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A couple of questions about batteries...
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OtleyLad



Joined: 13 Jan 2007
Posts: 2737
Location: Otley, West Yorkshire
PostPosted: Thu Jan 29, 15 11:08 am    Post subject: A couple of questions about batteries...  Reply with quote    

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.

dpack



Joined: 02 Jul 2005
Posts: 34188
Location: yes
PostPosted: Thu Jan 29, 15 12:09 pm    Post subject: Reply with quote    

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 systems

Falstaff



Joined: 27 May 2009
Posts: 1014

PostPosted: Thu Jan 29, 15 7:33 pm    Post subject: Reply with quote    

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 !

RichardW



Joined: 24 Aug 2006
Posts: 8427
Location: Llyn Peninsular North Wales
PostPosted: Thu Jan 29, 15 9:10 pm    Post subject: Reply with quote    

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.

OtleyLad



Joined: 13 Jan 2007
Posts: 2737
Location: Otley, West Yorkshire
PostPosted: Thu Jan 29, 15 9:22 pm    Post subject: Reply with quote    

RichardW wrote:
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?

OtleyLad



Joined: 13 Jan 2007
Posts: 2737
Location: Otley, West Yorkshire
PostPosted: Thu Jan 29, 15 9:25 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"

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.

Falstaff



Joined: 27 May 2009
Posts: 1014

PostPosted: Thu Jan 29, 15 9:41 pm    Post subject: Reply with quote    

OtleyLad wrote:
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"

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

RichardW



Joined: 24 Aug 2006
Posts: 8427
Location: Llyn Peninsular North Wales
PostPosted: Thu Jan 29, 15 9:47 pm    Post subject: Reply with quote    

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.

Hairyloon



Joined: 20 Nov 2008
Posts: 15033
Location: Today I are mostly being in Yorkshire.
PostPosted: Thu Jan 29, 15 10:44 pm    Post subject: Reply with quote    

RichardW wrote:
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?

OtleyLad



Joined: 13 Jan 2007
Posts: 2737
Location: Otley, West Yorkshire
PostPosted: Fri Jan 30, 15 6:34 am    Post subject: Reply with quote    

RichardW wrote:
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?

RichardW



Joined: 24 Aug 2006
Posts: 8427
Location: Llyn Peninsular North Wales
PostPosted: Fri Jan 30, 15 8:58 am    Post subject: Reply with quote    

Hairyloon wrote:


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.

RichardW



Joined: 24 Aug 2006
Posts: 8427
Location: Llyn Peninsular North Wales
PostPosted: Fri Jan 30, 15 9:02 am    Post subject: Reply with quote    

OtleyLad wrote:


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.

Hairyloon



Joined: 20 Nov 2008
Posts: 15033
Location: Today I are mostly being in Yorkshire.
PostPosted: Fri Jan 30, 15 11:44 am    Post subject: Reply with quote    

RichardW wrote:
Hairyloon wrote:


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.

RichardW



Joined: 24 Aug 2006
Posts: 8427
Location: Llyn Peninsular North Wales
PostPosted: Fri Jan 30, 15 2:08 pm    Post subject: Reply with quote    

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

Hairyloon



Joined: 20 Nov 2008
Posts: 15033
Location: Today I are mostly being in Yorkshire.
PostPosted: Sat Jan 31, 15 2:31 pm    Post subject: Reply with quote    

RichardW wrote:
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?

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