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Batteries: more frequent, shallower cycling vs. less frequent, deeper cycling


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6 minutes ago, magictime said:

Is that right? In the illustration in that Smartgauge article, isn't it because of sulphation (due to being more deeply discharged) that the smaller bank is said to take only 70% as long as the larger bank to reach end of life?

I took the article to be based on the 300 cycles to death which is then all about the things Tony talked about (plate shedding etc). Yes I am sure sulphation is part of the 300 cycles but there will be little sulphation happening in the two extreme cases they mention ie charge back to 100%. The 'go to 50% for best economics' seemed to me to ignore the failure mode that many peeps on here see when their LAs die in 12 months.

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2 minutes ago, Dr Bob said:

I took the article to be based on the 300 cycles to death which is then all about the things Tony talked about (plate shedding etc). Yes I am sure sulphation is part of the 300 cycles but there will be little sulphation happening in the two extreme cases they mention ie charge back to 100%. The 'go to 50% for best economics' seemed to me to ignore the failure mode that many peeps on here see when their LAs die in 12 months.

That is never what I understood the 50% rule to mean. I understood it to say that for optimum life try to never discharge below 50% which is rather different to "discharge to 50%". But that rule needs to be linked to the bit about recharge to as close to100% as soon as practicably possible. If you do not link the two then you will get early failure due to sulphation.

 

Whilst seem of us on here can discuss such things from a position of some knowledge and bend such rules to suit our own cases I would suggest we are in the minority. The majority need a few simple "rules" to guide them. If this were not so we would not get the regular "my new batteries won't hold a charge type question.

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8 minutes ago, Dr Bob said:

I took the article to be based on the 300 cycles to death which is then all about the things Tony talked about (plate shedding etc). Yes I am sure sulphation is part of the 300 cycles but there will be little sulphation happening in the two extreme cases they mention ie charge back to 100%. The 'go to 50% for best economics' seemed to me to ignore the failure mode that many peeps on here see when their LAs die in 12 months.

I have always assumed that the 300 cycles lifetime is due to plate shedding due to the lead oxide turning into (softer?) sulphate. This cycle life assumes the batteries are used "correctly" so there would be no significant sulphation as sulphation is really a form of battery abuse.

 

Hence 300 is the best you can get if you look after the batteries proper :).

 

............Dave

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9 minutes ago, Dr Bob said:

I took the article to be based on the 300 cycles to death which is then all about the things Tony talked about (plate shedding etc). Yes I am sure sulphation is part of the 300 cycles but there will be little sulphation happening in the two extreme cases they mention ie charge back to 100%. The 'go to 50% for best economics' seemed to me to ignore the failure mode that many peeps on here see when their LAs die in 12 months.

I'm not sure where the '300 cycles to death' thing is coming from (sorry), but I think you have in mind the heavy charge/heavy discharge example in that SG article? I was ignoring that one and going by the other one, where the shorter lifespan of the smaller battery presumably is due to sulphation from deeper discharge.

 

3 minutes ago, Tony Brooks said:

That is never what I understood the 50% rule to mean.

Probably very wise if I've been on the right track with my thoughts on that Smartgauge article, but somebody clearly thinks there's such a rule, because that article is explicitly about a 50% rule relating to the most economical use of batteries.

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5 minutes ago, dmr said:

I have always assumed that the 300 cycles lifetime is due to plate shedding due to the lead oxide turning into (softer?) sulphate. This cycle life assumes the batteries are used "correctly" so there would be no significant sulphation as sulphation is really a form of battery abuse.

 

Hence 300 is the best you can get if you look after the batteries proper :).

 

............Dave

 

I don't buy that.

 

First because it will be a conservative figure probably based on sample testing.

 

Secondly my 7.5 year old Exide leisure batteries are still performing adequately and they are rated at 300 cycles to 40%. I usually discharge to around 12.3V overnight and although a holiday boat in seven years we must have done far more then the equivalent of 300 cycles. I am however possibly a bit nerdish with the recharging each day.

 

Actually I am not even sure that 300 cycles is not a figure almost plucked out of the air to cover the supplier's back.

 

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11 minutes ago, Tony Brooks said:

First because it will be a conservative figure probably based on sample testing.

...

Actually I am not even sure that 300 cycles is not a figure almost plucked out of the air to cover the supplier's back.

Blimey, I hope Leoch's claims for their lead carbons are similarly conservative and back-covering, cos they quote something like 2000 cycles to 50% DoD!

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2 hours ago, magictime said:

if we're talking about identifying the most economical choice in an ideal world as a starting point, I don't see how SG's '50% rule' could ever point you in the right direction.

Plot it out like Gibbo suggested and see what you get. 

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58 minutes ago, Tony Brooks said:

 

I don't buy that.

 

First because it will be a conservative figure probably based on sample testing.

 

Secondly my 7.5 year old Exide leisure batteries are still performing adequately and they are rated at 300 cycles to 40%. I usually discharge to around 12.3V overnight and although a holiday boat in seven years we must have done far more then the equivalent of 300 cycles. I am however possibly a bit nerdish with the recharging each day.

 

Actually I am not even sure that 300 cycles is not a figure almost plucked out of the air to cover the supplier's back.

 

I also wonder about the specs. In my experience many specs are "optimistic marketing" and are what you might get on a good day, so if batteries are good for a lot more than 300 cycles I would expect the spec to say so. On the other hand in some industries specs need to be "real" but I suspect this does not apply to batteries.

I expect many purchasers look at things like the length of the guarantee rather than cycle life so maybe you are right and its a worse case figure just to cover their backs.

 

Do you know what the actual capacity of your batteries is at present?

 

Its quite possible that with cheaper batteries almost no testing is done and an old figure from a roughly similar battery, or even from a competitors specification is quoted.

 

Looking at the cycle life curve for Trojans etc there is no real advantage to stating above 50%, the cycle life vs depth of discharge exactly balances out. I do suspect that this is a known (or assumed) fundamental truth and that only one or maybe two points are actually measured and this theoretical curve then drawn

 

.............Dave

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and just for interest, if you discharge to 12.3 volts that equates to about 65% state of charge and if your batteries follow the same life cycle curve as Trojans etc this equates to about 440 cycles rather than 300.

 

...............Dave

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1 hour ago, Tony Brooks said:

That is never what I understood the 50% rule to mean. I understood it to say that for optimum life try to never discharge below 50% which is rather different to "discharge to 50%". But that rule needs to be linked to the bit about recharge to as close to100% as soon as practicably possible.

 

When I first started learning properly about batteries, I encountered the "never discharge below 50%" constantly but never saw "recharge to as close to 100% as soon as practicably possible". Few people ever say that until the person with the dead batteries says "but I NEVER discharged below 50%!", then out it comes. 

 

Until I read the Trojan data sheets that is, but even that's not quite what they say.

 

Trojan say words to the effect of "recharge immediately after discharge", IIRC. 

 

 

 

Edited by Mike the Boilerman
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7 minutes ago, Mike the Boilerman said:

 

When I first started learning properly about batteries, I encountered the "never discharge below 50%" constantly but never saw "recharge to as close to 100% as soon as practicably possible". Few people ever say that until the person with the dead batteries says "but I NEVER discharged below 50%!", then out it comes. 

 

Until I read the Trojan data sheets that is, but even that's not quite what they say.

 

Trojan say words to the effect of "recharge immediately after discharge", IIRC. 

 

 

 

but even that is open to interpretation.  Trojan 6volt jobbies are often known as golf cart batteries so it might mean that after driving about in a golf buggy thingy all afternoon put the batteries on charge that night rather than leaving them for a week or two???? It does not mean doing a quick recharge half way through the game of golf?

 

though in my opinion putting batteries on charge would actually be much more pleasurable than playing golf :).

 

...........Dave

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17 minutes ago, WotEver said:

Plot it out like Gibbo suggested and see what you get. 

I'm not in a position to do that, obviously. But I am in a position to question whether his own illustrative example (the 100Ah bank lasting 70% as long as a 200Ah bank, if repeatedly discharged slowly by 40Ah and then recharged to 100%), actually supports the '50% rule' he's talking about.

 

It doesn't, if he's taking 80% of original capacity to be the criterion for a battery bank's 'end of life'.

 

If, on the other hand, he's talking about the end of a battery bank's useful life to a person requiring that useable capacity - 40Ah - the illustration supports the 50% rule only if the following is correct: a 200Ah battery bank charged and discharged in a 100%-80%-100% pattern will lose capacity far faster than a 100Ah bank charged and discharged in a 100%-60%-100% pattern.  Specifically, it will take just about 40% longer for its capacity to drop by 60%/120Ah to 80Ah, than the smaller bank takes for its capacity to drop by 20%/20Ah to 80Ah.

 

Which doesn't look remotely plausible, surely? In fact, since it's being discharged less deeply, it should lose capacity a lot more slowly...

 

...which is why I think he must be using '80% = end of life', since that fits the illustration in which the bigger bank 'lives' about 40% longer than the smaller (= the smaller bank has 70% of the lifespan of the bigger).

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Firstly, what the OP is missing, is that no one fully understands everything about lead acid batteries, they really are a black art, sometimes performing in a  inexplicable way, especially when near their end of life.

 

Tony Brooks point 2 is correct on the failure mode for batteries which have been correctly charged and not suffered from sulphation. It is loss of active material from the plates, which often builds up on the bottom of the battery and shorts two or more plates. The shedding of active material also reduces the plates capacity.

 

The shedding occurs more on cheaper batteries because they have thinner plates which flex under high charge and discharge currents. Most leisure batteries are rebadged engine starter batteries, which have many more thinner plates to help provide the short term high currents necessary to start engines

 

Until the advent of VRSLA's (which turned commercial/industrial battery applications on its head, as they require almost no routine maintenance), wet lead acid batteries were the norm. These had plates typically 15mm thick and routinely lasted 25 years.

 

The number of cycles isn't relevant to most commercial/industrial applications, where the batteries are typically used for standby applications, like powering essential equipment or emergency lighting. I suspect it is the product of a marketing man trying to sell more batteries to leisure users. Commercial/industrial battery uses rarely involve  deep discharges, but occasionally will be discharged until completely flat, 0v unless a low voltage disconnect is fitted (rarely for critical power applications as it introduces another point of failure).

 

Finally the 50% "rule" is as has already been stated a rough rule of thumb, designed to help people choose the best compromise between longevity and operating costs. Again, never used in commercial/industrial applications.

 

Commercial /industrial users are very conservative and cost conscious,  which is why to date only VRSLA's have been adopted despite many new battery technologies. They offered lower overall costs by virtually eliminating maintenance but reduced operating life from 25 to 6 years. Lithium ion batteries are too expensive and have yet to demonstrate longevity, (Batteries being chemical devices are very much affected by mode of operation and the environment they are kept in so It is  difficult to use compressed longevity tests to ascertain lifespan), and lead carbon is too new but looks like it could be promising.

 

 

batt-melb-1913_orig.jpg

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23 minutes ago, WotEver said:

Yes of course he is. That is the generally accepted industry definition of EOL. 

I'm inclined to agree, obviously. But in that case the illustration doesn't support his '50% rule'.

 

It shows that if you compare a 100Ah battery bank to a double-the-cost, double-the-size 200Ah battery bank, the latter will not take double the time to hit 80% of capacity, if both are used to supply 40Ah of charge before recharging. OK, fine.

 

But that simply does not mean that the smaller bank is the more economical choice for someone requiring that 40Ah of useable capacity. It completely overlooks the fact that the bigger bank will still have 160Ah capacity remaining at 'end of life', and as such is surely likely to last at least as long again, and probably longer, before finally hitting the same 80Ah cut-off point for replacement as the smaller bank.

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14 minutes ago, magictime said:

I'm inclined to agree, obviously. But in that case the illustration doesn't support his '50% rule'.

 

It shows that if you compare a 100Ah battery bank to a double-the-cost, double-the-size 200Ah battery bank, the latter will not take double the time to hit 80% of capacity, if both are used to supply 40Ah of charge before recharging. OK, fine.

 

But that simply does not mean that the smaller bank is the more economical choice for someone requiring that 40Ah of useable capacity. It completely overlooks the fact that the bigger bank will still have 160Ah capacity remaining at 'end of life', and as such is surely likely to last at least as long again, and probably longer, before finally hitting the same 80Ah cut-off point for replacement as the smaller bank.

I think we all agree with that.

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And another thing.....or two

Capacity can drop quite significantly during a cold winter and this is also when sulphation is most likely to occur so more capacity is lost until its time for an equalistion, so we really need a lot of capacity in reserve if we are to avoid going below 50% :) on a daily basis. I suspect the 50% economic rule does not consider the realities of off-grid liveaboard boating.

 

..............Dave

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27 minutes ago, cuthound said:

Finally the 50% "rule" is as has already been stated a rough rule of thumb, designed to help people choose the best compromise between longevity and operating costs.

Yes, but a rough rule of thumb should be at least roughly correct! 

 

As I've said, it feels to me as if we're talking about two quite different rules here: one, try not to discharge your batteries below 50% if you want to get reasonable value/a reasonable lifespan out of them (which seems fair enough); and two, the most economical way to buy and use batteries is to aim for a 50% depth of discharge in each cycle (which for the reasons I've set out, seems to me to be possibly based on a misapplication of the technical definition of 'end of life').

9 minutes ago, Dr Bob said:

I think we all agree with that.

Phew!

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1 hour ago, dmr said:

I also wonder about the specs. In my experience many specs are "optimistic marketing" and are what you might get on a good day, so if batteries are good for a lot more than 300 cycles I would expect the spec to say so. On the other hand in some industries specs need to be "real" but I suspect this does not apply to batteries.

I expect many purchasers look at things like the length of the guarantee rather than cycle life so maybe you are right and its a worse case figure just to cover their backs.

 

Do you know what the actual capacity of your batteries is at present?

 

Its quite possible that with cheaper batteries almost no testing is done and an old figure from a roughly similar battery, or even from a competitors specification is quoted.

 

Looking at the cycle life curve for Trojans etc there is no real advantage to stating above 50%, the cycle life vs depth of discharge exactly balances out. I do suspect that this is a known (or assumed) fundamental truth and that only one or maybe two points are actually measured and this theoretical curve then drawn

 

.............Dave

 

No idea but they are almost certainly sulphated but still end up around 12.2 in the morning so acceptable (to me) capacity.

 

 

1 hour ago, dmr said:

and just for interest, if you discharge to 12.3 volts that equates to about 65% state of charge and if your batteries follow the same life cycle curve as Trojans etc this equates to about 440 cycles rather than 300.

 

...............Dave

 There are several rested voltage to state of charge tables available and they seem to differ from make of battery to make. As far as I am concerned 12.2v to 12.3v is best considered as around 50% charged. I certainly do not believe 12.3V normally equates to 65% charged.

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33 minutes ago, magictime said:

the most economical way to buy and use batteries is to aim for a 50% depth of discharge in each cycle

I don’t believe that’s the point that Gibbo was making. Another quote from the same page states:

Doubling the size of the battery bank could increase the life of the battery bank as a whole by say 3 times. So double the initial outlay (twice as many batteries to buy) results in a saving of 50% (they last 3 times longer).

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59 minutes ago, WotEver said:

I don’t believe that’s the point that Gibbo was making. 

Well I don't know how else to interpret the statements:

 

the most economical use of deep cycle batteries comes about when they are, on average, discharged to 50% capacity then recharged.

 

Discharging deep cycle batteries to 50% results in the most economical use of the batteries in terms of battery life and monetary outlay.
 

1 hour ago, WotEver said:

Another quote from the same page states:

Doubling the size of the battery bank could increase the life of the battery bank as a whole by say 3 times. So double the initial outlay (twice as many batteries to buy) results in a saving of 50% (they last 3 times longer).

Yes, I didn't know what to make of this. (For a start, that's a saving of 33%, not 50% - one larger battery bank costing say £400 lasting as long as three smaller battery banks costing £200 each.) I wasn't sure what depths of discharge we were supposed to be talking about in this illustration: is the double-sized, 400Ah bank supposed to end up at a 50% DoD after drawing 100A for an hour? If not, how does it illustrate the 50% rule? Why should we think it's not more economical to add 50% to the capacity, or 150%, rather than 100%? This is partly why I've stuck to the clearer (to me) illustration of a 200Ah bank vs a 100Ah bank with the same 40Ah taken out of each.

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The basic problem I've got with his reasoning comes out here:

 

However, reducing this battery bank by 50% (to a single 100Ahr battery) would perhaps only reduce the life of the bank as a whole by say 30% because the single battery would still be being relatively well treated. So in this case a saving could have been made by buying half as many batteries (so half the initial outlay) and getting a battery life of 70%. A saving of 30% in monetary terms.

 

Yes, you'll save money in the long term if you buy a new 100Ah bank every time your old one hits 80% capacity/end of life, rather than buying a new 200Ah battery bank every time your old one hits 80% capacity/end of life. That's what the illustration establishes. But if your cut-off point for your batteries needing replacement is that their capacity has dropped to 80Ah, why the hell would you buy a new 200Ah bank every time your old one hits 160Ah? It just makes no sense.

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Has anyone mentioned Peukert yet? I feel he deserves a mention. What self respecting battery thread would be complete without the mention of Wilhelm Peukert and his law?

 

Apologies if he has already been mentioned, or is irrelevant to the discussion.

 

There. At least he has had a mention, relevant or not.

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