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


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8 minutes ago, WotEver said:

Yes, I think the pertinent point in your scenario is that you’re not actually using the LAs. Not in any meaningful sense. 

....so then coming back to the point about a 50% DoD and charging every other day or 75% and charging daily, it seems to me that avoiding the 50% is far better as there will be less hard sulphate at 75%. Buy a bigger bank and discharge it less.

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Batteries are made to be used to enable us to enjoy living on a boat. If you feel a need to run the engine/generator everytime a battery gets slightly discharged then get a house, or at least a marina mooring. Running the engine everyday, or even twice everyday, uses diesel and puts more wear on the engine that will probably work out more expensive that the saving in battery life. I find it much easier to replace the battery bank than to replace the engine and I have done both.

 

To answer the question about cycles and DoD, then looking at the Trojan information suggests it works out more or less exactly the same, and also that NOTHING at all special happens at 50%, its just a continuous trade between cycle life and DoD right down to 80% discharge/20% charge.

 

No need to recharge till batteries are down to 50% (or 30% or whatever you feel comfortable with), even if this is 2 or 3 or 4 days. The only important thing is to get to 100% quite often, and in my case I am happy to do this once every couple of weeks or so, but 100% must be 100%, and this will need a suitable charging voltage and maybe even a little bit of equalisation.

 

The old rule, charge for a couple of hours to about 80% everyday and then to 100% once each week was not far from the truth. All this stuff about 100% everyday is rubbish invented by this forum :). In the summer if we are not moving I aim to run the engine every third day as that's when the hot water has got truly cold, and by then the batteries are usually getting a bit lower than 50%.

 

There is some evidence that batteries will suffer if they never see a significant discharge, but I am not sure about this.

 

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

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1 hour ago, Richard10002 said:

WIW My lay mans perception is that Lead Carbon batteries are either AGM batteries with a new marketing name, or are constructed in the same way as AGM batteries

Chap on the DBS Leoch stand, at the recent Caravan and Camping show, told me Lead Carbon are built the same way as AGM.

 

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8 minutes ago, dmr said:

The old rule, charge for a couple of hours to about 80% everyday and then to 100% once each week was not far from the truth. All this stuff about 100% everyday is rubbish invented by this forum

Yes to the first point and no to the second. This forum hasn’t invented anything, merely repeated Trojan’s advice that their batteries should be recharged ‘after every discharge’. That is patently absurd advice even if it would maximise battery life so we return to getting up to 100% at least once a week and as high as practical in between. 

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21 minutes ago, Col_T said:

Chap on the DBS Leoch stand, at the recent Caravan and Camping show, told me Lead Carbon are built the same way as AGM.

 

Lead carbon batteries that we are talking about are AGM batteries with the negative plates tweaked by using a lead carbon "alloy". These are the Leoch, Northstar and Victron types. Trojan are now marketing a lead carbon flooded deep cycle battery for solar and off grid for PSOC applications.

 

Therefore, in my view, these are not new chemistries, or all new types, but just incremental changes to normal batteries.

 

One thing that is perhaps making people cautious is the history of the Elecsol carbon batteries which failed to meet their promises. These had something to do with carbon fibre, I can not recall their details, but I believe the lead carbon is totally different.

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Yes, nobody is claiming lead carbon batteries aren't AGM lead acid batteries - they are. But they have different negative plates from other such batteries, and as such are claimed to charge faster and to be less prone to sulphation.

 

And they haven't appeared out of nowhere overnight as MtB seems to think. Variations on the basic idea of introducing carbon into a lead acid battery have been around for a while, although there's no high-profile brand in the UK as Firefly seems to be in the US or UltraBattery in Australia.

 

Dr Bob, insofar as I could understand any of that, the point about the more 'stubborn' sulphation appearing when batteries are deeply discharged did make sense! Makes me wonder if 50% DoD is the wrong place to draw the line, really.

 

And yes, I know you 'must' recharge to 100% every single day, but really... if you're not on shoreline, it's the dead of winter and you're not cruising 20 miles a day, it's just not realistic. Which I suppose is exactly my reason for deciding on lead carbons.

1 hour ago, PeterF said:

These had something to do with carbon fibre

Ah, I guess that would explain the carbon fibre preset on Smartgauges.

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2 hours ago, dmr said:

There is some evidence that batteries will suffer if they never see a significant discharge, but I am not sure about this.

 

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

 

This affects Valve Regulated Sealed Lead Acid batteries (VRSLA's - of which AGM's are a type) AKA "recombination batteries" because they do not contsin any liquid electrolyte and maintain electrolyte by recombining the oxygen and hydrogen evolved during charging. It is known as "Coup de Fouet" and is where the volt drop and subsequent recovery when the load is initially connected. VRSLA's kept on charge for long periods can drop their voltage below acceptable levels before recovering.

 

Coup de Fouet affects all lead acid cells to a small degree, but only VRSLA's can drop voltage to an unacceptable level before recovering.

 

The voltage drop to unacceptsble is easily overcome by periodically letting the batteries do some work.

Edited by cuthound
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25 minutes ago, magictime said:

Dr Bob, insofar as I could understand any of that, the point about the more 'stubborn' sulphation appearing when batteries are deeply discharged did make sense! Makes me wonder if 50% DoD is the wrong place to draw the line, really.

 

Maybe its ok for Tojans but not for cheapos

 

28 minutes ago, magictime said:

And yes, I know you 'must' recharge to 100% every single day, but really... if you're not on shoreline, it's the dead of winter and you're not cruising 20 miles a day, it's just not realistic. Which I suppose is exactly my reason for deciding on lead carbons.

Ah, I guess that would explain the carbon fibre preset on Smartgauges.

I'm sure you dont need to do it everyday ...but that's why I went to Lithiums.

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Having spent some time googling"lead carbon alloy" because Idid not think you could alloy carbon with lead it seems they are using graphene nanoparticles and yes they do alloy.

 

If thee is sufficient graphene particles to surround the lead atoms then conductivity would be maintained or possibly bettered but the lead particles would be very small and held apart by the graphene so they could not grow. From my understanding hence the smaller sulphate particles.   However that suggests to me that Ah for Ah the LC plates need to be larger because of the amount of inactive carbon.

 

Just my thoughts at this time, in other word there may be truth in the claims.

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4 hours ago, magictime said:

Makes me wonder if 50% DoD is the wrong place to draw the line, really.

It’s only a guide. With Trojans and their ilk you could probably go lower. With cheapos probably shallower. It’s nothing more than a rough guide balancing battery longevity against battery cost. 

http://www.smartgauge.co.uk/50percent.html

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8 minutes ago, WotEver said:

It’s only a guide. With Trojans and their ilk you could probably go lower. With cheapos probably shallower. It’s nothing more than a rough guide balancing battery longevity against battery cost. 

http://www.smartgauge.co.uk/50percent.html

The SG guide though is really only about the type of discharge - fast or slow - and doesnt cover sulphation as it just assumes the cycle is finished when back up to 100% based on tail current.

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14 minutes ago, WotEver said:

It’s only a guide. With Trojans and their ilk you could probably go lower. With cheapos probably shallower. It’s nothing more than a rough guide balancing battery longevity against battery cost. 

http://www.smartgauge.co.uk/50percent.html

This is exactly right, its about a rough guide to choosing the most economic size of a battery bank, but somehow its got miss-interpreted by Chinese whispers as saying  batteries will die if discharged below 50%.

Its a very rough guide, if diesel gets more expensive and batteries get better or cheaper the compromise will change, and individual styles of boating and types of engine/generator will also shift the balance.

 

I have chosen to have a battery bank that can last two or three days before getting much below 50%, some people like to run an engine twice a day, others want to go weeks between engine/generator runs. Solar might well have shifted the balance for some boaters.

 

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

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1 hour ago, WotEver said:

It’s only a guide. With Trojans and their ilk you could probably go lower. With cheapos probably shallower. It’s nothing more than a rough guide balancing battery longevity against battery cost. 

http://www.smartgauge.co.uk/50percent.html

That's the article I read a couple of days ago that got me thinking about this issue again.

 

So here's one of the things that's bugging me. One of the illustrations in that article imagines a 200Ah battery bank discharged slowly by 40Ah to a depth of 20% and then recharged slowly, over and over again. The claim is that a single 100Ah battery discharged by the same 40Ah to a depth of 40% and recharged the same way, over and over again, might last 70% as long but only cost half as much, hence being more economical. (We could say that if the 200Ah bank discharged by 20% lasts 50 months, the 100Ah bank discharged by 40% would last 35 months.)

 

So far, so good. But if the batteries' 'end of life' is being defined here by what I believe to be the industry standard - 80% of original capacity remaining - the 200Ah battery bank would have 160Ah capacity remaining at the end of that 50 months, while the 100Ah bank would have only 80Ah remaining after 35 months. 

 

Now the 100Ah bank really is on the point of becoming useless after 35 months, when it hits 80Ah capacity, since the user is only just able to get 40Ah out of it without going below 50% DoD every cycle and doing fairly rapid damage to it. But when the 200Ah battery bank hits 160Ah capacity after 50 months, the user can still get 40Ah out of it while only discharging to 75%! Even if it then deteriorates twice as fast, losing another 20% of its original capacity in just 25 months, it will still have 120Ah capacity after 75 months and a 40Ah discharge will take it to 67% DoD each cycle. If it then deteriorates twice as fast again, it will still only, finally, hit 80Ah capacity after 87.5 months - hence lasting 2.5 times as long, for the users' purposes, as the smaller bank costing half as much.

 

So the larger bank discharged to 80% would if fact be more economical in the long run than the smaller bank discharged to 60%, even if you make what look to me like some pretty extreme assumptions about the rapidity of its decline as its capacity begins to reduce.

 

What am I missing? I can't see how you escape the conclusion that a bigger battery bank with plenty of 'headroom' for the intended usage (and hence a shallow DoD) is generally going to be more economical in the long run than a smaller bank with just enough capacity (and a DoD closer to 50%).

Edited by magictime
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16 minutes ago, magictime said:

That's the article I read a couple of days ago that got me thinking about this issue again.

 

So here's one of the things that's bugging me. One of the illustrations in that article imagines a 200Ah battery bank discharged slowly by 40Ah to a depth of 20% and then recharged slowly, over and over again. The claim is that a single 100Ah battery discharged by the same 40Ah to a depth of 40% and recharged the same way, over and over again, might last 70% as long but only cost half as much, hence being more economical. (We could say that if the 200Ah bank discharged by 20% lasts 50 months, the 100Ah bank discharged by 40% would last 35 months.)

 

So far, so good. But if the batteries' 'end of life' is being defined here by what I believe to be the industry standard - 80% of original capacity remaining - the 200Ah battery bank would have 160Ah capacity remaining at the end of that 50 months, while the 100Ah bank would have only 80Ah remaining after 35 months. 

 

Now the 100Ah bank really is on the point of becoming useless after 35 months, when it hits 80Ah capacity, since the user is only just able to get 40Ah out of it without going below 50% DoD every cycle and doing fairly rapid damage to it. But when the 200Ah battery bank hits 160Ah capacity after 50 months, the user can still get 40Ah out of it while only discharging to 75%! Even if it then deteriorates twice as fast, losing another 20% of its original capacity in just 25 months, it will still have 120Ah capacity after 75 months and a 40Ah discharge will take it to 67% DoD each cycle. If it then deteriorates twice as fast again, it will still only, finally, hit 80Ah capacity after 87.5 months - hence lasting 2.5 times as long, for the users' purposes, as the smaller bank costing half as much.

 

So the larger bank discharged to 80% would if fact be more economical in the long run than the smaller bank discharged to 60%, even if you make what look to me like some pretty extreme assumptions about the rapidity of its decline as its capacity begins to reduce.

 

What am I missing? I can't see how you escape the conclusion that a bigger battery bank with plenty of 'headroom' for the intended usage (and hence a shallow DoD) is generally going to be more economical in the long run than a smaller bank with just enough capacity (and a DoD closer to 50%).

I agree. That's the way I have always looked at it and therefore went for 660Ahrs rather than 330Ahrs. Putting the nesspresso machine on takes 100A which is far better when you have twice the capacity. 

I don't understand what the failure mechanism is for these 300 cycles to death when the are fully charging them between cycles. For me the thing that kills lead acids quickly is sulphation ...happening when you don't fully charge them. Therefore if you are looking at cost effectiveness, you need to look at sulphation due to poor charging regimes rather than a comparison when you get 100% charging which is difficult or even impossible in a real world winter.

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10 hours ago, Dr Bob said:

I agree. That's the way I have always looked at it and therefore went for 660Ahrs rather than 330Ahrs. Putting the nesspresso machine on takes 100A which is far better when you have twice the capacity. 

I don't understand what the failure mechanism is for these 300 cycles to death when the are fully charging them between cycles. For me the thing that kills lead acids quickly is sulphation ...happening when you don't fully charge them. Therefore if you are looking at cost effectiveness, you need to look at sulphation due to poor charging regimes rather than a comparison when you get 100% charging which is difficult or even impossible in a real world winter.

I may well be wrong but when I worked in a battery shop the old hands all told me that the plates "danced" during heavy charging and discharging. I think they probably meant flexed.  This lead to two modes of failure:

 

1. The separators got worn so one punctured but during my time there a big leap in separator technology took place so I doubt that happens very often nowadays except in conjunction with 2 below.

 

2. As the plates flex some oxide is shed from the plates that falls into the sediment traps in the base of the case When this builds up sufficiently the plates short out or if a large lump gets stuck hard against the separator that are ribbed it can speed up the separator puncture.

 

I my view the fact that proper deep discharge batteries have thick plates while start batteries have thin plates tend to confirm this theory because the thicker the plates the more stress forms in the outer layers of the plates so the more they shed under high currents. Likewise long life is a attribute of spiral wound/tubular batteries that have plates in a tubular form and a tube much better resists flexing than a flat sheet type plate.

 

The higher the depth of discharge the higher the potential charging current that will flow subject to the charge source having the capacity. The part of this theory that worries me is why do not proper deep discharge (thick plates) batteries not die in a short time when regularly subject to high inverter loads.

 

As each cell in  large bank is subject to a lower discharge current for a given load than one in a smaller bank makes me suspect that a large bank will have a longer life than  a smaller bank if both are subject to adequate charging.

 

As far as I can see all the battery "rules" are nothing more than advice that are aimed at helping  optimise battery life and its perfectly acceptable to do something else as  long as you understand the potential consequences and accept them.

Edited by Tony Brooks
Remove a spuriouse pasted sentace that made no sense
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1 hour ago, Tony Brooks said:

I may well be wrong but when I worked in a battery shop the old hands all told me that the plates "danced" during heavy charging and discharging. I think they probably meant flexed.  This lead to two modes of failure:

 

1. The separators got worn so one punctured but during my time there a big leap in separator technology took place so I doubt that happens very often nowadays except in conjunction with 2 below.

 

2. As the plates flex some oxide is shed from the plates that falls into the sediment traps in the base of the case When this builds up sufficiently the plates short out or if a large lump gets stuck hard against the separator that are ribbed it can speed up the separator puncture.

 

I my view the fact that proper deep discharge batteries have thick plates while start batteries have thin plates tend to confirm this theory because the thicker the plates the more stress forms in the outer layers of the plates so the more they shed under high currents. Likewise long life is a attribute of spiral wound/tubular batteries that have plates in a tubular form and a tube much better resists flexing than a flat sheet type plate.

 

The higher the depth of discharge the higher the potential charging current that will flow subject to the charge source having the capacity.I don't understand what the failure mechanism is for these 300 cycles to death when the are fully charging them between cycles. The part of this theory that worries me is why do not proper deep discharge (thick plates) batteries not die in a short time when regularly subject to high inverter loads.

 

As each cell in  large bank is subject to a lower discharge current for a given load than one in a smaller bank makes me suspect that a large bank will have a longer life than  a smaller bank if both are subject to adequate charging.

 

As far as I can see all the battery "rules" are nothing more than advice that are aimed at helping  optimise battery life and its perfectly acceptable to do something else as  long as you understand the potential consequences and accept them.

That makes a lot of sense.

I particularly agree with the last 2 paragraphs.

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1 hour ago, Tony Brooks said:

As far as I can see all the battery "rules" are nothing more than advice that are aimed at helping  optimise battery life and its perfectly acceptable to do something else as  long as you understand the potential consequences and accept them.

Absolutely this. If you want to only take your batteries down to 30% DoD then they’ll have a happier life. Conversely if you regularly take them down to 30% SoC then they won’t last as long. The choice is yours. 
 

If you regularly charge up to 100% then the batteries will eventually die from corrosion. If you regularly fail to charge to 100% then the batteries will eventually die from sulphation. 
 

Use the 50% guide as a starting point then do whatever you feel happiest with. 

9 hours ago, Dr Bob said:

I can't see how you escape the conclusion that a bigger battery bank with plenty of 'headroom' for the intended usage (and hence a shallow DoD) is generally going to be more economical in the long run than a smaller bank with just eno

It will last longer for sure. Balancing the extra cost of the larger bank against the longer life that it affords is where the equation becomes more complex. 

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9 hours ago, Dr Bob said:

I agree. That's the way I have always looked at it and therefore went for 660Ahrs rather than 330Ahrs. 

Well that reassures me about my reasoning, but it does suggest that while 'don't discharge below 50%' might well be a useful rule of thumb for getting a decent lifespan out of your batteries, the '50% rule' talked about in that Smartgauge article -

 

"This is a rule that states quite simply, the most economical use of deep cycle batteries comes about when they are, on average, discharged to 50% capacity then recharged."

 

- isn't even close to being a useful rough guide. Because both uses you could practically make of it are going to lead you astray:

 

1 - If you happen to have a battery bank of a certain size already, the rule suggests you should deliberately let it get down to 50% before recharging in order to get the most economical use out of it (e.g. by charging every two days rather than every day). Wrong; you'll shorten their lifespan by leaving them more discharged, for longer.

 

2 - If you're determining the size of battery bank you require, the rule suggests you should buy something with a capacity of double your daily usage (or a bit more if you're not going to be able to get to 100% every recharge, I guess). Again, wrong. You should add plenty of headroom so that your bank (i) doesn't need to be discharged as deeply, and (ii) can afford to lose 40% or 50% of its original capacity before it becomes useless to you, rather than only 20% or 25%.

 

The only sense I can make of it is that it's based on a complete misapplication of the concept of 'end of life' (80% of original capacity). Yes, a 400Ah battery bank discharged to 75% (say) may take only 40% longer to reach 'end of life' than a 200Ah bank costing half as much and discharged to 50%; but that absolutely does not mean it would be more economical in the long run either for an owner of that 400Ah bank deliberately to discharge it to 50% instead, or for a user requiring 100Ah of useable capacity to choose a 200Ah bank over a 400Ah bank. Quite the reverse!

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10 minutes ago, WotEver said:

Absolutely this. If you want to only take your batteries down to 30% DoD then they’ll have a happier life. Conversely if you regularly take them down to 30% SoC then they won’t last as long. The choice is yours. 
 

If you regularly charge up to 100% then the batteries will eventually die from corrosion. If you regularly fail to charge to 100% then the batteries will eventually die from sulphation. 
 

Use the 50% guide as a starting point then do whatever you feel happiest with. 

It will last longer for sure. Balancing the extra cost of the larger bank against the longer life that it affords is where the equation becomes more complex. 

The quote you used was from magic time not me. I did agree with him though!

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Just to clarify, as people keep raising this point about rules being only a rough guide; I'm not taking issue with the rule of thumb that says you shouldn't generally discharge your batteries below 50% SoC if you want to get a decent lifespan out of them. That does indeed look like a useful starting point for making decisions about battery maintenance. It's for the user then to decide whether they feel it's worth their while to do better most of the time (discharging only to 60%, say), or worth the risk of going to 40% now and again, etc.

 

No, I'm talking about this '50% rule' as defined in that article on the Smartgauge site: 

 

"This is a rule that states quite simply, the most economical use of deep cycle batteries comes about when they are, on average, discharged to 50% capacity then recharged."

 

For the reasons set out above, this doesn't look even roughly correct to me - the figure isn't going to be anywhere close to 50% - and as such I don't see how it can be useful even as the roughest guide to economical use of batteries.

 

More than happy to be corrected if I've overlooked something, though.

 

 

Edited by magictime
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45 minutes ago, WotEver said:

It will last longer for sure. Balancing the extra cost of the larger bank against the longer life that it affords is where the equation becomes more complex. 

Yes, but as I say, even if you make some pretty extreme assumptions about how quickly that larger bank deteriorates once it gets below 80% of capacity, I don't see how it could ever be the case that the smaller bank should be the most economical choice. Yes there's the question of what you can afford and what you have room for, but 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.

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1 hour ago, magictime said:

 

"This is a rule that states quite simply, the most economical use of deep cycle batteries comes about when they are, on average, discharged to 50% capacity then recharged."

The only sense I can make of it is that it's based on a complete misapplication of the concept of 'end of life' (80% of original capacity). Yes, a 400Ah battery bank discharged to 75% (say) may take only 40% longer to reach 'end of life' than a 200Ah bank costing half as much and discharged to 50%; but that absolutely does not mean it would be more economical in the long run either for an owner of that 400Ah bank deliberately to discharge it to 50% instead, or for a user requiring 100Ah of useable capacity to choose a 200Ah bank over a 400Ah bank. Quite the reverse!

I agree with you that this 'rule' doesnt make sense based on your "complete misapplication ...of end of life".

For me it also doesnt make sense for two other reasons.

- If I have a 660 Ahr bank then it says the most economical use is to go down to 330Ahr before recharging. Bollox! That means I have to put nearly 400Ahr back in (CEF!!!). On my 90A alternator which struggles on LAs to put out 50A, that will take 10-12hrs engine running. In the real world in winter that will just not happen so you will not be back up to 100%. Ok, you dont have to run the engine every day (but I would want to be back to 100% at least a couple of times a week). For me, that 400Ahr is 3 days use. Ok, it is better to not run everyday as you are warming up a cold engine more (FOR CHARGING) but I run my engine for at least an hour every day to get hot water so that HAS to be taken into account.

- Linked to the first bullet, the rule totally ignores sulphation as I said previously. If you dont get back to 100% then the '50' sites will sulphate and be lost in a few weeks. Likely that Trojans will have better resistance here but cheapo LAs certainly wont. For me, that rule is shot to pieces for cheapo LAs. Sulphation is the reason all these batteries are failing in less than 2 years and keeping well above 50% DoD will help reduce sulphation. I know it did on my 660Ahr cheapo LA bank which is almost still full capacity after 2 years.

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13 minutes ago, Dr Bob said:

 the rule totally ignores sulphation as I said previously.

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?

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I also struggle with the 50% rule. It does say its for the most economic use of batteries, and it does say 50% on average, which are both good. Dunno how so many people interpret this as saying batteries are damaged if they go below 50%.

But to make an economic rule requires consideration of many other factors, like the expense of running an engine/generator to charge those batteries, and even the cost of battery replacement which can be anything from an easy DIY half day to a big expensive visit to a boatyard.

And then there is the cost of convenience, I want to have a lot more than "50%" capacity so that I don't have to run the engine everyday. A big long dog walk then an early evening drink is more fun than having to rush back to do an engine run before the 8pm deadline. The 50% rule does infer running the engine/generator everyday, though aiming for 50% discharge after the "desired discharge period" is a good way of looking at things. .

 

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

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2 minutes ago, dmr said:

I also struggle with the 50% rule. It does say its for the most economic use of batteries, and it does say 50% on average, which are both good. Dunno how so many people interpret this as saying batteries are damaged if they go below 50%.

It looks to me as if there are actually two different rules which can easily get confused because they have the '50% discharge' bit in common. One is a rule of thumb about the state of charge below which you shouldn't generally go if you want to get a decent lifespan out of your batteries; the other is alleged to be a more 'scientific' rule about the most economical pattern of cycling. I think you and I disagree about which one looks fair enough, and which one looks plain wrong!

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