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A couple of questions about batteries...
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RichardW



Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales
PostPosted: Sat Jan 31, 15 3:37 pm    Post subject: Reply with quote
    

Hairyloon wrote:

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.

OtleyLad



Joined: 13 Jan 2007
Posts: 2737
Location: Otley, West Yorkshire
PostPosted: Sat Jan 31, 15 3:43 pm    Post subject: Reply with quote
    

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.

Falstaff



Joined: 27 May 2009
Posts: 1014

PostPosted: Sat Jan 31, 15 4:08 pm    Post subject: Reply with quote
    

yes OL I saw that one too



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


RichardW 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.



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 ?

Hairyloon



Joined: 20 Nov 2008
Posts: 15425
Location: Today I are mostly being in Yorkshire.
PostPosted: Sat Jan 31, 15 6:32 pm    Post subject: Reply with quote
    

RichardW wrote:
Hairyloon wrote:

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.

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.

RichardW



Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales
PostPosted: Sat Jan 31, 15 7:44 pm    Post subject: Reply with quote
    

OtleyLad wrote:
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.

RichardW



Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales
PostPosted: Sat Jan 31, 15 7:54 pm    Post subject: Reply with quote
    

Falstaff wrote:


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.

RichardW



Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales
PostPosted: Sat Jan 31, 15 7:58 pm    Post subject: Reply with quote
    

Hairyloon wrote:

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

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.

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,

RichardW



Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales
PostPosted: Sat Jan 31, 15 8:16 pm    Post subject: Reply with quote
    

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.

Hairyloon



Joined: 20 Nov 2008
Posts: 15425
Location: Today I are mostly being in Yorkshire.
PostPosted: Sat Jan 31, 15 8:26 pm    Post subject: Reply with quote
    

RichardW wrote:
Hairyloon wrote:

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

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.

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?

Falstaff



Joined: 27 May 2009
Posts: 1014

PostPosted: Sat Jan 31, 15 9:11 pm    Post subject: Reply with quote
    

Falstaff wrote:
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 :-

RichardW 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 !

RichardW



Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales
PostPosted: Sat Jan 31, 15 10:32 pm    Post subject: Reply with quote
    

Falstaff wrote:


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.

RichardW



Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales
PostPosted: Sat Jan 31, 15 10:48 pm    Post subject: Reply with quote
    

Hairyloon wrote:

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.

Hairyloon



Joined: 20 Nov 2008
Posts: 15425
Location: Today I are mostly being in Yorkshire.
PostPosted: Sun Feb 01, 15 12:54 pm    Post subject: Reply with quote
    

RichardW wrote:
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?

RichardW



Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales
PostPosted: Sun Feb 01, 15 2:38 pm    Post subject: Reply with quote
    

Hairyloon wrote:
RichardW wrote:
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.

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