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OtleyLad

A couple of questions about batteries...

Seeking advice here - be gentle.

1. Lots of wiring diagrams for solar charging setups show the -ve terminal of the battery being earthed.
What is the effect of this and what happens if you don't earth it?

2. I'm looking for a meter to show me the state of my batteries in my solar setup. I'm determined to look after the batteries and would like somehting to show me the %charge/discharge, voltage, etc.
I've looked on fleabay and seen this at 10.89 here any good do you think?


Any advice will be much appreciated.no smilies
dpack

i cant help with those q's but i discovered that boat yards that deal with commercial fishing rather than pleasure boats are a good source of large gel batteries.iirc the caravan chandlers are also worth comparing in case they have special offers.

the type intended for constant load ie not for a starter motor for a few seconds are best for low voltage power systemsno smilies
Falstaff

The point of connecting one of the terminals to earth is to tether the voltage. If you connect the +ve to earth, then the battery operates from 0v to -12v and if you connect the -ve to earth the battery operates from +12v to 0v. If neither is connected, then there is a voltage difference of 12v, but it's value is not identified and can "float about"

I remember once I was trying to understand the operation of a chip and struggled for a couple of hours when the lecturer said "Would it help if I told you pin 2 was a virtual earth" ? That gave the whole circuit a reference point and the operation became obvious.

With simple dc stuff I don't think it would be critical to earth the system, but if you start interfacing with a domestic supply which IS earthed, it May become so.

More a matter of Good Practice I should say - but something which a Pro, looking at your circuitry would pick up on as looking "professional" and "when in Rome" eh ?

Don't know much about batteries and metering them I'm afraid - they either work or they don't as far as I'm concerned !no smilies
RichardW

I am sorry but that meter is crap.

Counting ah in & ah out is a sure fire way of killing your bats. It goes out of sync & needs constant re calibration if that is even possible.


The only meter that I know of the the Merlin SmartGage (not cheap but it does work & has other functions like alarms & ability to switch things relative to voltage or SOC). It can give you a % reading of the SOC of the batter bank capacity as it stands at that time. IE you start with a 200ah battery bank but after a year it has lost 20% so is only 160ah. The SG will tell you the SOC relative to its real capacity. The AH counters will tell you the balance relative to the size you tell it the bank is. Also they do not allow for the Perket factor or the fact that you cant use 1ah & then just replace it with 1ah. It might take up to 1.5ah to replace it depending on the capacity of the bank, its acid strength, speed of charge / discharge ect.no smilies
OtleyLad

I am sorry but that meter is crap.

Counting ah in & ah out is a sure fire way of killing your bats. It goes out of sync & needs constant re calibration if that is even possible.


The only meter that I know of the the Merlin SmartGage (not cheap but it does work & has other functions like alarms & ability to switch things relative to voltage or SOC). It can give you a % reading of the SOC of the batter bank capacity as it stands at that time. IE you start with a 200ah battery bank but after a year it has lost 20% so is only 160ah. The SG will tell you the SOC relative to its real capacity. The AH counters will tell you the balance relative to the size you tell it the bank is. Also they do not allow for the Perket factor or the fact that you cant use 1ah & then just replace it with 1ah. It might take up to 1.5ah to replace it depending on the capacity of the bank, its acid strength, speed of charge / discharge ect.


I did wonder about it - thanks for that info.

If I monitor the voltage would that tell me anything? Does the voltage fall off in a linear fashion as the battery discharges or does it suddenly drop off when its too late?no smilies
OtleyLad

The point of connecting one of the terminals to earth is to tether the voltage. If you connect the +ve to earth, then the battery operates from 0v to -12v and if you connect the -ve to earth the battery operates from +12v to 0v. If neither is connected, then there is a voltage difference of 12v, but it's value is not identified and can "float about"

I remember once I was trying to understand the operation of a chip and struggled for a couple of hours when the lecturer said "Would it help if I told you pin 2 was a virtual earth" ? That gave the whole circuit a reference point and the operation became obvious.

With simple dc stuff I don't think it would be critical to earth the system, but if you start interfacing with a domestic supply which IS earthed, it May become so.

More a matter of Good Practice I should say - but something which a Pro, looking at your circuitry would pick up on as looking "professional" and "when in Rome" eh ?

Don't know much about batteries and metering them I'm afraid - they either work or they don't as far as I'm concerned !


My other 1kw system is earthed - but both seem to chug along quite nicely whether they are earthed or not.no smilies
Falstaff

The point of connecting one of the terminals to earth is to tether the voltage. If you connect the +ve to earth, then the battery operates from 0v to -12v and if you connect the -ve to earth the battery operates from +12v to 0v. If neither is connected, then there is a voltage difference of 12v, but it's value is not identified and can "float about"

I remember once I was trying to understand the operation of a chip and struggled for a couple of hours when the lecturer said "Would it help if I told you pin 2 was a virtual earth" ? That gave the whole circuit a reference point and the operation became obvious.

With simple dc stuff I don't think it would be critical to earth the system, but if you start interfacing with a domestic supply which IS earthed, it May become so.

More a matter of Good Practice I should say - but something which a Pro, looking at your circuitry would pick up on as looking "professional" and "when in Rome" eh ?

Don't know much about batteries and metering them I'm afraid - they either work or they don't as far as I'm concerned !

My other 1kw system is earthed - but both seem to chug along quite nicely whether they are earthed or not.

A simple "thank you" would suffice 8)no smilies
RichardW

The issue with voltage monitoring is load / charging.

You cant take a valid reading whilst doing either of the above. You also should not take a reading till the battery has not been charged or discharged for 24 hours. Obviously thats not possible in a system that is in use. The best you can hope for is about an hour of limited charge or discharge.

A duff bat with a PV panel connected can have a health voltage. A good bat with a load on it can show a dead voltage (dep on load / bat size relationship).

What would be a light load on my nearly 4000Ah bank would kill a smaller bank.

Also each battery type has a slightly differing voltage range relative to its SOC.


Your big problems will be

1, a simple system that any one can use
2, cheap enough to be sell able

Whilst the SG fulfills 1 its only viable on bigger systems due to cost.

A simple volt meter needs understanding of loads / charging capacity ect for it to be remotely usable.

Re the earthing, a lot of systems will be fitted in vehicles / boats & so not earthing can lead to galvanic corrosion. Also if fitting an inverter then it reduces the possibility of shocks.no smilies
Hairyloon

I am sorry but that meter is crap.

Counting ah in & ah out is a sure fire way of killing your bats.
No it isn't, counting can't do anything.
Relying only on that count may well kill your batteries.
Quote:
It goes out of sync & needs constant re calibration if that is even possible.

Sorry, but what goes out of sync with what?

What actually is it that you want it to do?no smilies
OtleyLad

The issue with voltage monitoring is load / charging.

You cant take a valid reading whilst doing either of the above. You also should not take a reading till the battery has not been charged or discharged for 24 hours. Obviously thats not possible in a system that is in use. The best you can hope for is about an hour of limited charge or discharge.

A duff bat with a PV panel connected can have a health voltage. A good bat with a load on it can show a dead voltage (dep on load / bat size relationship).

What would be a light load on my nearly 4000Ah bank would kill a smaller bank.

Also each battery type has a slightly differing voltage range relative to its SOC.


Your big problems will be

1, a simple system that any one can use
2, cheap enough to be sell able

Whilst the SG fulfills 1 its only viable on bigger systems due to cost.

A simple volt meter needs understanding of loads / charging capacity ect for it to be remotely usable.

Re the earthing, a lot of systems will be fitted in vehicles / boats & so not earthing can lead to galvanic corrosion. Also if fitting an inverter then it reduces the possibility of shocks.

Ok, so if I was to measure the voltage after a couple of hours with the PV and load switched off would the voltage give me an approximate indication as to its SOC?no smilies
RichardW



No it isn't, counting can't do anything.
Relying only on that count may well kill your batteries.
Quote:
It goes out of sync & needs constant re calibration if that is even possible.

Sorry, but what goes out of sync with what?



With comments like that HG you obviously do not know what you are talking about.no smilies
RichardW



Ok, so if I was to measure the voltage after a couple of hours with the PV and load switched off would the voltage give me an approximate indication as to its SOC?

Yes, very roughly. So a reading taken say at 5 or 6am would be the best you can get.

If you count 12.7 to 12.8v at rest as fully charged then for standard Floaded Lead Acid bats of the deep cycle type (forklifts ect) then 12.2v would be considered 50% SOC.

That is handy as 50% SOC is also considered to be the sweet spot between battery life/charge cycles cost of recharge & cost of batter replacement. Keep higher than 50% & life goes up but so does recharge costs (IE from a genny) go lower than 50% & battery life reduces & replacement costs go up.

I can find you a chart but it will have to be later & I am just off out.no smilies
Hairyloon



No it isn't, counting can't do anything.
Relying only on that count may well kill your batteries.
Quote:
It goes out of sync & needs constant re calibration if that is even possible.

Sorry, but what goes out of sync with what?



With comments like that HG you obviously do not know what you are talking about.
That is why I am asking. :roll:no smilies
RichardW

The reading & reality.

Its no good the meter saying you have 50% left when:-

a, its telling you 50% of what the original capacity was (or what you have told it it was), not what is actually still available
b, the 50% wont be right anyway as the Ah counting drifts for the reasons I have already mentioned in previous posts
c, as the reading gets further away from reality it starts to encourage you to over use the bank so killing it quickerno smilies
Hairyloon

The reading & reality.

Its no good the meter saying you have 50% left when:-

a, its telling you 50% of what the original capacity was (or what you have told it it was), not what is actually still available
b, the 50% wont be right anyway as the Ah counting drifts for the reasons I have already mentioned in previous posts
c, as the reading gets further away from reality it starts to encourage you to over use the bank so killing it quicker
But is it not useful information as long as it is not the only information?
If the meter says you should have 50% and you only actually have 30%, does that not give an indication of how badly your batteries are deteriorating?no smilies
RichardW


But is it not useful information as long as it is not the only information?
If the meter says you should have 50% and you only actually have 30%, does that not give an indication of how badly your batteries are deteriorating?


If you think bad, wrong or misleading information is useful then yes.


How will you know you only have 30%?

I am guessing that OL wants a system that anyone can use & understand without needing to know any thing about Volts, Amps, Watts, Ah, Wh or Va.no smilies
OtleyLad

I've been doing a little searching and found this page.

It has some good explanations that even I can understand and this table:

Voltage SOC
12.57 100%
12.36 80%
12.15 60%
11.94 40%
11.73 20%

This for a battery 'at rest'. Luckily I've got a pretty accurate voltage reader I can use.no smilies
Falstaff

yes OL I saw that one too

8)

However I am struggling with Richard's opinion on the earthing situation.




.........Re the earthing, a lot of systems will be fitted in vehicles / boats & so not earthing can lead to galvanic corrosion. Also if fitting an inverter then it reduces the possibility of shocks.


Please explain if you'd be so kind Richard :-

a) HOW does "not earthing" lead to galvanic corrosion ?

b) Since an inverter is basically a chopper circuit running a 12v transformer backwards, the input side is therefore completely isolated from the output side. How on any level can earthing the Negative side of the battery have any effect whatever on the likelihood of a shock from the output circuit ?no smilies
Hairyloon


But is it not useful information as long as it is not the only information?
If the meter says you should have 50% and you only actually have 30%, does that not give an indication of how badly your batteries are deteriorating?


If you think bad, wrong or misleading information is useful then yes.
I had understood that the meter had been discussing logs Ah in and Ah out. I had not realised it did this badly. Perhaps you should say what you mean. :roll:

Quote:
How will you know you only have 30%?

How do you know when you've got 30%?
Quote:
I am guessing that OL wants a system that anyone can use & understand without needing to know any thing about Volts, Amps, Watts, Ah, Wh or Va.

Exactly so. Is that asking a lot?
It doesn't seem so to me, but I don't claim to know too much about batteries.no smilies
RichardW

I've been doing a little searching and found this page.

It has some good explanations that even I can understand and this table:

Voltage SOC
12.57 100%
12.36 80%
12.15 60%
11.94 40%
11.73 20%

This for a battery 'at rest'. Luckily I've got a pretty accurate voltage reader I can use.

Those voltages look a little out to me. You have to make sure that the chart you use matches the battery type & spec. Even similar bats can have different strength acid & that affects the voltage re SOC.

Also remember that the accuracy of the meter you use must be really good to 2 decimal places & have a small +- variation.no smilies
RichardW



However I am struggling with Richard's opinion on the earthing situation.


Please explain if you'd be so kind Richard :-

a) HOW does "not earthing" lead to galvanic corrosion ?

b) Since an inverter is basically a chopper circuit running a 12v transformer backwards, the input side is therefore completely isolated from the output side. How on any level can earthing the Negative side of the battery have any effect whatever on the likelihood of a shock from the output circuit ?

a, All circuits leak. If you dont have a fixed reference point (IE earth) then stray currents will result. Prob not much of an issue on land but boats and cars are seriously affected.

b, In most cases things are safe till a fault develops. If you get a fault then having the bat neg earthed will give a path for the current. Its similar to having the N & E in a mains system bonded so that the RCD can work properly.

Nearly all inverter makers recommend N&E bonding & bat neg to earth connection. So in effect the mains N & E & bat neg are all at 0v potential & connected together. Makes no difference to everyday use but in a fault situation it should be safer. Again this is all old hat for boaters.no smilies
RichardW


1, I had not realised it did this badly. Perhaps you should say what you mean. :roll:

2, How do you know when you've got 30%?

3, Exactly so. Is that asking a lot?

4, It doesn't seem so to me, but I don't claim to know too much about batteries.

1, If you had read all my posts you would have seen that I said Ah counting does not work well. :roll: :roll:

2, My meter accurately reads %SOC but then for me its worth spending more on the meter to protect a very expensive battery bank.

3, Yes it is. To do it well costs lots & lots.

4, :roll: :roll:no smilies
RichardW

Just for clarity.

The best way to know the state of your FLA's is the measure the specific gravity of the electrolyte using a known accurate tester & then to adjust for temp using an accurate temp meter. Oh & you will also need a full face mask, rubber long arm gloves & a rubber bib type apron. You also have to make sure that the cells have not stratified so a good hard charge to get the bubble to move the fluid about or use a de stratifying pump.no smilies
Hairyloon


1, I had not realised it did this badly. Perhaps you should say what you mean. :roll:

2, How do you know when you've got 30%?

3, Exactly so. Is that asking a lot?

1, If you had read all my posts you would have seen that I said Ah counting does not work well. :roll: :roll:
That is not the same as saying the data is bad, or indeed saying the data is useless. It is just saying that it doesn't do what you want it to do
Quote:
2, My meter accurately reads %SOC...

How? What is it actually measuring?
Quote:
3, Yes it is. To do it well costs lots & lots.

Why?
Chips that can accurately measure voltages, currents, etc things are cheap as, well, chips.
What is the difficult bit?no smilies
Falstaff

The point of connecting one of the terminals to earth is to tether the voltage. If you connect the +ve to earth, then the battery operates from 0v to -12v and if you connect the -ve to earth the battery operates from +12v to 0v. If neither is connected, then there is a voltage difference of 12v, but it's value is not identified and can "float about"


..........With simple dc stuff I don't think it would be critical to earth the system, but if you start interfacing with a domestic supply which IS earthed, it May become so.

More a matter of Good Practice I should say - but something which a Pro, looking at your circuitry would pick up on as looking "professional" and "when in Rome" eh ?



As you can see Richard, I had picked up Both of the points you made in your most recent post, in my original one, before you wrote :-


Re the earthing, a lot of systems will be fitted in vehicles / boats & so not earthing can lead to galvanic corrosion. Also if fitting an inverter then it reduces the possibility of shocks.

Which is entirely different and you are only now trying to move to a position which has some logic and which is trying to align with my original one !

a) However galvanic corrosion is a phenomenon which is entirely natural and unrelated to whether a boat contains any electrical equipment or not ! - There is now SOME concern that the use by some, of the hull as an "earth" actually exacerbates the process as you will see if you look up what galvanic corrosion actually IS !

b) You do at least seem now to understand that the "earthing" of the battery -ve does NOT in any way "Protect against shocks" as you originally thought and I'd be fascinated to hear your explanation as to how precisely coupling the neutral of the 230 V ac invertor output to the -ve terminal of the 12v dc battery in a boat as you suggest, is anything other than a potentially dangerous proposal !no smilies
RichardW



a) However galvanic corrosion is a phenomenon which is entirely natural and unrelated to whether a boat contains any electrical equipment or not ! - There is now SOME concern that the use by some, of the hull as an "earth" actually exacerbates the process as you will see if you look up what galvanic corrosion actually IS !

b) You do at least seem now to understand that the "earthing" of the battery -ve does NOT in any way "Protect against shocks" as you originally thought and I'd be fascinated to hear your explanation as to how precisely coupling the neutral of the 230 V ac invertor output to the -ve terminal of the 12v dc battery in a boat as you suggest, is anything other than a potentially dangerous proposal !

a, I am not suggesting that the boat hull is used as the -ve return path as it used to be on cars (I think now most use wires & not the chassis for the -ve return). That on a boat is bad news. Galvanic corrosion is due to the small current between dissimilar metals. That can either be a natural process or accelerated due to stray electrical paths. The accelerated version is also known as stray current corrosion but most boaters still call it galvanic corrosion, its just faster.

b, Bonding the N & E on the inverter is standard procedure where an RCD is fitted (this mirrors a standard domestic mains install where the bond is provided by the supply). All top brands (Victron, Mastervolt, Outback,ect) recommend it or come with it fitted internally as standard. My own one can have it switched in or out via software & can have it set one way when inverting & another when using shore power or genny inputs. Bonding the E of the inverter to the hull is standard procedure. Bonding the bat -ve is standard procedure.no smilies
RichardW


How? What is it actually measuring?
Why?
Chips that can accurately measure voltages, currents, etc things are cheap as, well, chips.
What is the difficult bit?

It measures what ever the designer designed it to measure. Why not ask them & see how far you get?

Why? Well I guess R&D cost lots of money & just cos the chip is cheap the data it contains was costly to produce.

Getting reliable data from a battery in use is difficult. How can the meter know if the battery is discharging, resting or charging when it takes a reading?

A single reading without reference is of no use.no smilies
Hairyloon

Getting reliable data from a battery in use is difficult. How can the meter know if the battery is discharging, resting or charging when it takes a reading?
Measure the current at the same time?
If you know the load and the voltage, can you not calculate the state of charge?no smilies
RichardW

Getting reliable data from a battery in use is difficult. How can the meter know if the battery is discharging, resting or charging when it takes a reading?
Measure the current at the same time?
If you know the load and the voltage, can you not calculate the state of charge?

Measuring current can be helpful. However knowing the current flow will not help find SOC. Like I mentioned earlier a 50 amp current on my bank would be quite small in comparison to its capacity so it will hold the voltage. If you load up a small bank at the same current the voltage could collapse. It can help you find the state of health of the bat if you also know the bat size & how long the load has been applied & the starting & ending SOC & voltages.

Whilst it seems like an easy thing to do to get a SOC reading in practice its much much harder.

Its not like a fuel gauge for a tank of fuel. If you take 6L out of a full 10L tank & put 3L back you know you have 7L left. With FLA's the speed you take that fuel out is also important as is the temp & how quick you put the fuel back in.no smilies
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