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How to tell the state of batteries (in a PV storage system)?

 
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OtleyLad



Joined: 13 Jan 2007
Posts: 2737
Location: Otley, West Yorkshire
PostPosted: Wed Aug 28, 13 10:19 am    Post subject: How to tell the state of batteries (in a PV storage system)? Reply with quote
    

I've been thinking of adding batteries to my existing system and wanted to know the state of the ones I already have.
I have a multifunction test meter - is there someway of testing the battery with it?

Hairyloon



Joined: 20 Nov 2008
Posts: 15425
Location: Today I are mostly being in Yorkshire.
PostPosted: Wed Aug 28, 13 10:28 am    Post subject: Reply with quote
    

I don't know, but I was reading recently that it is possible (not necessarily easy) to take batteries apart and salvage the good cells.

Of course, in the old days, batteries were made as individual cells that were intended to be easily reconditioned.
I'm surprised that nobody has started making those again.

vegplot



Joined: 19 Apr 2007
Posts: 21301
Location: Bethesda, Gwynedd
PostPosted: Wed Aug 28, 13 10:29 am    Post subject: Reply with quote
    

You can measure the open circuit voltage but that doesn't tell you much. A better way is to test the battery under a specified load for a specified period and measure the voltage at that point. The battery manufacturer should be able to provide details of the test and the expected results.

Bing/Google are good...

https://www.diy-nos.freeserve.co.uk/testing-battery.htm

vegplot



Joined: 19 Apr 2007
Posts: 21301
Location: Bethesda, Gwynedd
PostPosted: Wed Aug 28, 13 10:30 am    Post subject: Reply with quote
    

Hairyloon wrote:
I don't know, but I was reading recently that it is possible (not necessarily easy) to take batteries apart and salvage the good cells.

Of course, in the old days, batteries were made as individual cells that were intended to be easily reconditioned.
I'm surprised that nobody has started making those again.


They do. Long-life deep cycle industrial batteries are produced in 2v cells.

Hairyloon



Joined: 20 Nov 2008
Posts: 15425
Location: Today I are mostly being in Yorkshire.
PostPosted: Wed Aug 28, 13 10:33 am    Post subject: Reply with quote
    

vegplot wrote:
Hairyloon wrote:
I don't know, but I was reading recently that it is possible (not necessarily easy) to take batteries apart and salvage the good cells.

Of course, in the old days, batteries were made as individual cells that were intended to be easily reconditioned.
I'm surprised that nobody has started making those again.


They do. Long-life deep cycle industrial batteries are produced in 2v cells.

Are they designed to be easily reconditioned?

RichardW



Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales
PostPosted: Wed Aug 28, 13 12:43 pm    Post subject: Reply with quote
    

You can measure the specific gravity. Best after a full long charge cycle. Compare that to its spec. Then do a controlled discharge to about 50% then measure the SG again. You can then calc the true capacity of the bank & compare that to new specs.

As has been mentioned on the Navitron forum your array is well over sized to your battery bank.

IE you have 1000watts peak into just 200ah of battery. Thats 83 amps of charge. Well in excess of the 10% rough rule of thumb (20% is fine too).

I have 1200 watts peak into 4200ah of battery. I am under sized.


Not sure what your load size is but that is the place to start the calculations on system size.

OtleyLad



Joined: 13 Jan 2007
Posts: 2737
Location: Otley, West Yorkshire
PostPosted: Wed Aug 28, 13 1:09 pm    Post subject: Reply with quote
    

RichardW wrote:
You can measure the specific gravity. Best after a full long charge cycle. Compare that to its spec. Then do a controlled discharge to about 50% then measure the SG again. You can then calc the true capacity of the bank & compare that to new specs.

As has been mentioned on the Navitron forum your array is well over sized to your battery bank.

IE you have 1000watts peak into just 200ah of battery. Thats 83 amps of charge. Well in excess of the 10% rough rule of thumb (20% is fine too).

I have 1200 watts peak into 4200ah of battery. I am under sized.


Not sure what your load size is but that is the place to start the calculations on system size.


The batts are sealed so cant do the SG.

RichardW



Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales
PostPosted: Wed Aug 28, 13 1:19 pm    Post subject: Reply with quote
    

In that case then you need to charge them with a 3 stage charger for at least 48 hours with no load.

Then let them sit for 24 hours with no load & no charging.

Then take a voltage reading & compare that to the makers specs.

Then do a controlled discharge with a fixed load for a fixed time.

Wait 24 hours then take a voltage reading & compare to makers specs.

Using the pre & post voltages & specs with the known discharge amount you can say how much capacity they still have.

OtleyLad



Joined: 13 Jan 2007
Posts: 2737
Location: Otley, West Yorkshire
PostPosted: Thu Aug 29, 13 10:06 am    Post subject: Reply with quote
    

RichardW wrote:
In that case then you need to charge them with a 3 stage charger for at least 48 hours with no load.

Then let them sit for 24 hours with no load & no charging.

Then take a voltage reading & compare that to the makers specs.

Then do a controlled discharge with a fixed load for a fixed time.

Wait 24 hours then take a voltage reading & compare to makers specs.

Using the pre & post voltages & specs with the known discharge amount you can say how much capacity they still have.


Thanks for that Richard.

RichardW



Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales
PostPosted: Thu Aug 29, 13 12:50 pm    Post subject: Reply with quote
    

Lol, that was not the response I was expecting.


More on the lines of life is to short or bloody nora thats a faf.


Measuring banks whilst in use is hard. A Smartgauge meter will do it (at a premium cost). How ever it will only tell you the battery SOC % based on its current real size so it might read 100% but that could be 100ah & not the as new value of 220ah.

Again using that and some know load you can calc the actual size based on the discharge curve.


The SG meter is a very handy tool if you are having to use a costly power source like a genny to recharge the bats.

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