Economic Generation

[Robin2]

But unless I'm missing something, you're comparing apples to oranges here. Assuming you've taken 20% out of the SOC then the small bank (which only ever gets charged to 80%, which is the point of the exercise) will be at 60% SOC. The large bank (for the same discharge) will be at 90% SOC. After a couple of hours, you may have got the small bank back to 80% whilst the large bank will have gone up to say 93%.

 

How is the small bank better?

 

Tony

 

There are crossed wires somewhere ...

 

Both the large and small batteries get charged to over 99% - the question is how many hours (daily average) of engine running are needed. To make an allowance for the Peukert effect I assumed that there would be less amp-hours taken from the large bank.

 

The point about having 2 (or 3 or 4) equal sized battery banks is to make best use of engine hours. Very simply, if you run the engine for (say) 1 hour each day which is enough to bring today's battery back to 85% charge, at the same time it will be bringing the other batteries up over 99% charge and there is no need for the long engine run that would be needed to bring a single battery (of whatever size) back to full charge.

 

I think I am correct but, if not, I am hoping someone will put forward a convincing counter argument before I invest in another battery.

[nb Innisfree]

There are crossed wires somewhere ...

 

Both the large and small batteries get charged to over 99% - the question is how many hours (daily average) of engine running are needed. To make an allowance for the Peukert effect I assumed that there would be less amp-hours taken from the large bank.

 

The point about having 2 (or 3 or 4) equal sized battery banks is to make best use of engine hours. Very simply, if you run the engine for (say) 1 hour each day which is enough to bring today's battery back to 85% charge, at the same time it will be bringing the other batteries up over 99% charge and there is no need for the long engine run that would be needed to bring a single battery (of whatever size) back to full charge.

 

I think I am correct but, if not, I am hoping someone will put forward a convincing counter argument before I invest in another battery.

 

Ok here's my effort.

 

Day1 starts with 4x100ah 100% charged batts at 1800hrs

Day2 - 1700 batt 1 is down to 70%, switch to batt2 and run engine for 1 hour at safe max charge of 30 amp.

Day3 - 1700 batt2 down to 70% switch to batt3 run engine 1 hr

Day4 - 1700 batt3 down to 70% switch to batt4 run engine 1 hr

Day5 - 1700 batt4 down to 70% switch to batt1.....which has only had 3 hours charge but needs another 5 hrs to reach 100% SoC...

 

Or..

 

Day 1 starts with 4x100ah as single bank 100% charged at 1800hrs

Day2 1700 down to 92.5% charge 1 hour to, say, 95%?

Day3 1700 down to 87.5% charge 1 hr to 93%?

Day4 1700 down to 86.5% charge 1 hr to 92%?

Day5 1700 down to 84.5% charge 1 hr to 90%?

Day6 1700 down to 82.5% charge 1 hr to 88%?

Day7 1700 down to 80.5% charge 1 hr to 86%?

Day8 1700 down to 78.5% charge 1 hr to 85%?

Day9 1700 down to 77.5% charge 1 hr to 84%?

Day10 1700 down to 76.5% charge 1 hr to 83%?

Day11 1000hrs down to, say, 76%, by this time an 8hr absorption to 1800hrs is probably needed to bring batts back to 100% SoC, 17 hrs charging in 11 days. First example needs 8 hrs every 5 days which equals the second example but when estimating usage in the second example I assumed the same rate of percentage usage as first example without allowing for the larger bank taking less percentage due to Peukert, so second example wins!

 

Waiting for Gibbo to shoot me down in flames!

Edited November 24, 2010 by nb Innisfree

[Nickhlx]

How about using solar PV to provide the last part of the charging cycle ? Depending on the time of year, even at this time you will be getting 4 to 6 hours, especially if you can re-orientate at around mid-day and late afternoon, ready for the next morning. Its not as reliable a supply as starting the engine, but potentially ( IYPTP) I think solar can offer a valuable contribution, especially for topping off the charge of batteries...

 

Nick

 

 

[WotEver]

Waiting for Gibbo to shoot me down in flames!

I don't think he will, because what you write just 'feels' right. He may have some real world figures to nudge your example into more accuracy but I'm sure the principal is correct.

 

If it wasn't for Peukert then it wouldn't make any difference either way. But Peukert is real, so it does.

 

Tony

[Sunset Rising]

So this saves engine running time which saves fuel and an inestimable amount of money in engine deterioration...... but how long would you have to keep this up to reciver the cost of all the extra batteries ? What would the payback time be ?

[nb Innisfree]

So this saves engine running time which saves fuel and an inestimable amount of money in engine deterioration...... but how long would you have to keep this up to reciver the cost of all the extra batteries ? What would the payback time be ?

 

Impossible to predict as the quality and condition of new batteries isn't known, but my example doesn't take into account erratic cruising habits etc, it's just a theoretical scenario but IMO it is a reasonable balance between fuel and battery costs and with cheap FLAs I would guess they last 2 years or more. No one can say with any certainty what the outcome of any charging regime will be, a bit like life in general I suppose, unpredictable and paradoxical?

[Robin2]

Ok here's my effort.

 

Day1 starts with 4x100ah 100% charged batts at 1800hrs

...

 

Thanks, you have obviously put a lot of effort into this - while you remain sceptical, perhaps you would like my idea to work???

 

While I believe my idea is correct I am reading all these comments carefully so that I don't waste my money - either on fuel or batteries.

 

Two things strike me about your analysis, one of them is the trivial point that battery A will have had 4 hours charging before going back into service. The more substantive issue is why you think a total of 6 hours charging is necessary to get back to 100%. Nothing I have read states the problem in a finite way like that.

 

In my model I assume that if the SoC is below 80% then 80% of the available amps will be taken up by the battery. If the SoC is between 80% and 90% then 70% of the amps will be taken up and if the SoC is over 90% the accepted amps = 50% of the deficit (so in theory it never gets to 100%). I also have safeguards so that below 80% it can't get above 85% and below 90% it can't get above 95%.

 

The results of the model coincide with the actual behaviour of my present batteries which are not in good condition and with the chart in the Sterling brochure showing the behaviour of their Alternator to Battery charger. I accept that this is a very small comparative sample and if I can get more data elsewhere it would be great.

 

I don't know why you limit your amps to 30 - I have assumed 70 in my model but it does not make any difference if I reduce it to 30 because the batteries are at the stage where they cannot accept a lot of power.

 

I would very much appreciate all further comments.

[nb Innisfree]

Thanks, you have obviously put a lot of effort into this - while you remain sceptical, perhaps you would like my idea to work???

 

While I believe my idea is correct I am reading all these comments carefully so that I don't waste my money - either on fuel or batteries.

 

Two things strike me about your analysis, one of them is the trivial point that battery A will have had 4 hours charging before going back into service. The more substantive issue is why you think a total of 6 hours charging is necessary to get back to 100%. Nothing I have read states the problem in a finite way like that.

 

In my model I assume that if the SoC is below 80% then 80% of the available amps will be taken up by the battery. If the SoC is between 80% and 90% then 70% of the amps will be taken up and if the SoC is over 90% the accepted amps = 50% of the deficit (so in theory it never gets to 100%). I also have safeguards so that below 80% it can't get above 85% and below 90% it can't get above 95%.

 

The results of the model coincide with the actual behaviour of my present batteries which are not in good condition and with the chart in the Sterling brochure showing the behaviour of their Alternator to Battery charger. I accept that this is a very small comparative sample and if I can get more data elsewhere it would be great.

 

I don't know why you limit your amps to 30 - I have assumed 70 in my model but it does not make any difference if I reduce it to 30 because the batteries are at the stage where they cannot accept a lot of power.

 

I would very much appreciate all further comments.

 

Battery A will have had 3 hrs not 4, but that is only a detail.

To maximise a battery life it must be recharged to 100% SoC after every discharge, not practical of course if off grid, so an occasional full recharge has to suffice, but for any battery to fully recharge takes several hours there's no other way, the charge will automatically reduce and the rate of charge becomes progressively slower, a sign of it being fully charged is when the charge rate flattens out, in fact this won't take several hours but probably 24hrs or more, I've watched this lots of times when we were on shorepower.

 

I limited the charge to 30 amps because the max safe charge a batt can take is 1 amp per 1% of depth of discharge, i.e. a 100ah battery at 70% SoC is at 30% DoD so 30x1=30amp or a 500ah batt at 50% DoD will take 250A. After 80% SoC battery resistance reduces charge current anyway (absorption charge)

 

In my experience it takes 8 hrs to go from approx 90%+ SoC until charge current reduces to 2% of batt capacity which is acknowledged to be nominally fully charged, not truly fully charged which equalises the cells. Sticking permanently to 2 or 3 hrs charging will reduce batt life drastically by sulfation (unless they are discharged by a small amount such as a starter batt)

 

There is a myth that a battery will never reach 100% SoC if voltage is below 14.4, that an older alternator at 13.8v can't achieve this, but it will it just takes longer, a lot longer. Also as Gibbo has pointed out there is nothing special about 50% DoD, lots of people think 50% is ok but 49% isn't, any discharge is harmful. Nothing is set in stone, parameters are shifting all the time

 

All this isn't my opinion it is a well known phenomena, trying to do otherwise is really trying to invent perpetual motion

 

Edited to add:

 

Yes I would love your idea to work as it would revolutionise off grid usage, boaters would be in heaven.

Edited November 25, 2010 by nb Innisfree

[WotEver]

Still no observations from Gibbo. That's a shame.

 

Tony

[Chris Pink]

Impossible to predict as the quality and condition of new batteries isn't known, but my example doesn't take into account erratic cruising habits etc, it's just a theoretical scenario but IMO it is a reasonable balance between fuel and battery costs and with cheap FLAs I would guess they last 2 years or more. No one can say with any certainty what the outcome of any charging regime will be, a bit like life in general I suppose, unpredictable and paradoxical?

 

Well the complex integral maths and the logging of the charging patterns, the length of time and the difficulty of comparing one set of batteries with the next all make this a difficult call.

 

One of the flaws in your analysis above is that one of the reasons for having a bank cycling from 50% to 80% SOC is that in this range the batteries are on bulk charge and it terms of raw amp hours will take so much more during their hour of glory. If you are continually dropping to 90% or thereabouts and recharging the current will be quite low. But then no manufacturer gives figures for cycles 50% to 80% with a full charge once every 5 cycles. I think the main argument for 2 banks is that in a 'normal' abusive environment - ie the real world, people I know - a true 100% charge is a rare thing.

[Robin2]

 

All this isn't my opinion it is a well known phenomena, trying to do otherwise is really trying to invent perpetual motion

 

 

The only reference material I have is (in no particular order) (i) The Sterling brochure charging graph, (ii) a document on charging that I got from Surette who make the Rolls batteries and (iii) the Boatowner's Mechanical and Electrical Manual by Nigel Calder. My calculations seem to be consistent with all three. If you have any other references I would appreciate links to them.

 

It is important to keep my objective in mind - the cheapest way to meet my electricity needs.

 

A lot of the theory seems to be about protecting the life of batteries without considering the

overall economic situation. For example saving 20 minutes of fuel usage each day (say 20p) would pay for a £75 battery in a year. The batteries are only a means to an end, and a very inefficient means at that. Also, I have become very sceptical about the life expectancy of batteries. I don't want to invest a huge amount (capital cost and charging fuel) only to find that the battery wears out after 2 years anyway.

 

Having said all that I will re-visit my calculations and slow down its charge rate.

Edited November 26, 2010 by Robin2

[nb Innisfree]

The only reference material I have is (in no particular order) (i) The Sterling brochure charging graph, (ii) a document on charging that I got from Surette who make the Rolls batteries and (iii) the Boatowner's Mechanical and Electrical Manual by Nigel Calder. My calculations seem to be consistent with all three. If you have any other references I would appreciate links to them.

 

It is important to keep my objective in mind - the cheapest way to meet my electricity needs.

 

A lot of the theory seems to be about protecting the life of batteries without considering the

overall economic situation. For example saving 20 minutes of fuel usage each day (say 20p) would pay for a £75 battery in a year. The batteries are only a means to an end, and a very inefficient means at that. Also, I have become very sceptical about the life expectancy of batteries. I don't want to invest a huge amount (capital cost and charging fuel) only to find that the battery wears out after 2 years anyway.

 

Having said all that I will re-visit my calculations and slow down its charge rate.

 

My first source was Energy Unlimited well worth a read, and my second source Smartgauge these together with invaluable info from grumpy old Gibbo himself and other various contributors on this forum and my own 5 years experience living aboard have taught me the basics of battery charging. Like a lot of subjects teasing out the reality from lots of confusing and conflicting information is a long and difficult process (for instance optimistic advertising from battery manufacturers) some translate things differently and find themselves in trouble, depends how thorough and logical you are, for me Gibbo is very strong in these areas and my money is on him (even though he is sometimes a bit grumpy, bless him!)

 

Even authoritative sources disagree, for instance Energy Unlimited recommend partial state of charging with a monthly equalisation charge but my experience suggests this is insufficient, a weekly one being far more realistic.

Edited November 26, 2010 by nb Innisfree

[Alan Saunders]

I seem to remember a nicely argued suggestion that separating the batteries into two banks to achieve a full charge came up a year or two back and Gibbo said he would reconsider and modify his advice if it proved advantageous. No such assurance has been received.

 

I have tried a spreadsheet to compare the two options. Assuming 4x100Ah flooded cell batteries are cycled between 90% and 50% using 40Ah/day. N.B. In either case, all four batteries receive a charge every day (period); the previous table omitted the charge on 'day 5' (my 'day four') and commenced with 100% charge which is an exceptional case.

Discharging One (of Four) Batteries in Each Period.
Period:      0    1     2     3     4
Bat1 SOC%    90 50/60 60/70 70/80 80/90
Bat2 SOC%    80 80/90 50/60 60/70 70/80
Bat3 SOC%    70 70/80 80/90 50/60 60/70
Bat5 SOC%    60 60/70 70/80 80/90 50/60

Discharging Four Batteries in Parallel.
Period:      0    1     2     3       4
Bank SOC%    90   80    70    60    50/90

'/' = charging period.

So far as I can determine: (NB The available charge to the batteries will be reduced by demand from other sources, e.g. lights, 'fridge, TV etc.)

• If you prefer to charge frequently for a short time or your charger is of limited capacity the 'split scheme' may be advantageous.
  
• If you prefer to recharge infrequently (e.g. one or two long runs at the weekend) the 'parallel scheme' is better.
  
• Assuming that you are happy to charge at C/5 to C/3 the initial charge to the 'parallel scheme' can be much greater; 80A to 130A and therefore uses less fuel and incurs less engine wear.

 

The complications of a split bank are difficult to justify and reliability drops alarmingly with each added component. Simpler to size the battery bank and charger(s) to suit your circumstances.

 

The Victron Energy Book 'Energy Unlimited' is interesting; in particular with regard to the Peukert factor. Reinout Vader says "In fact a battery which has been discharged at a very high rate will recover over time and the remaining capacity can be retrieved after the battery has been left at rest for several hours or a day." This implies that a battery bank having occasional loads with intervening rest periods will supply exactly the quoted 20hr power. If this is true, batteries in a boat that is left all day (or all week) with little or no demand are not affected. A 100Ah battery will supply 5A continuously for 20hrs and, after a rest period, a few more Ahs. Whatever the demand, you need only put back what you took out plus an allowance (10%?) for inefficiency.

 

Reinot Vader also endorses Nigel Corder's book “Boatowners Mechanical and Electrical Manual” - I endorse that recommendation; it is definitive. Borrow or buy it now, £25 well spent.

 

I thought I was good at maths and arithmetic (my calculus is rusty) but this is making my brain hurt. I would rather take Gibbo's advice as gospel. As usual, especially with batteries and charging, it depends on your demands, cruising pattern and the price of lead, diesel and engine maintenance.

 

Finally, a battery supplier will usually consider a battery to have reached 'end of life' when its capacity is 80% of the original. I guess that most of us replace our batteries when they are <50% of original capacity, maybe <30% (i.e. when capacity = daily demand). So, doubling capacity from 1 x 110Ah to 2 x 110Ah means a 50% reduction in capacity results in the same capacity you had with one good, new battery. 110Ah is the cheapest per Ah but I recently found that my battery compartment can accomodate 135Ah batteries; same W x H as the 110Ah but 2cm higher. 20% more capacity and lifetime with no expensive modifications.

 

Alan Saunders

[blueprince]

Interesting thread, I run multiple banks, 4x4(16), and I use diesel, solar PV, and a wind genny to charge, I swap banks for charging and usually charge for 3/4 hours a day, my main drive is electric (two milk float motors) and I can get an hour or two just on PV in summer, I also use electrolyte swapping and a constant temp chamber for the batts, (it also moves to trim the boat.) I must admit it's a bit complicated, more so since I installed an expert system to run the charging, (I used manual switching till a year ago!) but it seems to work, I can't reduce charging times, but I can make sure that when the system is charging off diesel, all the horsepower is going to the batts, also pumping the electrolyte around gives about a 30% improvement. I dread to think what my next inspector will say, as it looks like I'm gearing up for a frankenstein moment. I'm also pretty sure that drilling holes in the casings has invalidated my warranty.

[Robin2]

Thanks Innisfree and Alan. Your extensive replies have got my mind working on some interesting new thoughts.

 

I think it is even more important to be clear about the question that needs to be answered. And I think there may be 2 different questions.

 

The "proper" question is like this. If I were building a brand new boat I could decide to have no domestic battery and just run my engine whenever I need electricity. (Ignore noise restrictions for now). This would be perfectly feasible but would use a lot of diesel. Now, if I installed some domestic batteries would I be able to meet my electricity need at lower total annual cost (batteries + diesel)? And if so, what arrangement of batteries and charging regime would minimize the annual cost.

 

The second (improper? but relevant) question is something like this. If I have a set of batteries which are functional but heavily worn by an inadequate charging regime which costs £X/year without counting the cost of battery depreciation what is the most economical way of improving the situation. And "improving" must mean keeping the future total annual cost (batteries + fuel) as low as possible.

 

It will probably be Sunday or Monday before I am prepared to put my conclusions in print.

 

 

In the meantime I would be interested to hear your views on these points which will probably be a part of my conclusions:

 

(i) What lifetime do you expect from your existing batteries (since purchase) using your existing charging regime?

 

(ii) If the weekly 8 hour charge was reduced to once every 2 weeks or once every 4 weeks how would the battery life expectancy change?

 

(iii) If the weekly 8 hour charge was reduced to 6 hours or 4 hours how would the battery life expectancy change?

 

Even an informed guess would be useful.

Edited November 27, 2010 by Robin2

[nb Innisfree]

Thanks Innisfree and Alan. Your extensive replies have got my mind working on some interesting new thoughts.

 

I think it is even more important to be clear about the question that needs to be answered. And I think there may be 2 different questions.

 

The "proper" question is like this. If I were building a brand new boat I could decide to have no domestic battery and just run my engine whenever I need electricity. (Ignore noise restrictions for now). This would be perfectly feasible but would use a lot of diesel. Now, if I installed some domestic batteries would I be able to meet my electricity need at lower total annual cost (batteries + diesel)? And if so, what arrangement of batteries and charging regime would minimize the annual cost.

 

The second (improper? but relevant) question is something like this. If I have a set of batteries which are functional but heavily worn by an inadequate charging regime which costs £X/year without counting the cost of battery depreciation what is the most economical way of improving the situation. And "improving" must mean keeping the future total annual cost (batteries + fuel) as low as possible.

 

It will probably be Sunday or Monday before I am prepared to put my conclusions in print.

 

 

In the meantime I would be interested to hear your views on these points which will probably be a part of my conclusions:

 

(i) What lifetime do you expect from your existing batteries (since purchase) using your existing charging regime?

 

(ii) If the weekly 8 hour charge was reduced to once every 2 weeks or once every 4 weeks how would the battery life expectancy change?

 

(iii) If the weekly 8 hour charge was reduced to 6 hours or 4 hours how would the battery life expectancy change?

 

Even an informed guess would be useful.

 

 

Crystal ball required here I think.

 

(i)Three and a half years old 8x125ah AGMs already well below half of original capacity which need 2hrs daily charge to meet 24 usage (70ah @24v) seem to be holding stable so anybody's guess how long they will last. didn't look after them properly earlier on.

 

(ii) Moderately worse?

 

(iii) Drastically I would imagine.

[Alan Saunders]

Crystal ball required here I think.

 

(i)Three and a half years old 8x125ah AGMs already well below half of original capacity which need 2hrs daily charge to meet 24 usage (70ah @24v) seem to be holding stable so anybody's guess how long they will last. didn't look after them properly earlier on.

 

(ii) Moderately worse?

 

(iii) Drastically I would imagine.

If it were possible to give a sensible estimate then Gibbo would not be busy destroying new batteries to get some answers.

 

So we only have rules of thumb, intuition and experience to rely on. Innisfree's batteries are doing well! I believe AGM's cannot be revived by overcharging, i.e. 'equalisation'. OTOH they are more tolerant to deep discharge.

It may be that leaving a battery to rest between charges reduces the length of the absorbtion charge but the only way to check that absorbtion is complete is to monitor the SG - not possible for a sealed battery. The best you can do is to monitor the capacity or internal resistance of the batteries and change the charging regime appropriately.

 

Two advantages of the split bank have just occurred to me:

1. When you make the big mistake, e.g. unable to recharge due to engine problems or leave the tunnel light on for a day or two, you will only wreck one battery/bank.
   
2. Since you have the ability to isolate individual banks it will be easy to find a poor cell that is dragging all banks down.

 

I am still learning about battery charging. For instance what does 'bulk' and 'acceptance' mean? Why are 80%, 90% and 95% SOC significant? I now believe that the only significant SOC is (depending on battery chemistry) ~95% when acceptance drops to 1% > 3% C (or ~half DOD), 'absorbtion'. 'Acceptance' means that however hard you try the battery will only accept a certain current, maybe equal to the DOD? Applying a higher voltage will only increase the temperature of the battery. 'Bulk' means that the battery is being charged at less than the maximum current that it could 'accept'.

 

My last set of batteries, cheap flooded cell, were discharged to <20% several times and otherwise recharged daily from ~50% to 80% with no absorbtion charges unless I was cruising. After a year they were probably about 30% of original capacity and then failed. I needed to run the engine faster to maximise the charge voltage. In discharge they quickly dropped to <12V but would go on supplying >10V (minimum for TV) for many hours.

 

I believe we understand the problem; to optimise the total cost of batteries, fuel and maintenance. The real cost of running the engine, not just diesel, say £1/hr far exceeds the cost of replacement batteries. By omitting the absorbtion charge I may have saved £400 last year 2x 135Ah batteries (70Ah/day @ 12v in winter) cost me £220. Maybe Innisfree could manage with 4x 125Ah batteries?

 

Most of us believe the answer is to have a single battery bank with a capacity of 2x > 3x the demand between charges. Recharge from ~50% to ~80% and every x charges add an extra 8hrs. In practice I might get to the boat at 7pm and only charge for an hour, charge for 2hrs the next morning. Sometimes I can go 48hrs between charges. Occasionally I am out all day from 8am to 10pm and the batteries are only 40% the next morning.

 

I believe I could (did?) reduce costs by abusing the batteries but I don't want the inconvenience of replacing them more often.

 

Alan

[Nickhlx]

Surely this weather ( when its under approx 3 deg C outside) people can / could / do switch the fridge off and put food that needs to be refrigerated in a suitable container outside, to protect it from wildlife, like in the old days when houses had "larders" or small rooms adjoining the kitchen with a couple of windows made from fine mesh that were pretty cool in the winter months ? The roof or fore / aft decks might be coolest, and if you have a cratch, under there a bit warmer, or at least not quite as cold... that would save having to generate... and with led lights, no fridge and low e.g TV etc use, a minimal consumption of amp hours per day if you are out for long periods....

 

Nick

[Robin2]

I've been through a few iterations of my ideas before settling on what's here. In fact it was only yesterday that I recalled an important experience that is pushing me towards a multi-battery solution.

 

My experience is that a battery needs a 3 hour charge between discharges if its SoC is not to decline in a one-step-forwards two-steps-back manner. Maybe I am unlucky with my batteries but 18 months ago I replaced the leisure batteries that were on Leopard when I bought it. I expected the new golf-buggy batteries to reduce charging times and save fuel but they behaved much the same asthe old ones. What's especially irritating is that after the first hour the batteries only accept 10 amps or so which is a very inefficient use of fuel.

 

To cut a long story short:

 

If I were to generate power as needed and have no domestic batteries lets assume I would need engine runs of 8 hours per day for 6 months in winter and 5 hours per day in summer. At 0.9 litres per hour and £0.75 per litre that would be an annual cost of £1603.

 

If I installed a 200ah battery (2 x £75) and replaced it every 2 years I could reduce my daily engine run to 3 hours plus a 5 hour run once every 3 weeks (Surette, the makers of the Rolls batteries say once every 30 days is enough). The total annual cost would reduce to £873 - as expected, batteries are a good investment.

 

Now how could I get the annual cost lower still?

 

If I doubled the size of my battery bank the batteries would last longer - say 5 years (i.e. more than double) and with no change in the charge regime the annual cost would be £858 - a further annual saving of £15.

 

But suppose I treat the new batteries as a second bank and alternate between the two each day. My battery life should be double the single battery case and I could reduce my daily charge to 1.5 hours (the batteries would still have 3 hours charge between uses). So the annual cost would be £504. This is £354 cheaper than using the batteries as a single bank.

 

And if I had a third battery bank I could reduce my daily charge to 1 hour. Even if I assumed the extra batteries did not extend battery life the annual cost would be £419 saving a further £85 per year.

 

I do hope my arithmetic is correct (You should have all the data you need if you wish to check it)

 

****************

 

Obviously daily switching between battery banks takes some effort of will or memory but the physical switching can be simple.

 

And apart from the economics, keeping the banks separate has these advantages:-

 

(i) the batteries have time to settle - which seems to be universally considered beneficial.

 

(ii) it is possible to monitor their state while settled, whether by voltage or specific gravity.

 

(iii) it is unlikely that all three banks would suffer a major failure at one time.

 

(iv) two or more banks could occasionally be coupled - e.g. if one wanted to skip a day's charging.

 

(v) a higher-voltage equalisation charge could be applied to an off-line bank without risk to appliances or inconvenience to one's lifestyle.

 

(vi) it would be possible to compare the performance of different battery brands in different banks.

 

(vii) when it is necessary to replace batteries they would not all need to be replaced at the same time.

 

************

 

All comments will be very welcome ...

Edited November 28, 2010 by Robin2

[nb Innisfree]

Good luck

[Robin2]

Good luck

 

I'm not sure if that means you agree, you disagree, or you don't care

[Chris Pink]

I'm not sure if that means you agree, you disagree, or you don't care

 

I think it means that the range of potential error in your calculations is greater than the range of savings due to the number of variables omitted to arrive at them therefore the only person who will ever know is you once you have gone through each possibility is yourself.

 

Please report back on your findings in 10 years time.

 

 

I am still learning about battery charging. For instance what does 'bulk' and 'acceptance' mean? Why are 80%, 90% and 95% SOC significant? I now believe that the only significant SOC is (depending on battery chemistry) ~95% when acceptance drops to 1% > 3% C (or ~half DOD), 'absorbtion'. 'Acceptance' means that however hard you try the battery will only accept a certain current, maybe equal to the DOD? Applying a higher voltage will only increase the temperature of the battery. 'Bulk' means that the battery is being charged at less than the maximum current that it could 'accept'.

 

 

 

bulk is terminal voltage below charger regulation (eg 14.3V) so current limited by charger (to roughly 80% SOC)

acceptance is terminal voltage at charger regulation voltage so current limited by battery internal resistence (80-100% SOC)

 

charging in acceptance will always be at a lower current than bulk - and it diminishes as SOC increases and not linearly. Gibbo's analysis shows that the total time taken to charge a set of batteries to 100% has a theoretical minimum (8-10 hours) as the fast the bulk charge the slower the acceptance (massively over-simplified explanation - it's around somewhere).

 

My take is that the maths revolves around a balance between keeping your charging within bulk and battery life (cost). This is missing from both your analysis earlier and Innisfree's figures.

[Sir Nibble]

Still no observations from Gibbo. That's a shame.

 

Tony

I may be wrong but I seem to recall that Gibbo's take on this was along the lines of "It won't work it can't work wishfull thinking is not science". However, such is the lure of the thing that it keeps going round and who wants to spend their lives constantly repeating themselves to those who do not listen?

I hope I do not misrepresent the bloke but I doubt he will engage with this having already proved it's rubbish, even if it's proponents have subsequently managed to prove it works by the repeal of mathematical and physical laws.

[Robin2]

I may be wrong but I seem to recall that Gibbo's take on this was along the lines of "It won't work it can't work wishfull thinking is not science". However, such is the lure of the thing that it keeps going round and who wants to spend their lives constantly repeating themselves to those who do not listen?

I hope I do not misrepresent the bloke but I doubt he will engage with this having already proved it's rubbish, even if it's proponents have subsequently managed to prove it works by the repeal of mathematical and physical laws.

 

So where is my maths faulty?

[Sir Nibble]

 

All comments will be very welcome ...

Nothing wrong with the maths as far as I can see, it's just based upon a collection of false premises. I really cannot be bothered to go into it deeper.