Battery discharge test

[wpowokal]

I decided it was about time that I conducted a discharge test on one of my battery banks, it's at least 2 years since the last. Now my batteries are not new, the electrical dept. estimated they were 13 years old when I stole them in 2001.

They are 650 A/h  Exide Fanur'e X , the case is clear plastic and is fretting on the bottom of a number of batteries, this is a pain and has created a cesspool of acid in my battery room. I repair each wayward battery with auto bog, seems to work fine, then refill it with new acid.

In preparation for this test I charged up all batteries and floated them for 5 hours @30V (24V system) I then isolated the bank to be tested. The test started early morning Sunday last, so early that I had to use a light to take readings (it's mid winter here minus the rain).

So taking voltage readings on each 2V cell then specific gravity (SG) I discovered that #1 battery had, apparently, only water in it???  Its voltage was 2.053, slightly under the others, so bugger what to do. I moved the cables to the adjacent bank, which had not been rested over night, but having made up my mind to conduct this test, having obtained the Sunday paper carton of Moselle and cheese platter it had to proceed.

So a new set of pre test readings was taken on the alternate bank, bugger one cell had lower volts and SG than all others, obviously a week link.  So just hit the cask or press on regardless, with the placement of the fuse the test began.

I had a dump load resistor unit good for 40 A @ 24v and made up the remaining 25 amps with a resistor bank and jumper lead, sort of a variable resistor one could say.

Readings were taken every 15 min. of cell volts and SG, the test lasted 7.5 hours and was terminated when the weak ling reached 1.75v, the industry standard cut off point. At that point I had extracted 475 A/h, which simplistically equates to 75% capacity. However if one discounts the weak link and looks at the rate of voltage decay for the next lowest cell the test could reasonably have continued for another 30 minuets. Which would have equaled 80% capacity, however industry rating is at 25 deg C, my batteries were at 16 deg C (taken half way through the test), now applying the rate of 1% less capacity per deg C that's another 9%, if I keep going I will have 120% capacity batteries.

Now industry standard recommends replacing batteries @ 80% efficiency, however I have 96 of these suckers, in 8x 24 V banks so none of them are exactly overworked. So apart from the ongoing task of repairing casings I reasonably expect to get at least another two years from them.

The results in PDF format is quite a large file (780k),

http://www.otherpower.com/images/scimages/74/battery_test_results.pdf

On another point this may interest some new commers designing their own system. It relates to charge efficiency near top end of battery voltage, people like me miss out on about half the incomming amps at the higher volts if all batteries are in service.

http://www.sandia.gov/pv/docs/PDF/batpapsteve.pdf

[SamoaPower]

WOW! A 5200 Ah (rated) battery bank. Impressive. What do you have for charge sources? By my reckoning, you would need about 300 amps to maintain them well.

Your test is equally interesting. It certainly shows that good batteries, well maintained, can have an extended service life. I'm wondering why you chose to use a 10 hour discharge rate? I thought a 20 hour rate was the norm. Your results would have been even better at the 20 hour rate. For even greater accuracy, you could adjust your "variable" resistor throughout the test to maintain a near constant current. Also, the discharge current should be based on the estimated actual capacity, not the rated capacity.

"Now industry standard recommends replacing batteries @ 80% efficiency ..."

What nonsense! The industry just wants to sell you more batteries. I see no reason not to use them down to the 50% point. I think you're being pessimistic about an additional two years. I bet you can do better than that if you can keep the cracks under control.

A suggestion. If your total capacity is much greater than your actual need to match the rest of your system, split your banks into active and standby units. Float the standby unit at about 26.2 volts to compensate for self discharge and don't cycle them. They will last almost indefinately like this. Continue to use the active bank normally and when they drop to 50% capacity, replace them with the standby unit. I wouldn't be surprised if you got another 10 years out of the total. It also has the benefit of reducing your charge current requirement.

Lastly, thank you so much for that last link. I was aware of the effect but not the degree. A real eye-opener. It speaks even more highly to using load scheduling to improve on the energy harvest. Again, thank you.

[willib]

you mentioned that your in winter .

your midwinter eary morning temp was about 60 F .

i'm guessing by the sand you are on a beach in Fiji

[scottsAI]

Hello wpowokal,

Only one cell plate wares out over the life.

Most deep cycle batteries, both plates are the same.

Fully charge the battery.

Fully discharge the battery.

Reverse charge the battery, and keep it this way.

Claims of near 2x life have been reported.

(can't be true, how do you know when to reverse? 80%)

You hear of doing this?

Have fun,

Scott.

[Flux]

I have seen starter batteries reversed and still work, no idea how good they were, engine still started.

I think Allan's batteries have Plante positives and most likely Faure ( pasted) negatives, I wouldn't consider trying to reverse them.

The earlier versions of these often went on working until the positive plates expanded to the extent that they split the glass jar. You should have a few more years yet. Not sure if your leaking jars is the same problem or a physical problem with the plastic.

The odd cell will die before the rest. Some batches failed through breaks in the positive link bar, you most likely haven't got them as they failed at about 10 years and when they failed you could feel the positive terminal post moving about.

I agree about the low battery efficiency, having too much battery capacity is a major problem especially with wet antimony batteries. Unless you have some back up way of charging them to gassing point at intervals they get progressively discharged. If you include the energy required to keep them in condition then the normally quoted 80% efficiency is way too high.

Flux

[wpowokal]

No south west Australia actually, we do get temps down to 2-3 DegC here sometimes but...

regards allan

[wpowokal]

Hi SomoaPower,

I have around 1.2Kw of sola and 3 wind turbines to keep these suckers on track.

I choose the 10 hour rate because that is this battery's rating, it does at least give me an indication of their remaining capacity.

Industry standard 80% is for industrial use, sorry did not mean to misslead, just my background showing through. I do tend to agree with you, I use 2 years remaining quite loosley.

The reason I generally run with all banks I/S is I am lazy and have no dump load set up, so they can absorb anything throwen at them.

At this time of year I can run with just two banks in service, which does asist the recharge rate.

[wpowokal]

Hi Flux,

         I suspect the casing fretting problems are purley physical, although I suspect there may be a abuse factor.

The first ones to leak were all in one bank, now each pallet held 12 batteries. So as collected and off loaded into the battery room this tends to indicate damage on the pallet. Each of these suckers weigh 60Kg, so it is easy to damage the casing.

Positive terminals are breaking off due to spongy expansion, this was the reason they were retired from their previous life. But I can live with one terminal.

My batteries get gassed more than often enough, so all are well maintained in that respect.

regards Allan

[wpowokal]

As an adendum to this post, for anyone thinking of reparing battery casings as I do.

It is necessary to make the bog at least 6mm thick and preferably continue it up the side a little.

My batteries weigh 60Kg each so it is easy to see that if I did not sand the bottom smooth it would place stress on the casing/bog.

regards allan