Author Topic: The power of half  (Read 2732 times)

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Volvo farmer

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The power of half
« on: January 31, 2012, 10:25:03 PM »
Howdy friends.

Been living with 800Ahr of battery for the last 5.5 years. One of the cells in my Surrette Rolls S530 bank has failed. The manufacturer tells me it's OK to put a new battery in the mix, despite having 2 parallel strings and the difference in resistance between new and old batteries. My system is 24 volt, eight L16 size batteries in two parallel strings. I tend to disagree that a new battery will behave well with these old ones in this configuration.

So I picked the best batteries in terms of SG and made a single series string, supposedly at 400Ahr/20hr rate 5 years ago. Now I am discovering the power of half.  We are typically 100-125 ahr out overnight on a tri-metric, so might actually get 2 days out of this capacity with no solar/wind charging o a good day. 

I know what is coming, as I traded a monthly electric bill for a battery replacement bill years ago. However...

Who has replaced a failing battery bank in an off grid system, and what was your criteria for replacing the thing?  I recently read that some fellow got a forklift battery with some extra cells and changed out the bad cells as they failed with the good ones. This seems to be an excellent use of resources in my book. 

Our lights have not gone out yet, though a water pump, hair dryer, and several lights early in the morning have brought the battery voltage down some.  Do you wait till the inverter shuts off at 21V under heavy load repeatedly before knowing your battery is shot? 

I want more years from my $3500 investment, and even 2 years more means a lot in terms of dollars/year.

Thanks VF
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richhagen

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Re: The power of half
« Reply #1 on: February 01, 2012, 12:14:49 AM »
I have less experience than you in this regard.  I have grid power, so my off-grid systems are less critical.  I found that I loose more power to self discharge steadily as they age and they will hold steadily lower voltages when left standing.  The first batteries to fail were cheap batteries on a small 12V system, and then the first 48V batteries which were 'marine deep cycle' type.  The golf cart type batteries I have still hold a charge but with reduced capacity.  I am getting to know the routine.  The capacity reduces as they age, and generally not linearly, but at an accelerating pace.  You have much better quality batteries, but I suspect the trends will be the same.  I think you will definitely accelerate the demise of the surviving half of the string because of the increased depth of discharge it will now have to endure each cycle. 

When to replace probably depends upon how much failure is tolerable.  If the power only went out every great once in a while, you might milk them along, but the situation will steadily get worse until you have to act to keep the family happy. 

I don't know the conditions or life expectancy of your surviving batteries, but at present you have paid about $636 per year.  If you were to pay $437.5 or a replacement battery and you got even one additional year after the purchase of the new battery from the bank as a whole you would be ahead of the game economically as compared to replacing the whole bank now, provided you could keep the two strings working.  If you got one year out of running it on a half a string or two years more if you replace the one bad battery, you would still be ahead economically.  Mixing old and new is certainly not ideal, but it might be the cheapest option in terms of long term costs given what you have to start with.  Just my 2 cents, Rich
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hydrosun

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Re: The power of half
« Reply #2 on: February 01, 2012, 08:25:23 AM »
. coincindentally, I have the exact same configeration of 8 surrette batteries at 24 volts.   Mine are about 3 years old and I've noticed a reduction of capacity.    I've had to do some longer than normal(6 hours) equalization to get the specific gravity even in all the cells.  In the past I've done like you have and reduced the bank to one  string if a cell went out. Some customers have replaced an L16 with a new golf cart battery as the capacity would be similar to an aging L16. In a  older bank it's Ok to mix and match a bit more as the batteries are not all alike anymore anyway. More equalizaton has to be done to make sure all the batteries get fully charged. Or at least push the bulk voltage up a bit so all the batteries get charged regularly, Just check the water more frequently  as some batteries will be overcharged and use more water. In your case I'd go with a golf cart battery as the lower cost way to get more use out of the remaining L16 bank. Unless you can find a used L16 battery somewhere.
Chris

DanG

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Re: The power of half
« Reply #3 on: February 01, 2012, 09:20:17 AM »
Did you get the 1200 cycles to 50% depth of discharge they advertise?

Does the Surrette-Rolls 2 year replacement, 5 years prorated warranty cover anything in this? Seems like an easy way to anchor you in a perpetual loop with that product.

If you have the Hydro-caps that can be transferred to the same make & line of battery that is one small cost saved by staying with that line.

At 117-pounds each that is $27 each /$215 scrap value (best MN quote). Nothing to brag about but money in your pocket once the new battery delivery van departs without them.

A new set of HuP Solar-One 845AH/20hr batteries will set you back $4 a pound delivered. ($5940 for  2100 Cycles To 80% DOD over 10 Years, cycles to 50% DOD 4000).

Okay, fine, I am a trouble maker, but I >really< am in lust with that line of bettery :)

ChrisOlson

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Re: The power of half
« Reply #4 on: February 01, 2012, 11:15:28 AM »
The manufacturer tells me it's OK to put a new battery in the mix, despite having 2 parallel strings and the difference in resistance between new and old batteries

It's OK to do that to get the rest of the life out of your old batteries as long as you have the realization that you will not the full life from the replacement battery.  Parallel batteries tend to support one another and are quite forgiving of a "lazy" battery in the configuration.  Series strings are very unforgiving of a "lazy" battery in the string.

I think I'd charge them up and equalize them, then load test each one with a set load over time and see how well they hold up their voltage with the test load.  Heavy loads up to 10% of the ah rating of the battery work best to weed out the men from the boys for a load test.  Put the three strongest ones on the string with the new battery, and put the other four on the other string.

VF - maybe you already know this, but using 28.4 for absorb voltage on Surrette batteries will deficit charge them and cause them to die early from sulfation.  29.8 (temperature compensated) is what I was told it takes to properly desulfate and re-mix the electrolyte in Surrette batteries during absorb.  Using the "normal" 28.4 like you would with Trojans et al will require frequent equalization cycles.  During equalization check the SG on the cells and don't stop until the SG stops rising in the cells, in relation to one another.  If I don't equalize for 3-4 months with my Surrette bank it takes 8-9 hours before all the cells get up to spec.

I would guess your battery, assuming it's not open, died because of sulfation.  It wouldn't hurt to do a corrective equalization on it and see if it comes back.  It might take a couple days at 8 volts but at that age quite a few can be revived yet if they just got choked off due to being deficit charged.
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Chris
« Last Edit: February 01, 2012, 12:09:57 PM by ChrisOlson »

dnix71

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Re: The power of half
« Reply #5 on: February 01, 2012, 03:37:11 PM »
I replace my batteries when they fail. If one goes bad in the string it pulls down the resting voltage enough to notice. There is no way to make batteries last long enough to make it pay to go off grid. A decent size 34 marine hybrid deep cycle is $100 with trade. I had a couple go bad last year, but they were an off brand, I don't by Big Name stuff, can't afford it. I ruined 2 adding tetra sodium edta this year, but it also seemed to boost the capacity of the remaining, so I'm running off 7 instead of 8 in parallel and rarely go below 12.5 volts unless we get overcast and no wind for 2 or more days in a row.

Volvo farmer

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Re: The power of half
« Reply #6 on: February 02, 2012, 09:53:58 PM »
Thanks for all the replies.

Hydrosun, Your story is remarkably similar to stories I have read on the NAWS forum. Seems like Surrette has been getting a reputation for early failures in RE installations in the last few years. I am certainly considering a different brand on my next purchase.

Chris, I have run these batteries hot since I have owned them. Usually had a 29.0-29.2 Absorb voltage for 2 hours, and hardly ever over 120 Ahr out of a 800 ahr bank before recharging.

DanG, good point. I think the pro-rated warranty on one battery is less than scrap value at this age. 1200 cycles is only 3.2 years. If I had sized my bank to 400 ahr in the beginning, I think I might be dollars ahead at this point as my daily DOD was in the 15% rather than 50% range.

I have found a temporary cheap solution to my problem. A local solar installer has an old scrap Trojan L16 that holds voltage that he will trade me for the equivalent lead in scrap. I have a few old car batteries laying around that I can donate. I'm going to jam this old Trojan in with the old Surrettes and shoot to get through next winter, which would get me to my seven year goal. 

I was really hoping to be smarter than the average off-grid Joe and buy good batteries, treat them right, and get ten years out of them. It appears I have failed to achieve my goal on the first try, so here we go again! :)

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ChrisOlson

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Re: The power of half
« Reply #7 on: February 02, 2012, 10:10:21 PM »
I was really hoping to be smarter than the average off-grid Joe and buy good batteries, treat them right, and get ten years out of them. It appears I have failed to achieve my goal on the first try, so here we go again! :)

It's my theory, though not totally proven, that series configurations and slight mismatches between batteries leads to their early demise in a lot of systems.  With our old 12 volt system we just had Farm & Fleet Group 29 marine deep cycle batteries, 125 ah.  They were cheap - only about $75 each.  The original four that I bought in June 2003 lasted 8 years and they still held their own against the newer ones that I added later.

We replaced our bank with new Surrettes last April because we needed more power to run our new 24 volt inverters, so I used the old ones for cores.  When I serviced the bank last September, now with series connections for the 24 volt system, I had batteries with SOC all over the map in the bank.  Just simply due to slight mismatches between batteries, and no two batteries are ever identical.  I reconfigured my new bank to get rid of half the series connections in it and that seemed to help a lot the last time I serviced them now.

So I don't know, but I'm leary of series connections in battery banks and think they should be avoided like the plague.
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Chris

PeterDe

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Re: The power of half
« Reply #8 on: February 17, 2012, 02:05:44 AM »
With any battery that is made up of multiple cells in a single container (jar) such as the S530, where you do not have the ability to measure (or charge or discharge) any individual cell in the container, you will have some cells that are better performers and some that are worse.  In my opinion giving up the ability to be able to address each individual cell in any battery string is not worth the dollar savings of purchasing a battery string made up of multi-cell containers.

It was mentioned to use a lift truck battery and that might be a good idea, but it needs to be understood that normally when a battery that has been repalced in a lift truck fleet is replaced it normally has already been used up.  Remember that NORMALLY with any lead acid battery, the amount of the positive grid corrosion (normal aging mechanism) is what disctates when it is at the end of its normal life.  However with that said, these can still be used with lower draws for a number of years.  One might want to look at used VRLA 2 volt cells from Telecom applications.  They normally fail from what is commonly called dry-out, but this failure can be recovered.

If I was to go off grid and wanted a battery that would outlive me I would use Nickel-Iron cells, as we got some out of an old huntimg lodge in the Adirondacks which are all over 80 years old, and through electrolyte replacement and some cherge discharge cycles were able to get them to support a derated load for the original 5 hour run time.  Buying new ones is expensive, but if you can find old discarded ones they would be my first choice.