on a Battery Monitor.
Another very confusing post - voltmeters don't need a shunt to measure voltage! Two wires connected to the battery is all that is needed - not even A level practical physics - anyone can do it which is the big advantage of a SmartGauge Battery Monitor.
Digital Battery monitor meters are very accurate at reading Volts and Amps - they are just not very accurate at counting Ahs for all the reasons discussed in this thread.
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Amulet
Well-Known Member
You're right. You probably know more than me.
Things I am sure of: 1. It's pretty easy to build a monitor which tells you accurate current and voltage, and, because I like data I check everywhichway. My data say that if all you want is voltage and current, then you can expect even a pretty cheap system to give it to you, and it won't make any difference whether whether you grovel in your battery locker with a meter or read it off your monitor unless you have made a spectacular pig's ear of installing your monitor. (And by the way, how do you measure current with your multimeter? Hall effect clamp meter? Undo your battery leads?) 2. It is impossible to accurately predict remaining battery capacity in a real life situation. I could rabbit on about the evidence for this but I am becoming boring and anyone can do a literature search if they care. The 1897 Peukert model is an empirical equation (which is a mathematical way of saying a rule of thumb).
Apologies for being pompous and NASA centric. NASA kit is cheap and of mixed quality, but their HQ is just down the road from my UK home and they are very happy to chat to visitors.
I'll shut up now... promise... no more ... ask sailinglegend in future.
Amulet
Well-Known Member
It's the drink, I swear. Did I really type that? I do know that ammeters need a shunt, not volt meters. Apologies. I'll go and lie down. Actually we agree about one thing. It's no problem measuring voltage and current, and capacity is hard. And I accept that some systems for measuring capacity are better than others (but none are great). And this time I will leave off.
GrahamM376
Well-Known Member
Whilst the Smartgauge may be a bit better than the NASA as far as determining capacity remaining, it's useless if you want to analyse what's going on i.e. what's flattening the batteries so quickly? With no ammeter, you've no idea how much charge is going in or what current the various loads are taking out.
GHA
Well-Known Member
+1
Why fit something which doesn't tell you what's going on??
Sight drift but some useful data here re temp and voltage under load against state of charge . Not an accurate way of judging state of charge but handy for a quick glance.
http://offgridcabin.wordpress.com/2012/03/14/batteries-new-sg-soc-charts/
http://autonopedia.org/renewable-energy/energy-storage/soc-vs-volts/
GHA
Well-Known Member
Isn't the generally accepted ballpark 1 or 2% of capacity at 14.4v? So a battery SOC meter is wrong if you set it up wrong. Hardly the fault of the meter.
How do you know when your batteries are 100%? Temp adjusted SG measurement?
sailinglegend420
Well-Known Member
My Lifeline AGM manual says 0.5% of capacity at 14.3v.
The problem is you can't set up an Ah meter to this value because the charger will have dropped down to a float voltage long before the current reaches 0.5% of capacity at 14.4 volts. That is why this has to be manually controlled if you really want to check when your batteries are back to 100%. Let them stay at a float voltage for another 12 hours to get fully charged, but don't say your solar panels have fully charged your batteries by lunchtime because you controller has dropped down to float mode!
When my SmartGauge tels me, or when they've been on shorepower for 24 hours which I do once every 2-3 weeks.
Temp adjusted SG measurement will tell you if you have unbalanced cells that need equalizing. SG measurements are no better that Open Circuit voltage measurements to establish the battery SoC.
This is another controversial subject in the “electrickery game”, so extreme care needs to be taken when using SG to measure SOC. Also severe care needs to be taken with the hydrometer. It should be kept in a sealed container to avoid contamination which over time can affect the electrolyte.
Measuring SG of an open wet lead acid battery accurately may be a difficult and time consuming procedure. SG readings have been traditionally used to measure the state of charge, but unless the battery is healthy they may not be the best method because the SG reading gets lower as the battery ages.
The SG of each cell at a 100% SOC should be 1.265 or higher. If the cells in the battery are not equal then an average of all cells has to be made. If individual cells are lower it means their Ah capacity is reduced due to sulfation; a value of 30 points between cells indicates that the battery should be equalized.
For accurate results the SG must only be taken when the cells have been topped up with distilled water and then only after the batteries have been charged, this stirs up the electrolyte to give a true SG reading for each cell. Another major problem is the SG reading must only be taken when the charged battery has been allowed to rest with no charge or discharge for several hours. This is not very practical on a typical cruising boat, so the potential for errors can give poor results. The temperature of the battery must also be measured and compensation has to be applied to all readings.
If the battery is known to be fully charged because it has been on shorepower for a couple of days the SG readings taken then will represent 100% charged. If these are all lower than the readings when the batteries were new that indicates they have lost some of their Ah capacity. Every time the battery is discharged and charged again these SG values will reduce very slightly.
It's worthing noting that you can have a very high state of charge and a relatively high SG, but the battery may not have enough residual capacity to last long at anchor.