State of Charge Determined by Under Load Voltage

[Dockhead]

I notice that our own MaineSail has put up this very interesting article on this subject:

https://marinehowto.com/under-load-b...oltage-vs-soc/

I note with satisfaction how closely this tracks my own chart:



I am reinforced in my belief that tracking voltage is better than using an amp-counting battery monitor.

I have a SmartGauge, which in contrast to the amp-counting meters, determines SOC using voltage. It is really convenient to just push a button and see the SOC in %, instead of having to look it up on a chart.

But so far I haven't seen anything the SmartGauge can do, which I can't do with my simple voltmeter.

[john61ct]

The SG calculates based off a heck of a lot more than just voltage.

[daletournier]

Originally Posted by Dockhead

I notice that our own MaineSail has put up this very interesting article on this subject:

https://marinehowto.com/under-load-b...oltage-vs-soc/

I note with satisfaction how closely this tracks my own chart:

Attachment 145448

I am reinforced in my belief that tracking voltage is better than using an amp-counting battery monitor.

I have a SmartGauge, which in contrast to the amp-counting meters, determines SOC using voltage. It is really convenient to just push a button and see the SOC in %, instead of having to look it up on a chart.

But so far I haven't seen anything the SmartGauge can do, which I can't do with my simple voltmeter.

I'm with you in on this one DH, I tend to look at volt meter when I get up just before light, don't really pay attention to amp counter anymore.

Sent from my vivo Y35 using Cruisers Sailing Forum mobile app

[Dockhead]

Originally Posted by john61ct

The SG calculates based off a heck of a lot more than just voltage.

I've heard that. Whether it's true or not, I can't say.

In any case, the read out of SOC tracks my voltage chart exactly. When the batteries are being discharged, of course. The Smart Gauge attempts to guess at SOC achieved during charging, something you can't do with voltage, but these guesses are very weak.

[conachair]

I'm with mainesail on this one... Far too many variables to get any kind of accurate figure , temperature being a major one. Though living aboard I think most get an instinctive guess pretty quickly of SoC, just don't kid yourself it's anywhere near accurate - but does it need to be anyway as long as the batteries are regularly getting back to full charge...

Also, if there's any solar or wind power going in then the voltage is all over the place,

Measuring A Lead Acid Battery State of Charge Photo Gallery by Compass Marine How To at pbase.com

EDIT 2/13/11: As an update I have since applied many different loads to remove the surface charges or about 6 or seven different batteries, ages and types.. 5 amps, 10 amps 20 amps etc. etc., and all for varying times.

The problem is that none of the surface charge removing loads give me an accurate reading that agrees & matches up with a 24+ hour rest. I suppose you could do some rather lengthy experiments and find the exact load to remove a surface charge, that works for your bank, at a certain state of charge, but this would be complicated at best. The closest I got was within 8-10%. A 10% variance on a 400 Ah bank is 40 Ah's or nearly a days worth of power for us. Having & interpreting my bank, reading the equivalent a whole day off, can be and would be rather frustrating.

TEMPERATURE: I have also experimented at length with resting voltages and temperature. At 95F a battery can come to rest in as little as 12 hours. When cold during the winter this can take 10+ days!! Temperature plays a huge role in when you will achieve a "resting" voltage.

[Dockhead]

Originally Posted by conachair

I'm with mainesail on this one... Far too many variables to get any kind of accurate figure , temperature being a major one. Though living aboard I think most get an instinctive guess pretty quickly of SoC, just don't kid yourself it's anywhere near accurate - but does it need to be anyway as long as the batteries are regularly getting back to full charge...

Also, if there's any solar or wind power going in then the voltage is all over the place,

Measuring A Lead Acid Battery State of Charge Photo Gallery by Compass Marine How To at pbase.com

Yes, MaineSail mentions surface charge in the article I linked. Obviously you need to have any surface charge off, before you read anything into your battery voltage, but I think most of us have a good feel for this.

And obviously, solar or wind going in will throw off everything. However, if your battery voltage is say 12.5 volts even with solar going in, is this really a problem? I don't think you can damage your batteries if the voltage is like that. Then you will know where you are when the solar comes off.

It seems to me that if you keep battery voltage above 12.1 no matter what, you should be ok. You just don't have access to anything like precise SOC unless there is no charging going on, and the surface charge is off. The same conditions which are necessary for the SmartGauge to give an accurate reading.

[conachair]

Originally Posted by Dockhead

Yes, MaineSail mentions surface charge in the article I linked. Obviously you need to have any surface charge off, before you read anything into your battery voltage, but I think most of us have a good feel for this.

And obviously, solar or wind going in will throw off everything. However, if your battery voltage is say 12.5 volts even with solar going in, is this really a problem? I don't think you can damage your batteries if the voltage is like that. Then you will know where you are when the solar comes off.

It seems to me that if you keep battery voltage above 12.1 no matter what, you should be ok. You just don't have access to anything like precise SOC unless there is no charging going on, and the surface charge is off. The same conditions which are necessary for the SmartGauge to give an accurate reading.

Think we're sort of in agreement, the bit I don't think is possible is getting any kind real accuracy of SoC using voltage alone on a real boat, though IMHO it's not really that vital. Or possible with amp counters without a true capacity test (can any of us apart from mainesail do that without a temperature controller environment and some lovely electrical test equipment?) . Smartgauge does seem to be accurate in discharge but with no indication of actual capacity so limited use on its' own.

Saying that -how much does it matter as long as you have a good feel for it? - a nice big ebay panel voltmeter easily visible around the cabin is an extremely useful piece of battery kit IMHO . I get twitchy under 12.4v

[Dockhead]

Originally Posted by conachair

Think we're sort of in agreement, the bit I don't think is possible is getting any kind real accuracy of SoC using voltage alone on a real boat, though IMHO it's not really that vital. Or possible with amp counters without a true capacity test (can any of us apart from mainesail do that without a temperature controller environment and some lovely electrical test equipment?) . Smartgauge does seem to be accurate in discharge but with no indication of actual capacity so limited use on its' own.

Saying that -how much does it matter as long as you have a good feel for it? - a nice big ebay panel voltmeter easily visible around the cabin is an extremely useful piece of battery kit IMHO . I get twitchy under 12.4v

Indeed.

I think I've posted this before, but here is my logic about simple voltage readings for SOC:


1. Based on my own observations, including SG readings, comparison to different battery monitors (including SmartGauge), and MaineSail's tests -- resting voltage vs. SOC charts are quite accurate to show the real SOC in the absence of surface charge and any charging.

2. Resting voltage is ALWAYS higher than loaded voltage. Therefore, you CANNOT overestimate SOC by using the resting voltage chart. Any error is harmless. For larger banks and smaller loads, the difference anyway is very small.

3. You are absolutely correct -- you do NOT need to know SOC precisely anyway. What you do want to know is WHEN TO CHARGE. You simply can't go wrong, if you make it a firm rule to charge no later than when you you see 12.1v (just what MaineSail advises). On the condition, of course, that there is no surface charge and no charging going on.


Even WHEN TO CHARGE you don't actually need to know so precisely -- because cycle life doesn't fall off a cliff before 50% DOD. My batteries according to the Trojan chart still get half the cycles if you discharge them 80%, which is almost worth it, since you cycle fewer times if you discharge more deeply. So if you occasionally discharge 60% or 70% and avoid some cycles as a result, you may even come out ahead in terms of battery life.



THEREFORE, on the basis of all of these things, I believe that a simple voltmeter is really all you need on a cruising boat. It will give you all the information you really need to manage your batteries, even considering the issues with surface charge and solar or wind charging going on.

[conachair]

Originally Posted by Dockhead

THEREFORE, on the basis of all of these things, I believe that a simple voltmeter is really all you need on a cruising boat. It will give you all the information you really need to manage your batteries, even considering the issues with surface charge and solar or wind charging going on.

On a 1 to 5 .....

5 - very much disagree..

Voltage on its' own is limited without an ammeter to show what's actually going in or out, know both and yes, with a bit of experience that gives plenty info to keep a happy battery bank



Slight drift, but I've just started playing around with a little setup using a esp8266 board and ADS1115 which will hopefully record voltage & broadcast it as a web page, hopefully amps as well.

Data is lovely!

[Dockhead]

Originally Posted by conachair

On a 1 to 5 .....

5 - very much disagree..

Voltage on its' own is limited without an ammeter to show what's actually going in or out, know both and yes, with a bit of experience that gives plenty info to keep a happy battery bank



Slight drift, but I've just started playing around with a little setup using a esp8266 board and ADS1115 which will hopefully record voltage & broadcast it as a web page, hopefully amps as well.

Data is lovely!

I certainly agree that amps are useful to know, and I share your love of knowing more vs knowing less ("data is lovely!" ).

But if your goal is knowing WHEN to charge, why do you need amps? Other than to anticipate how soon you might need to charge?

[IdoraKeeper]

Voltage is great as long as the batteries are healthy. As they age voltage will mask the presence of deteriorating cells, depending on when you check during a discharge cycle. Not having a smart monitor, I can only infer battery health by judging the rate of voltage decay under what is a normal load on my system. My starting battery is used only for cranking. Decay there is masked by surface charge resulting in a non start sometimes. It's a problem.

[conachair]

Originally Posted by Dockhead

I certainly agree that amps are useful to know, and I share your love of knowing more vs knowing less ("data is lovely!" ).

Knowing more just shows you how much of what you think you know is wrong,,

But if your goal is knowing WHEN to charge, why do you need amps? Other than to anticipate how soon you might need to charge?

If that's all then maybe not, but if the goal is having happy batts then knowing amps in/out is , imho, fairly essential. Like when suns up but voltage is a tiny bit less than you'd expect.. check what's coming in then go out and clean that little layer of dust off the panels and repoint towards the sun a bit.. Or too many lights left on?, fridge wiring is getting corroded so taking more juice than it should?... etc etc.

[Dockhead]

Originally Posted by IdoraKeeper

Voltage is great as long as the batteries are healthy. As they age voltage will mask the presence of deteriorating cells, depending on when you check during a discharge cycle. Not having a smart monitor, I can only infer battery health by judging the rate of voltage decay under what is a normal load on my system. My starting battery is used only for cranking. Decay there is masked by surface charge resulting in a non start sometimes. It's a problem.

Yes, this is true and it is worth saying that this method is useless for start batteries.

For start batteries, you should use an impedence test or just replace as soon as you feel the battery start to struggle at all. Start batteries on my boat last seemingly forever (10+ years) because I have no loads whatsoever on the start battery except for the starter and engine management system. I have dual alternators and my engine start bank and system is totally separate from the house bank and system with no interconnection at all (they are even different voltages).


For deep cycle batteries in house bank service, though, this is not a problem. The true state of charge becomes evident as soon as the surface charge is off. A big voltage decline quickly when surface charge comes off is a very good way to recognize shot batteries.

[Dockhead]

Originally Posted by conachair

Knowing more just shows you how much of what you think you know is wrong,,




If that's all then maybe not, but if the goal is having happy batts then knowing amps in/out is , imho, fairly essential. Like when suns up but voltage is a tiny bit less than you'd expect.. check what's coming in then go out and clean that little layer of dust off the panels and repoint towards the sun a bit.. Or too many lights left on?, fridge wiring is getting corroded so taking more juice than it should?... etc etc.

I completely agree with all of this, anyway for a boat with solar.

The first sentence is especially true

[conachair]

Originally Posted by Dockhead

The first sentence is especially true

Best mantra IMHO is "I'm certain to be wrong about everything, it's just how wrong and does it matter?"

The earth is almost but not quite flat..