Thread: Battery monitors

I understand perfectly well that many people simply can not shut down their systems to make resting voltage measurements to establish state of charge.

And Yes an ammeter is a valuable item in getting some idea of demand and charge progress.

An accurate voltmeter used in combination with an ammeter can give a pretty fair indication of charge progress.

But none of this will achive the function of a simple fuel guage that so many desire and a number of battery monitors purport to provide.

The multitude of variations are just to complex for even sofisticated computer modeling to deal with.

And all this can be done with a simple voltmeter and ammeter and in the reigon of $100 done and dusted.

But still battery monitors promising much and priced in the hundreds of dollars sell like hot cakes.

cheers

I tend to agree battery monitors promise a lot that they cannot provide and cost a heap. But I guess we have to agree to disagree, as I believe a voltmeter and ammeter still can't provide true state of charge easily in a working system, and the monitor is the only way I know how of giving the best estimate.

We will have to get together when I get back to discuss, including the entire rig I have these days, in around a month or so when I am due back

Cheers

Originally Posted by dai sensei

But I guess we have to agree to disagree, as I believe a voltmeter and ammeter still can't provide true state of charge easily in a working system, and the monitor is the only way I know how of giving the best estimate.

You may disagree, but I can assure you that you are wrong. Been working with this stuff for years. Instrument fitter by trade, studied elec engineering at uni, done heaps of industrial remote solar powered instrumentation. Trust me, we know what works and what doesn't. You know what we use? Dataloggers with voltage, current and temp.

BTW, the original battery monitor you posted is a rebadged xantrex, so might pay to go price up a xantrex unit before you jump in.

I also had a look at the details of the xantrax, it does not load the battery when measuring, so I would question its usefulness at all. To get an accurate reading of SOC the measuring device needs to temporarily load up the battery.

Originally Posted by Pearo

You may disagree, but I can assure you that you are wrong.

I am a bit confused, you say I am wrong, when what I am saying is that you need more than just a voltmeter (or voltage and ammeter meters alone). You yourself now say you need a datalogger with voltage current and temp, but isn't that what a Battery Monitor is just combined into one unit?

I am no expert and know and respect Soundman's opinion. I have the experience of basically living in my camper for the past 2 years and want to learn and understand more if I am barking up the wrong tree.

Originally Posted by dai sensei

I am a bit confused, you say I am wrong, when what I am saying is that you need more than just a voltmeter (or voltage and ammeter meters alone). You yourself now say you need a datalogger with voltage current and temp, but isn't that what a Battery Monitor is just combined into one unit?

I am no expert and know and respect Soundman's opinion. I have the experience of basically living in my camper for the past 2 years and want to learn and understand more if I am barking up the wrong tree.

I did not say you need a datalogger, I said thats what we use commercially. Why? So we can track trends and preemtivly diagnose a failing battery. Guess how we determine SOC? Voltage, thats it, nothing more.

Given you use your camper on a regular basis, a simple voltmeter is more than enough. Check voltage when fridge is running or when battery is loaded and you have a good indication of how much power you have left. Once you get below about 11.5V you are damaging your battery.

As for dataloggers, you are it. If you charge your battery each day for 8 hours with a solar panel and the typical battery voltage is 13.8V you can assume the battery is charged. If after a few years, it starts only charging to 13.7V then you know something is not right and it might be time to check the battery with a hydrometer. Getting to understand what voltages to expect after certain conditions is key to understanding you battery, its life and its SoC.

I know all too well that these battery threads can get a bit out of control.

On 4wd, boating and camping forums...battery threads are like sharpening threads on woodwork forums....only thing is in sharpening there are a number of different ways that work.

In the case of batteries.....the reality.... while full of uncontrolled variables, is quite simple and the number of actual viable choices is small.


Unless you have access to the electrolite to measure SG ( non sealed batteries)....the only reliable and accurate indication of state of charge is resting unladen terminal voltage....temperature compensated if you want to be fussy.

State of charge is a relativly simple concept.....it is simply an assessment of how a chemical process is proceeding in the battery.

State of charge will actually tell you reasonably little......it will tell you ( for want of a better similie) how full the tank is.

The problem is the capacity of the tank and how much can be put into and drawn from the tank varies...and the tank leaks.

then there is the added complexity that the tank that is full and contains 100 units today ( an leaks) may contain 80 or 120 units tomorrow under different conditions without having any put in or taken away.

to complicate the matter even more.....if I start with a tank that has 100 units...then draw 50 units out realy fast.......it may only have 10 units left.......however if I draw slowly on that 100 unit tank...I may get 130 units out of it .....but If I let it get much below 50% full on a regular basis the tank is damaged .

AND the complications go on.




Unless you put a battery into some sort of controlled environment to reduce the number and effects of the variables....its all beyond modeling.



what a lot of people want to know is beyond "state of charge"......but the problem is..."state of charge" is all that we have that is, at all accurate or reliable.


A simple ammeter is the next most usefull thing to a voltmeter......but again, it tells only a little and can not account for the many variables.

It CAN tell you that ya banging good charge in.....but it cant tell you how much stuck.

It can tell you how much you are drawing out.....but it cant tell you how much is left.

In conjunction with the volt meter it give an impression of how charging is proceeding.....but it is an impression not an accurate or reliable picture.

As has been mentioned there are all sorts of very expensive battery condition gadgets that produce very convincing stories about how much has been used or how much is left.

But they are so inaccurate..due to the many variables....they are near to useless.


In a non attended battery installation..particularly one with a controlled environment...they may give good indications....if the data is correctly interpreted....of the battery condition, capacity and projected life......but they are just that indications.

people so badly want batteries to be predictable and accurately quantifiable....like their petrol tank...but they just aren't and never will be.


cheers

Soundman got it.

I disagree about the unloaded measurements, but thats getting academic and probably WAY out of the realm of this topic!!


May I point out one thing. Using the fuel guage example, think about you car. Put a generic fuel guage on your tank, how does it now how full it is? You have to calibrate it to the size of the tank. Do any of these battery monitors allow you to input the battery size?

The best thing you can do to make your battery last is to:

• Dont overcharge it. If its a brand name battery, get the datasheet and check the max charge voltage. Check against your various charging mechanisms. Over voltage will boil the electrolyte which is a sure fire way to kill a battery.
• Dont let it go to flat. 11.5V should be the lowest you go, but if you can keep above 12V you will ensure your battery remains healthy. Excessive discharge results in sulfate build up on the electrodes which kills batteries.
• Keep the electrolyte levels topped up.

Soundman I cannot fault what you are saying, but I am not about to give up and just take the risk, I must try and get the best knowledge I can to make decisions. Hence a battery monitor is better than nothing, certainly better than a voltage meter on it own, even though it may be inaccurate. I do however ignore the hours to charge, discharge etc, only voltage/charge/SOC.

You are right about threads on this though and I feel I am beating my head against a wall . I have formed my own opinions through talking to many many actual camper users. Most installers that have great knowledge of installations, and fixing things when they go wrong, give little/misleading/hard to understand/crap guidance for the "working system" to avoid it failing.

Originally Posted by Pearo

I did not say you need a datalogger, I said thats what we use commercially. Why? So we can track trends and preemtivly diagnose a failing battery. ...

So we, the owners aren't allowed to want the same knowledge, ie do I have a failing battery or going to cause it to fail

Originally Posted by Pearo

Guess how we determine SOC? Voltage, thats it, nothing more.

Given you use your camper on a regular basis, a simple voltmeter is more than enough. Check voltage when fridge is running or when battery is loaded and you have a good indication of how much power you have left. Once you get below about 11.5V you are damaging your battery.

I am now (compared with last year ) aware of only using voltages whilst it is under load, but with only voltage, I have no idea of if it is being charged giving a false voltage reading except at night.

Back last year when I had just a volt meter (no generator either) I was reading 12.2V and worried so used my car to charge it till I was reading what I thought was a reasonable 12.6V. I went to bed and woke up to 11.6V and I am sure damaged the battery. It is now my understanding the voltage may have been reading 12.6V but this was probably false and the SOC was probably far less than 60%. I actually have no idea and only guessing, but the voltage dropped far more than it should have overnight. I also know now a quick charge with a car will give a quick drop when under load.

Now, if my monitor says I have 60% SOC in the morning and I know it is going to be a less than perfect solar charging day, I will run the generator all morning (activating the charger in the camper) to bring it up to 90%. If I don't make that decision in the morning, I'm going to p_ss off a lot of fellow campers nearby running the generator at night if I don't get a decent solar charge.

Originally Posted by Pearo

If you charge your battery each day for 8 hours with a solar panel and the typical battery voltage is 13.8V you can assume the battery is charged. If after a few years, it starts only charging to 13.7V then you know something is not right and it might be time to check the battery with a hydrometer. Getting to understand what voltages to expect after certain conditions is key to understanding you battery, its life and its SoC.

All good useful information. I must admit when the voltage shows more than 13V I have been ignoring it as being a charging voltage and not really real, but will now take more notice.

When charging on 240V should it be the same maximums etc?

Originally Posted by dai sensei

Soundman I cannot fault what you are saying, but I am not about to give up and just take the risk, I must try and get the best knowledge I can to make decisions. Hence a battery monitor is better than nothing, certainly better than a voltage meter on it own, even though it may be inaccurate. I do however ignore the hours to charge, discharge etc, only voltage/charge/SOC.

You are right about threads on this though and I feel I am beating my head against a wall . I have formed my own opinions through talking to many many actual camper users. Most installers that have great knowledge of installations, and fixing things when they go wrong, give little/misleading/hard to understand/crap guidance for the "working system" to avoid it failing.



So we, the owners aren't allowed to want the same knowledge, ie do I have a failing battery or going to cause it to fail



I am now (compared with last year ) aware of only using voltages whilst it is under load, but with only voltage, I have no idea of if it is being charged giving a false voltage reading except at night.

Back last year when I had just a volt meter (no generator either) I was reading 12.2V and worried so used my car to charge it till I was reading what I thought was a reasonable 12.6V. I went to bed and woke up to 11.6V and I am sure damaged the battery. It is now my understanding the voltage may have been reading 12.6V but this was probably false and the SOC was probably far less than 60%. I actually have no idea and only guessing, but the voltage dropped far more than it should have overnight. I also know now a quick charge with a car will give a quick drop when under load.

Now, if my monitor says I have 60% SOC in the morning and I know it is going to be a less than perfect solar charging day, I will run the generator all morning (activating the charger in the camper) to bring it up to 90%. If I don't make that decision in the morning, I'm going to p_ss off a lot of fellow campers nearby running the generator at night if I don't get a decent solar charge.




All good useful information. I must admit when the voltage shows more than 13V I have been ignoring it as being a charging voltage and not really real, but will now take more notice.

When charging on 240V should it be the same maximums etc?

Dont get me wrong, if you want to go down the commercial path and use dataloggers you can. Rest assure, that commercial solutions cost many thousands of dollars. Check out alber or generex gear. I am sure its possible to pick up a cheap voltage and current datalogger that will do the same, and you dont need fancy software to analyse the results, excel will do just as good a job.


Ok, here in lies the big issue with loaded v unloaded voltage measurements (something you battery monitor is incapable of dealing with).

If you stick a charger on a battery and fully charge it, the voltage as measured with a voltmeter (once the charger is removed) will read about the same voltage as your charger which will be about 14V, depending on the battery chemistry and the charger. Let the battery sit for an hour or so, and this voltage will drop to about 2.15V a cell (once again, depending on chemistry), or 12.9V for a 12 volt battery. If you load the battery up straight after you remove the charger, you will read close to that 12.9V give or take a bit. In practice, the most accurate way to measure the SOC is to disconnect the battery and let is rest for an hour, but this is totally impracticable in almost all circumstance other than when you are testing a battery. These measurement will be the same regardless of how you charge it, provided the charger you are using is half decent.


At the end of the day, the battery monitor will not give up much useful info other than voltage and current. You will still need to learn how to interpolate that data and how it applies to your setup.


BTW, the last place you want to gain knowledge is from other camper users. I am not privy to any caravan or camper site, but I can only guess at the pseudoscience that gets circulated in those realms!. I would also question a lot of installers, most are just low level techs and dont have a real understanding of the physics. If you were to chose between the 2 for advice, I would go a tech as they do it every single day, just be warned that a lot of them are salesman too, trying to drive their business.



On a side note, I also test 12V battery chargers out of personal interest. I have done a few now, from supercheap chargers to CTEKS to dispel some myths. I just bought an aldi charger to see how they stack up, but I am yet to put it to the test. I basically load test them, plus check ripple and charge characteristics.

I've been doing some more research on the subject of battery monitoring, and it seems that a battery should be rested for at least 12 hours after charging to obtain a true reading from a volt meter, totally unacceptable in a motor home/caravan situation.

I find it difficult to believe that many RV manufactures would fit battery monitors if they did not give some sort of reliable information about the state of the house batteries. If these monitors didn't work then we would have many unhappy campers with flat or dead batteries complaining to the RV manufacturers. Furthermore I find it hard to believe that a company like Xantrex would risk their good name on an unreliable product.

More reading:

http://hobohome.com/news/?p=679

http://www.xantrex.com/documents/Acc...O_LinkLITE.pdf

http://www.campertrailers.org/enerdrive_elite.htm

http://hobohome.com/news/?p=685

The first and most important thing to get past is that idea that...if someone is selling it or installing it it must be good or usefull.

The fact is that many camper or motorhome manufacturers know squat about electrics and those they have installing know even less.

The fact that many boat, caravan, camper or motorhome manufacturers install barreries in enclosed unventilated places proves that.

The fact that they also install gas bottles, housing generators and petrol containers in poorly ventlated or poorly protected places reinforces the matter.

These manufacturers are not rocket scientists at best they are coach and body builders or panel beaters...at worst they are not even qualified welders.

The majority of the caravan, camper and motorhome manufacturers are consumers and simply rely on what the salesman ( company rep) tells them or what they read on the internet.


AND let me tell you there is some hokum comming out of some of the electronics manufacturers and their representatives...even the better ones.


On the matter of measuring resting voltage......

remember this is all we have, that is realy accurate....AND.... the battery manufacturer documents and the text books will confirm this.

When a few other things are known, resting battery voltage is a pretty accurate indicator of "State Of Charge'.

Remember too that a lot of what is written by the reliable sources is for those in controlled situations looking for accuracy.

The longer the battery is rested the more accurate the state of charge can be assessed using voltage measurement.

BUT

Even after a short period of rest, voltage can be a good indicator of state of charge.
Right from the instant the battery is isolated from charge and load....the error factors begin to rapidly deminish.

After 15 minutes or half an hour of resting, a voltage measurement can give a pretty fair indication of the state of the battery.....the longer the battery is idle the more accurate the indication.

As soon as any load is applied the voltage will drop....and this drop will depend on how heavily the battery is loaded

As soon as a battery is put on charge the voltage will rise...this rise will depend on how high the charger voltage is, how much current it is capable of delivering and the state of charge of the battery.



As far as the viability of battery monitors.

My problem with them is that people take what they indicate as gospel.....and with no further thaught or observation, this leads to problems.

As for capacity settings and other "calibtarions".

Some of the better battery monitors do provide facilities to set the battery capacity and battery type and a very few, quite a bit of profiling.

Some of them may even have a fair go of making an estimate of state of charge while on charge or under load......but don't dream that it will be accurate.


Neil......I know you as an observant man with an engineering background.....I have no doubt that understanding the limitations, you will make good use of whatever instrument is provided.

But you aint the average consumer.

just remember that if the monitor is reading 60%..it may be anywhere between 40% and 80%....of whatever the battery capacity might be.




One thing that concerns me is.

That there is a lot said about batteries in general...and by some reasonable sensible and reliable people.

BUT
There is currently so much variance in batteries available.....the simple stereotypes of the past and the simple assertions are just not reliable.

Many battery chargers and monitors will have a profile for flooded batteries and another for AGM...and maybe a profile for small Sealed batteries.

A flodded battery profile will set the float voltage at arround 13.8V, the AGM float voltage at arround 14.5 volts and the small sealed battery profile will be current limited to a couple of amps.

But we now have flodded batteries that employ most of the technological advances of AGM.

SO do you charge them as an AGM or flooded.

Most of the small sealed batteries are AGM in all but name....and some will cop good hard charge rates...so we do we use the sealed battery programe or the AGM.....or the manufacturer may recommend a 13.8 volt float voltage...do we put it on the flodded cycle.

Then there is such a wide variation appart from that.
If we are to make sensible choices we realy need to know what these over simplistic selections mean, and know the specificaton of the batteries involved.

Of course lots of people simply don't want to know...they just want to know their battery is half full.

Giving them a guage that tells em what they want to know, keeps them happy......regardless if it being at all accurate or the truth.


If concrete was a variable and as vague as the lead acid battery, I don't think we would be building bridges out of it.

cheers

So, Soundman, are you saying that the elite battery monitor I am thinking of installing is useless? That it is not capable of giving accurate information about the condition of my battery/ies?

A flodded battery profile will set the float voltage at arround 13.8V, the AGM float voltage at arround 14.5 volts and the small sealed battery profile will be current limited to a couple of amps.

Totally wrong, the float voltage of an AGM is 13.6 V. In float mode the amperage going into all batteries varies between 0.1-2 amps depending on loads at the time.

A battery monitor (the top end ones )monitor more than battery voltage if the parameters are set correctly at installation.

They measure, depending on whether or not you have solar, charging via a battery charger, generator or by vehicle alternator, amperage in and out of the batteries, percentage of amp/hours remaining and also the voltage.

They also factor in Peukerts law to account for appliance usage to give a highly accurate view of the power situation.

Granted that if you had time to sit and watch an ammeter and a volt meter and take in all parameters of a vehicle that may be in Tassy one month, the bogong high plains next month and then Darwin good onya.

Most Rver's don't understand how voltage and amps work let alone how to work it out but they do understand a meter that says you only have 25% power left in your batteries before things die, the beer gets hot and you can't pump water.

I consider them a must in all RV.

Oh yeah, I do low voltage installations and set yachts and caravans and motorhomes up for self sufficiency.