Smartgauge percentages

[Theo]

and it wont volt drop at 10m ? I thought any voltage will drop as you distance yourself. or is their a sensor at the battery itself?

The voltage drop is proportional to the current through the wire. If there is negligible current there will be negligible voltage drop. Even with thin wires.

 

N

[by'eck]

and it wont volt drop at 10m ? I thought any voltage will drop as you distance yourself. or is their a sensor at the battery itself?

Volt drop in cabling is a function of current. The Smartgauge's input impedance is high enough to create virtually no current draw so volt drop is not an issue even with the specified thin wires, although there will be an ultimate limit.

[londonron]

great news. I think i can cross something else off my list. Doesnt seem to be gettting any smaller my list you know

[Phil.]

Ok, even more confused now.

This morning SG was reading C58. I then plugged in on shorepower and the mastervolt combi 2500/100 commenced charging at a voltage of 14.50. After 5 hours the smartgauge reading was now C100. This I thought meant the batteries are now full, however the mastervolt was still charging, indicating that it was still in the absorption stage. Voltage was now 14.70. The mastervolt continued like this for a further 3 hours until float stage was reached and voltage had dropped to 13.40.

So my question is why did SG say batteries at 100% before they actually are. Does this mean SG is knackered or does it need a reset. If SG is doing this it will mean that when charging using engine I will be stopping engine before batteries are full even though SG indicates they are potentially damaging them.

[nicknorman]

Ok, even more confused now.

This morning SG was reading C58. I then plugged in on shorepower and the mastervolt combi 2500/100 commenced charging at a voltage of 14.50. After 5 hours the smartgauge reading was now C100. This I thought meant the batteries are now full, however the mastervolt was still charging, indicating that it was still in the absorption stage. Voltage was now 14.70. The mastervolt continued like this for a further 3 hours until float stage was reached and voltage had dropped to 13.40.

So my question is why did SG say batteries at 100% before they actually are. Does this mean SG is knackered or does it need a reset. If SG is doing this it will mean that when charging using engine I will be stopping engine before batteries are full even though SG indicates they are potentially damaging them.

A few points:

 

The Combi is only aware of the total current it is supplying. It doesn't know how much of the current is going into the batteries and how much going into the boat to run the fridge, lights etc. I find our MV Combi tends to linger in absorption mode for this reason. It is several hours after 100% on the SG before it finally goes to float even though the current going into the batteries has been very low for a long time.

 

Secondly, there is no clear definition of 100% SoC and it is never reached. 100% SoC is only approached asymptotically (that's my fave word again) so it just depends on how close to 100% you want to consider to be "fully charged". In my experience the SG makes a reasonable stab at it. And don't forget the SG only claims to be within 10% during charge, although in my experience it is much closer than that by the time 100% is reached.

 

There is always a compromise between time spent charging (running a generator etc) vs squeezing the last bit of charge into the batteries for no great reason. Yes battery life might be shortened a bit by only reaching 99% but the generator running time saved more than makes up for the slightly shorter life of the batteries.

 

In summary I would stop charging from a genny / static engine when the SG gets to 100%, but don't be surprised if the batteries can take a little more charge if the charging voltage is applied for a few hours more - during which time just a couple of amp hours will have been added.

 

If you really want to get a handle on what is going on with the batteries you really need to monitor the charging current as well as the Smartgauge.

Edited November 8, 2014 by nicknorman

[Phil.]

Thanks Nick I understand that now

[by'eck]

Thanks Nick I understand that now

As Nick suggests the tail charging current whilst still in higher voltage absorb mode is probably the most important parameter in judging a fully charged battery. Its the premise most shunt based (current measuring) battery monitors work on to indicate 100% SOC.

 

Another point is that many mains chargers and external alternator regulators have a fixed timed period in absorb mode. Early ones required manual pre-set according to bank capacity, later ones are adaptive but they still work with a timer so its not unusual for them to stay in absorb mode longer than might be expected whilst the batteries draw minimal current.

[Theo]

I am trying to get a handle on how accurate the Smartguage is.

 

1) How long do the batteries need to be rested before a sensible reading can be taken to estalish SoC?

 

2) Having turned of the engine and disconnected the solar panels I believe that I need to discharge the batteries a little bit before I take the readings. How much?

 

N

[nicknorman]

I am trying to get a handle on how accurate the Smartguage is.

 

1) How long do the batteries need to be rested before a sensible reading can be taken to estalish SoC?

 

2) Having turned of the engine and disconnected the solar panels I believe that I need to discharge the batteries a little bit before I take the readings. How much?

 

N

I presume you are referring to checking the voltage using a voltmeter, vs what the Smartgauge is showing? Because of course to take a epreading from the SG no resting etc is required. With that caveat

 

1) it depends on the extent and rate of the previous discharge. If you just had a couple of LED lights on, then just a couple of minutes. If you have just been running the washing machine (ie high current drain for an extended period) then an hour or two. Or something in between. Of course if you monitor the voltage repeatedly over say 10 minute intervals, you should get a sense of when the voltage has stabilised. If you want to be clever plot the readings on a graph (excel) and work out the final voltage - it will be (wait for it...) an asymptotic curve.

 

2) With charging, again it depends in how fast and how long. 24 hrs, or turn on a reasonable load say 5 amps (tunnel light) for 30 mins, wait another 30 mins. Something like that, but it is not an exact science.

Edited November 8, 2014 by nicknorman

[Theo]

I presume you are referring to checking the voltage using a voltmeter, vs what the Smartgauge is showing? Because of course to take a epreading from the SG no resting etc is required. With that caveat

 

1) it depends on the extent and rate of the previous discharge. If you just had a couple of LED lights on, then just a couple of minutes. If you have just been running the washing machine (ie high current drain for an extended period) then an hour or two. Or something in between. Of course if you monitor the voltage repeatedly over say 10 minute intervals, you should get a sense of when the voltage has stabilised. If you want to be clever plot the readings on a graph (excel) and work out the final voltage - it will be (wait for it...) an asymptotic curve.

 

2) With charging, again it depends in how fast and how long. 24 hrs, or turn on a reasonable load say 5 amps (tunnel light) for 30 mins, wait another 30 mins. Something like that, but it is not an exact science.

I suspect that the asymptotice curve will be exponential

 

That is nice and clear and what I would have expected before Mr Gibson told me that I need to leave the batteries resting for a couple of days. I hope that I am recalling his advice correctly. It was given about six years ago when I sent him no end of calibration curves of SoC (Smartguage) v, SoC (Rest Voltage). I was using off spec standby batteries and tested SG on each of the battery types to get the best match. The results were all over the place and he sent me a reasoned explanation of why SG was actually working correctly even though my graphs seemd to indicate that this was not the case.

 

N

[nicknorman]

I suspect that the asymptotice curve will be exponential

 

That is nice and clear and what I would have expected before Mr Gibson told me that I need to leave the batteries resting for a couple of days. I hope that I am recalling his advice correctly. It was given about six years ago when I sent him no end of calibration curves of SoC (Smartguage) v, SoC (Rest Voltage). I was using off spec standby batteries and tested SG on each of the battery types to get the best match. The results were all over the place and he sent me a reasoned explanation of why SG was actually working correctly even though my graphs seemd to indicate that this was not the case.

 

N

No, an asymptotic curve is not the same as an exponential one! Anyway, the Smartgauge is not perfect but it is pretty good. Usually it tracks our Mastershunt within a few % on discharge. It lags behind on fast charge by more than 10% but it has caught up by the time 100% is reached.

[Theo]

No, an asymptotic curve is not the same as an exponential one! Anyway, the Smartgauge is not perfect but it is pretty good. Usually it tracks our Mastershunt within a few % on discharge. It lags behind on fast charge by more than 10% but it has caught up by the time 100% is reached.

Perhaps I am just forgetting my maths but I thought that asymptotic described curve which approached, but never actually reached, a line. So an exponential curve like a pd v. time curve for a capacitor resistor discharge would approach, but never reach the time axis, and was, therefore asymptotic to it.

 

But getting back to the point, I am wondering why my set up seems so erratic. I have 7 off 110Ah batteries (SFL) and I have them wired up with equal length identical csa cables hydraulically crimped to their connecting lugs. The cables are connected to bus bars and I have tried the SG connected to the bus bars (To which all loads and batteries are connected and it gave possibly erratic results. I then did lots of measurements of currents through each of the batteries. The currents were different so the pd's between the battery terminals and the busbars were different by some millivolts. This lead me to think that the better idea was to connect the SG dirrectly to one battery on the assumption that the SoC of one battery was likely to be similar to the SoC of the whole bank. I am still getting funny results...

 

...but I should not hijack this topic. I will shut up now and complete all the measurements that I intend to take and come back on yet another battery topic to see if the cognoscenti can help me to find out what is going on.

 

More later in a brand new topic.

 

N

Edited November 9, 2014 by Theo
Must read the post before hitting send...

[bottle]

You obviously know more than I do but

 

Is it not in the instructions, for connecting the Smartgauge, that the connections must be made directly to the battery terminals at each end of the bank no where else, I think the point is repeated a number of times in the instructions.

 

Could this be why you are getting, funny results.

[nicknorman]

An asymptotic curve and an exponential curve look similar, but the shape is different. An exponential function is by definition of the form x to the power y, whereas an asymptotic curve has a different function (can't remember exactly what). In particular an exponential curve doesn't approach zero, although of course it's reciprocal does.

 

Anyway, I have the SG connected to one battery in the bank, not across the diagonal of the whole bank. This seems the logical way to do it if interconnect resistances are to be eliminated, although the SG website has it the other way (Gibbo was not always right!).

[ditchcrawler]

Just a thought .......

 

When the designer of the Smartgauge used to post on here nobody had a bad word to say about the SG. Now .....

 

 

 

Dave

Shhhh

[Theo]

You obviously know more than I do but

 

Is it not in the instructions, for connecting the Smartgauge, that the connections must be made directly to the battery terminals at each end of the bank no where else, I think the point is repeated a number of times in the instructions.

 

Could this be why you are getting, funny results.

The improtant thing AIUI is that electrical loads don't affect the voltage seen by the SG. On my set up, which has the batteries connected radially to the busbars, I don't have "ends" of the batteries. I thought, until I measured some voltage drops under load, that the pd between the positive and negative busbars would be the same as the pd between the ends of the batteries. But it is not exactly the same, hence my reconnecting to the ends of one of the batteries.

[Detling]

As I understand it Smartgauge is not meant to be read like a 'Bible' and mine certainly doesn't work as one. If I run some equipment for a known time I know I have used X percent of the 550 Amphours my batteries theoretically can hold. Smartgauge does not drop by X percent straight away but wait a while and it will be very close.

 

When we get back to the marina after a good run and SG states C100, when I connect shorepower the Victron goes Bulk, Adsorption to float in about 5 minutes so the batteries are pretty full.

 

What I do know is that if it says C60 in the morning I need a good charge period today, if it says C78 then in summer the solar will probably be enough. It is also useful when you look at 5:00pm and realize that all is not well when it reads C65 so you had better run the engine for an hour or so to survive the night. It doesn't matter if C65 is actually 65 percent or not, it indicates that your batteries are below 70 percent and that may be important at 5:00pm otherwise Murphy states that by 8:30 your inverter will beep and turn off in the middle of that film / match. and it is too late to charge your battery!

[bottle]

The improtant thing AIUI is that electrical loads don't affect the voltage seen by the SG. On my set up, which has the batteries connected radially to the busbars, I don't have "ends" of the batteries. I thought, until I measured some voltage drops under load, that the pd between the positive and negative busbars would be the same as the pd between the ends of the batteries. But it is not exactly the same, hence my reconnecting to the ends of one of the batteries.

 

Ah! I can see the problem but do not have an answer.

 

As I understand it Smartgauge is not meant to be read like a 'Bible' and mine certainly doesn't work as one. If I run some equipment for a known time I know I have used X percent of the 550 Amphours my batteries theoretically can hold. Smartgauge does not drop by X percent straight away but wait a while and it will be very close.

 

When we get back to the marina after a good run and SG states C100, when I connect shorepower the Victron goes Bulk, Adsorption to float in about 5 minutes so the batteries are pretty full.

 

What I do know is that if it says C60 in the morning I need a good charge period today, if it says C78 then in summer the solar will probably be enough. It is also useful when you look at 5:00pm and realize that all is not well when it reads C65 so you had better run the engine for an hour or so to survive the night. It doesn't matter if C65 is actually 65 percent or not, it indicates that your batteries are below 70 percent and that may be important at 5:00pm otherwise Murphy states that by 8:30 your inverter will beep and turn off in the middle of that film / match. and it is too late to charge your battery!

 

That is probably the best explanation I have seen and is how I use it.

[George94]

It's best to run the engine/genny in the morning, when the batteries are well down and will take plenty of amps, and then let the solar panels take over and trickle charge the last 10 or 20% over a number of hours. If you use the engine/genny for the final top up, you will use much more fuel than if you had done it the other way around.