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Splitting a battery bank

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Not trying to invent the wheel but.....

Say, you have a domestic battery bank of 3 batteries - which is big enough for your electrical needs, would it be worth installing 4 and isolating 1 battery in turn out of the bank every 7 days to be fully charged for a week on it's own smaller solar panel?

This would ensure each battery of the bank would get 100% fully charged for a few days every month.

There must be times when normal battery banks don't actually get a regular 100% charge, say, when CCing.

Does this make sense and do you think it would increase lead-acid battery lifetime by the 25+% needed to make it financially viable? Or am I  just thinking too much again?

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3 hours ago, smiler said:

There must be times when normal battery banks don't actually get a regular 100% charge, say, when CCing.

Surely there is more chance of a battery being fully charged by a CCer than by a 'weekender' (unless you really means a CMer / Bridge hopper)

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3 hours ago, smiler said:

Say, you have a domestic battery bank of 3 batteries - which is big enough for your electrical needs, would it be worth installing 4 and isolating 1 battery in turn out of the bank every 7 days to be fully charged for a week on it's own smaller solar panel?

 

The advantage of that approach over just adding a fourth battery and keeping them all in service all the time will be pretty marginal - and a lot less trouble.

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2 hours ago, smiler said:

Not trying to invent the wheel but.....

Say, you have a domestic battery bank of 3 batteries - which is big enough for your electrical needs, would it be worth installing 4 and isolating 1 battery in turn out of the bank every 7 days to be fully charged for a week on it's own smaller solar panel?

This would ensure each battery of the bank would get 100% fully charged for a few days every month.

There must be times when normal battery banks don't actually get a regular 100% charge, say, when CCing.

Does this make sense and do you think it would increase lead-acid battery lifetime by the 25+% needed to make it financially viable? Or am I  just thinking too much again?

 

But its up to you to organise things so the batteries are charge to pretty much 100% at least once a week. I suspect the less discharge % on a four batter bank plus extra solar would do as much to optimise the batteries life. Given typical battery location and wiring I suspect the disconnecting and connecting may be time consuming.

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Thanks for confirming, Tony - that's pretty much what I was getting at, but I confess was focussed on the general buggerance factor of faffing about with battery connections. even if it was only going to involve throwing switches every so often.

4 minutes ago, Loddon said:

Popcorn anyone ?

Don't like the stuff. Packet of crisps any time!

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10 hours ago, Machpoint005 said:

Thanks for confirming, Tony - that's pretty much what I was getting at, but I confess was focussed on the general buggerance factor of faffing about with battery connections. even if it was only going to involve throwing switches every so often.

Don't like the stuff. Packet of crisps any time!

 

That's the point - switches in the plural. It would demand bus bar style interconnects on the positive with a switch on each and the same for the feeds from the charging source.

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2 hours ago, Tony Brooks said:

 

That's the point - switches in the plural. It would demand bus bar style interconnects on the positive with a switch on each and the same for the feeds from the charging source.

Four good quality high current switches would be very expensive too. Using the horrible cheap and nasty red key ones would give intermittent and unpredictable batteries dropping out. Unbolting and reconnecting link cables would be very annoying and time consuming, with a small, but real risk of accidentally shorting out the bank with an errant cable, or dropped spanner each time you do it.

Jen

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I think this is a bad idea......for all of the reasons above and especially the horrid switching arrangement that would be needed which could fund another battery to make it 5 in the bank!

I'd like to throw in 2 other inputs.

First, why are you thinking of this idea? If it is that you are not charging your batteries properly now, then how is this going to make it better? One battery might get charged properly once per month (but maybe not Nov – Feb!) but the other 3 will have 3 weeks where you are not taking them to full – because your 'ace' plan of charging one every month is good. 3 weeks is too long time without full charge and sulphation will take place. See below. You will get lulled into a false sense of security - I dont have to worry about my batteries not getting to 100% as they will be fully charged by solar in 3 weeks time!

Secondly, there is a technical input. Now, I am not a battery expert, but I am a trained chemist and see a big problem. This isnt something I have seen written about, but as a chemist I understand chemical reactions and what makes them happen and battery chemistry is no different. I would happily debate the following with anyone.

A typical lead acid battery is designed with it's plates having a very large surface area. The surface area is increased by having 'pores' in the plates. The production of electrons from the plates is a surface reaction and needs chemical species to travel to that 'site' on the surface or deep in the pores to complete that reaction. On discharge from full, the first electrons appear as a surface charge. It is very easy for each 'site' on the 'easy to get at' surface to give up its electron in the chemical reaction. Once the surface charge is gone then the next 10% of charge out of the battery comes from sites very near the tops of the pores. Lets call them the 90% sites. When the next 10% charge comes out (so the SoC is dropping from 90% to 80%), it is more difficult for the chemical reaction to happen as the reaction is likely in the pores themselves and some of the energy released is needed to provide the activation energy to make the electron releasing reaction take place. (Is this why the voltage drops with SoC????). Anywho, this goes on down until we get to say 50% SoC. By this time, the chemical reaction is taking place half way down the pores so steric hinderance is a key issue and the reaction is much slower needing a bigger activation energy for the 50% sites. Ok that is discharge.

We now charge back up. Which sites get converted back first? The 99% or 100% sites on the surface or the 50% sites deep down the pores?

From a chemical point of view it is certain it is the 100% sites as they are not sterically hindered and need the least activation energy to turn the lead sulphate back to lead. Once they are full, the charge then does the 90% sites, then the 80% sites and so on. The last sites to be converted as the SoC approaches 100% are the 50% sites deep in the pores and this is why everyone says you must charge with 14.4V. You need the high voltage to provide the high activation energy. Charging the easy to get at sites which give up their electrons easily when discharging from full, charge back up easily with just over 13 Volts unlike the sites deep in the pores.

How does this affect my charging then? Well in the OPs case of discharging 3 batteries and only getting up to 100% charge on one offline battery every 4 week, then discharging to 50% and only say getting back to 95% means that the 5% deficit of sites deep in the pores are not going to see a charge until their turn for the 4 week solar boost and will certainly form a hard sulphate deposit that will be very very difficult to remove. Is solar really going to hack it and get the high volatage in to convert that sulphate?

Me?, I'd sooner invest in the 4th battery to make my bank bigger so instead of discharging to 50%, I would only be discharging to 63% (ish) and maybe spend all the money you are using on switches to get 5 batteries so discharging to only 70%. All the battery companies show the number of cycles to death increasing for less discharge but these figures usually do not take into account sulphation. The need for higher activation energies for the sterically hindered sites deep in the pores though means that they must be more difficult to react so sulphation will be a key input. Put the biggest bank in you can for longest life. You've got more chance of avoiding sulphation by not discharging as deeply therefore needing less energy to recharge that last 1% of capacity deep in the pores.

Problem is, have you got room for 5 batteries?

Who's got the popcorn?

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6 minutes ago, Dr Bob said:

I think this is a bad idea......for all of the reasons above and especially the horrid switching arrangement that would be needed which could fund another battery to make it 5 in the bank!

I'd like to throw in 2 other inputs.

First, why are you thinking of this idea? If it is that you are not charging your batteries properly now, then how is this going to make it better? One battery might get charged properly once per month (but maybe not Nov – Feb!) but the other 3 will have 3 weeks where you are not taking them to full – because your 'ace' plan of charging one every month is good. 3 weeks is too long time without full charge and sulphation will take place. See below. You will get lulled into a false sense of security - I dont have to worry about my batteries not getting to 100% as they will be fully charged by solar in 3 weeks time!

Secondly, there is a technical input. Now, I am not a battery expert, but I am a trained chemist and see a big problem. This isnt something I have seen written about, but as a chemist I understand chemical reactions and what makes them happen and battery chemistry is no different. I would happily debate the following with anyone.

A typical lead acid battery is designed with it's plates having a very large surface area. The surface area is increased by having 'pores' in the plates. The production of electrons from the plates is a surface reaction and needs chemical species to travel to that 'site' on the surface or deep in the pores to complete that reaction. On discharge from full, the first electrons appear as a surface charge. It is very easy for each 'site' on the 'easy to get at' surface to give up its electron in the chemical reaction. Once the surface charge is gone then the next 10% of charge out of the battery comes from sites very near the tops of the pores. Lets call them the 90% sites. When the next 10% charge comes out (so the SoC is dropping from 90% to 80%), it is more difficult for the chemical reaction to happen as the reaction is likely in the pores themselves and some of the energy released is needed to provide the activation energy to make the electron releasing reaction take place. (Is this why the voltage drops with SoC????). Anywho, this goes on down until we get to say 50% SoC. By this time, the chemical reaction is taking place half way down the pores so steric hinderance is a key issue and the reaction is much slower needing a bigger activation energy for the 50% sites. Ok that is discharge.

We now charge back up. Which sites get converted back first? The 99% or 100% sites on the surface or the 50% sites deep down the pores?

From a chemical point of view it is certain it is the 100% sites as they are not sterically hindered and need the least activation energy to turn the lead sulphate back to lead. Once they are full, the charge then does the 90% sites, then the 80% sites and so on. The last sites to be converted as the SoC approaches 100% are the 50% sites deep in the pores and this is why everyone says you must charge with 14.4V. You need the high voltage to provide the high activation energy. Charging the easy to get at sites which give up their electrons easily when discharging from full, charge back up easily with just over 13 Volts unlike the sites deep in the pores.

How does this affect my charging then? Well in the OPs case of discharging 3 batteries and only getting up to 100% charge on one offline battery every 4 week, then discharging to 50% and only say getting back to 95% means that the 5% deficit of sites deep in the pores are not going to see a charge until their turn for the 4 week solar boost and will certainly form a hard sulphate deposit that will be very very difficult to remove. Is solar really going to hack it and get the high volatage in to convert that sulphate?

Me?, I'd sooner invest in the 4th battery to make my bank bigger so instead of discharging to 50%, I would only be discharging to 63% (ish) and maybe spend all the money you are using on switches to get 5 batteries so discharging to only 70%. All the battery companies show the number of cycles to death increasing for less discharge but these figures usually do not take into account sulphation. The need for higher activation energies for the sterically hindered sites deep in the pores though means that they must be more difficult to react so sulphation will be a key input. Put the biggest bank in you can for longest life. You've got more chance of avoiding sulphation by not discharging as deeply therefore needing less energy to recharge that last 1% of capacity deep in the pores.

Problem is, have you got room for 5 batteries?

Who's got the popcorn?

Thanks Dr B. Very nicely explained. I may well crib that when appropriate.

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10 minutes ago, Tony Brooks said:

Thanks Dr B. Very nicely explained. I may well crib that when appropriate.

Tony, Thanks for the nice comment but remember I am not a battery expert.

My note above was based on my theoretical knowledge of chemical reactions and you cant argue with activation energies in this case. It would be interesting to hear other views on this ....but I think I am right.

I also do have a bit of experience now with the way my Lithium batteries interact with the Lead acids they are paralleled up to. It is really interesting to watch how easy it is for the LAs to give up their surface charge when a large load is applied (despite the Li's being at a higher potential) and equally interesting to see how that surface charge is replaced with just 13.1V (or less) from the Li's. I often see the case where the LAs are depleted by 5% charge and this goes back in very quickly.....and far more quickly than the last 5% of charge when you have taken the LAs down to 50% and bring them up again.

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One reaction that you missed out (although not immediately pertinent to the point you were making) is pore clogging, which occurs rapidly with a high discharge current. The pores get clogged with sulphate and the only way to remove that sulphate is with a 14.4V+ charge. It won’t recover on its own. That is one of the reasons why you have to put more energy back into the battery after a fast discharge than you do after a slow discharge. Yet another argument for having as many batteries as practical. 

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1 hour ago, WotEver said:

One reaction that you missed out (although not immediately pertinent to the point you were making) is pore clogging, which occurs rapidly with a high discharge current. The pores get clogged with sulphate and the only way to remove that sulphate is with a 14.4V+ charge. It won’t recover on its own. That is one of the reasons why you have to put more energy back into the battery after a fast discharge than you do after a slow discharge. Yet another argument for having as many batteries as practical. 

Oooo - never knew that  - but then I don't have LA batteries (I use an older technology....)

 

It show that the only way is to have an adequate battery bank size and a proper charging regime. The cost of an extra battery in a bank is small / affordable when you see folks rattling on about how much data they use on mobile contracts.....

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1 hour ago, WotEver said:

pore clogging, which occurs rapidly with a high discharge current

How much is "high" in this context? On, say, a typical 110Ah leisure battery.

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4 minutes ago, Keeping Up said:

How much is "high" in this context? On, say, a typical 110Ah leisure battery.

4 feet 7 inches.

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6 minutes ago, TheBiscuits said:

 

Please tell!  

 

https://en.wikipedia.org/wiki/Lead–acid_battery

The lead–acid battery was invented in 1859 by French physicist Gaston Planté and is the earliest type of rechargeable battery. 

I believe my technology was invented / perfected by a Mr. T. Edison.

I should have said "made the technology commercially viable" or some such.

You may guess and deride, but my cells have done the jobe for me with the same set for 20+ years...

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4 minutes ago, OldGoat said:

I believe my technology was invented / perfected by a Mr. T. Edison.

I should have said "made the technology commercially viable" or some such.

You may guess and deride, but my cells have done the jobe for me with the same set for 20+ years...

 

NiFe then.  No derision, I was just intrigued by your comment. 

 

If they are still doing the job after 20+ years you'll not get any mockery from me.  Do you have to do anything complicated with them in high or low temperatures?

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6 minutes ago, TheBiscuits said:

 

NiFe then.  No derision, I was just intrigued by your comment. 

 

If they are still doing the job after 20+ years you'll not get any mockery from me.  Do you have to do anything complicated with them in high or low temperatures?

I do get some stick (not that I mind...) when I mention them..

High temperature problems - I don't think so,  but last week was the first time we've been down to the boat for months (lockdown and the heat wave) and several things  behaved in a peculiar fashion even after routine checks.

The engine boiled

The ancient fridge was a bit dodgy

The fresh water pump started to leak.

 

The battery bank (50 cells giving 500 A/H at 24v charged OK but we didn't go far away from shore power)

 

Apart from the outside heat, the electolyte being a 25% (?) alkaline solution isn't going to freeze in a hurry and the batteries are below the waterline where the Thames never goes below 4 degrees C.

 

Short answer - complication - No.  Given that the capacity is more than adequate for a leisure user's needs and are not stressed they hardly ever need topping up.

They're just large and heavy and have a high charge voltage, thus I can only use 19 cells in each bank.

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1 hour ago, Keeping Up said:

How much is "high" in this context? On, say, a typical 110Ah leisure battery.

C/10 instead of C/20 will demonstrate it. It’s only slight but it’s there. You also lose energy by heating the battery more on a higher discharge. 

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Thanks for all the replies folks.

Some interesting and very informative posts.

I guess I've been thinking too much.😉

I was wondering about when CCing, say every other day for 2 or 3 hours then the batteries will get most of the bulk charge in from the engine, then relying on solar to fill the last, say 10-20% durimg the rest of the day. My question was on grey days would the batteries really get to 100% SOC bearing in mind still using inverter, fridge etc while there is relatively low solar power coming in.

I thought isolating the one battery would at least ensure each battery got to 100% at least once, twice or maybe three times a month(depending on routine).

Like I said maybe I'm overthinking a problem that doesn't exist.

Edited by smiler

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Physically juggling battery connection just to do this sounds like a faff and an unproductive use of time. Do a bit more work instead and get that extra battery as advised. :)

 

On a related point... is two weeks between 100% charges going to knacker my lead acids? I've been tear-arsing it across England all summer but when I'm static I only do a couple of hours charging a day.  Every fortnight I go into a marina to do my washing and get 24h on hookup which I assume probably gets the batteries to 100%. I have a big 4 panel solar array going on in a couple of weeks which should reduce my concerns but I rather suspect a new set of batteries will be needed before too long. 

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10 minutes ago, The Gravy Boater said:

is two weeks between 100% charges going to knacker my lead acids?

They’ll get knackered sooner or later whatever you do. Fully charging them only fortnightly will simply hasten that a bit. You’re right that solar will make a huge difference. 

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