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Battery Charging Analogy


Richard10002

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Just saw this in a facebook post - I particularly like the sponge analogy bit near the end...

 

"

For those who find understanding batteries and charging confusing. I found this and thought it might help.

Battery Charging
As we plan to have a large (and expensive) domestic battery bank I’ve been reading about charging lead acid batteries. No equipment lasts forever, but I do want to protect our investment. The first thing I’ve identified is batteries can be damaged if they are discharged more than 50%. Next, it appears recharging can be quite complex. I’m going to explain my understanding of the problem and solution using an analogy. Readers with more experience are welcome to correct me by leaving a comment.

Consider the battery bank to be a water tank. Water (electricity) is drawn off and the tank gradually empties. The tank shouldn’t be emptied more than 50%.

When the tank has to be refilled the water can either be trickled in or poured in under pressure. Obviously the more pressure and volume of water being poured into the tank then the quicker it will fill (with electricity, volts = pressure and amps = quantity). However if there is too much pressure or too large a quantity of water the tank might burst. In a battery the electrolyte can boil, the plates warp or the case split. So too much, too quickly, can be a bad thing. Too little may result in the process taking too long or the tank not being filled to maximum capacity.

However the water tank isn’t an empty tank. It’s actually a tank filled with a sponge. In the initial stages of refilling the tank the sponge readily and quickly absorbs the water. Eventually it will reach a saturation point where it doesn’t matter how much pressure and volume of water you attempt to force into the tank it simply can’t be absorb in that volume or pressure. Nevertheless there are pockets of air in the sponge, so the tank still hasn’t been filled. But if we are prepared to be patient and trickle a small amount of water into the tank it will all gradually be absorbed until the sponge is completely saturated. If we don’t completely fill the sponge in the tank then those dry pockets will eventually become permanently unavailable and the total capacity of the tank will be reduced. This is a cumulative process until the tank is essentially worthless.

So our battery bank is going to need a large amount of electricity to recharge the batteries at the beginning of the process and then a “trickle” charge to complete the final top up. Running the engine and alternators during this latter stage is a waste of energy (diesel). For the top up stage we need to consider either using shore power or perhaps solar panels. Alternatively, we can accept the battery life will be reduced through failing to fully recharge the batteries."

 
 
 
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8 hours ago, Richard10002 said:

Just saw this in a facebook post - I particularly like the sponge analogy bit near the end...

 

"

For those who find understanding batteries and charging confusing. I found this and thought it might help.

Battery Charging
As we plan to have a large (and expensive) domestic battery bank I’ve been reading about charging lead acid batteries. No equipment lasts forever, but I do want to protect our investment. The first thing I’ve identified is batteries can be damaged if they are discharged more than 50%. Next, it appears recharging can be quite complex. I’m going to explain my understanding of the problem and solution using an analogy. Readers with more experience are welcome to correct me by leaving a comment.

Consider the battery bank to be a water tank. Water (electricity) is drawn off and the tank gradually empties. The tank shouldn’t be emptied more than 50%.

When the tank has to be refilled the water can either be trickled in or poured in under pressure. Obviously the more pressure and volume of water being poured into the tank then the quicker it will fill (with electricity, volts = pressure and amps = quantity). However if there is too much pressure or too large a quantity of water the tank might burst. In a battery the electrolyte can boil, the plates warp or the case split. So too much, too quickly, can be a bad thing. Too little may result in the process taking too long or the tank not being filled to maximum capacity.

However the water tank isn’t an empty tank. It’s actually a tank filled with a sponge. In the initial stages of refilling the tank the sponge readily and quickly absorbs the water. Eventually it will reach a saturation point where it doesn’t matter how much pressure and volume of water you attempt to force into the tank it simply can’t be absorb in that volume or pressure. Nevertheless there are pockets of air in the sponge, so the tank still hasn’t been filled. But if we are prepared to be patient and trickle a small amount of water into the tank it will all gradually be absorbed until the sponge is completely saturated. If we don’t completely fill the sponge in the tank then those dry pockets will eventually become permanently unavailable and the total capacity of the tank will be reduced. This is a cumulative process until the tank is essentially worthless.

So our battery bank is going to need a large amount of electricity to recharge the batteries at the beginning of the process and then a “trickle” charge to complete the final top up. Running the engine and alternators during this latter stage is a waste of energy (diesel). For the top up stage we need to consider either using shore power or perhaps solar panels. Alternatively, we can accept the battery life will be reduced through failing to fully recharge the batteries."

 
 
 

These points seems to contradict each other, although in the final paragraph shore power and solar panels are mentioned which charge for an indefinite period.

Edited by blackrose
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1 hour ago, blackrose said:

These points seems to contradict each other, although in the final paragraph shore power and solar panels are mentioned which charge for an indefinite period.

The problem with describing lead acid battery charging is that it doesn't lend itself very well to any analogy. The 'sponge' analogy is often used, but it falls down in several areas.

Another analogy that works (but again, not very well) is to imagine a very wide but very shallow bucket with a small hole in it. This bucket is made of a product that slowly dissolves into the water when exposed to the air. The small hole represents self-discharge. The 'slowly dissolving stuff' represents sulphation and general battery erosion. If you fill the bucket too fast, after the first hour or so the water just splashes out. If you fill the bucket too slowly you don't wash away the sulphation and might not even counter self-discharge. If you fill the battery 'just right', starting fast and then slowing down as time progresses you will wash away the sulphation and fill the bucket to the brim - but the last little trickle takes a long time. As I said, still not a great analogy. 

Edited by WotEver
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Why not just spend a bit of time watching a voltmeter and ammeter during charge and discharge.

There's plenty of charts and stuff on google, all worth a look, but nothing beats a bit of real life observation, surely.

I s'pose some people insist 'it's all too complicated' or even that 'technical stuff is above/beneath them' but we all have to start somewhere... :)

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2 hours ago, smileypete said:

Why not just spend a bit of time watching a voltmeter and ammeter during charge and discharge.

There's plenty of charts and stuff on google, all worth a look, but nothing beats a bit of real life observation, surely.

I s'pose some people insist 'it's all too complicated' or even that 'technical stuff is above/beneath them' but we all have to start somewhere... :)

I think it needs a combination of real life experience, (watching the dials), and explanation from others, (why reinvent the wheel?).

 

If you put someone brand new to boating and batteries, and sat them in front of the dials 24/7, (or even once an hour for a few days, other than sleep), they would probably know as much about batteries and charging at the end of any period as they did at the start - nothing :)

I've been destroying batteries since 2006. I've watched dials, monitors, (NASA BM2 and Smartguage), read loads of stuff on this forum and ybw, and by various experts, and have only really just become aware of what might be the secret... it might even have been yourself and MtB who helped me to what might have been my lightbulb moment :)

When I read the sponge thing, I related to it immediately - particularly the bit about not filling it full enough, often enough, such that the empty spaces become useless.

Just thought it might help someone else to understand better, and I'm reasonably sure it will :)

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3 hours ago, smileypete said:

Why not just spend a bit of time watching a voltmeter and ammeter during charge and discharge.

There's plenty of charts and stuff on google, all worth a look, but nothing beats a bit of real life observation, surely.

I s'pose some people insist 'it's all too complicated' or even that 'technical stuff is above/beneath them' but we all have to start somewhere... :)

He lost me with his insistence that discharging below 50% SOC would 'cause damage' rather than being a point on a DOD versus cycles graph.

Like you I struggle with water analogies, why not describing a water system with an analogy of electricity? Plumbing, head, pressure and flow restriction are just as complicated when looked at in anything other than a superficial way. 

There's also no place in this analogy for the most important battery charging fact; that if you leave them partially discharged then 'damage' will occur. 

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36 minutes ago, hounddog said:

He lost me with his insistence that discharging below 50% SOC would 'cause damage' rather than being a point on a DOD versus cycles graph.

Like you I struggle with water analogies, why not describing a water system with an analogy of electricity? Plumbing, head, pressure and flow restriction are just as complicated when looked at in anything other than a superficial way. 

There's also no place in this analogy for the most important battery charging fact; that if you leave them partially discharged then 'damage' will occur. 

He didnt "insist", he said they "can be damaged if discharged below 50%", (and can doesn't mean will ), so what he says seems OK to me.

On partial discharge, he says that, if you don't fill all the holes in the sponge, they eventually become useless.....

If it doesnt help you, or anyone, that's fine... I posted it because it might help "someone". It certainly helped me :)

Given what MtB and myself learned a few weeks ago, he could have made it, "if you dont fill all the holes in the sponge regularly, they eventually become useless. Regularly means more often than weekly, and daily where possible".

My own rules are now:

Try not to discharge below 50%, (but it's not critical if you do). It's OK to rely on Smartguage to monitor this percentage.

Charge up regularly after any use, to as full as you can, (daily if possible, and no longer than 2 or 3 days). It is not OK to rely on Smartguage to monitor this percentage.

At least every 3 days of use, charge up fully, such that the Amps drawn are 2% of capacity, or less. 

(I probably do other things subconciously but cant think what they are right now).

 

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9 minutes ago, Richard10002 said:

He didnt "insist", he said they "can be damaged if discharged below 50%", (and can doesn't mean will ), so what he says seems OK to me.

Any lead acid battery WILL be damaged by ANY discharge, no matter to what depth. If it didn't happen they'd last forever if well looked after. All that happens with a discharge to below 50% SoC, as Hounddog  said, is the point on the cycles graph moves along a bit. So it's even more incorrect to say 'may' than 'will'.

The 50% 'knee' on the graph is also not set in stone for all batteries - differrnt batteries have the knee in a different place. Trojan for instance appear to have it at around 40% SoC for T105s. 

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2 hours ago, Richard10002 said:

Given what MtB and myself learned a few weeks ago, he could have made it, "if you dont fill all the holes in the sponge regularly, they eventually become useless. Regularly means more often than weekly, and daily where possible".

So how is that different to 'charge hard to 80% then carry on charging slowly to 100%'? No analogy needed. Especially in these days of 40A chargers and solar power. The reality is, if you don't live aboard the 2nd stage is easy, if you do you'll never complete it. 

Seems easier to understand to me. I'm not quite sure how i fill a sponge fast to 80% and then why do i need to trickle the rest? and why will it clog up if i don't fill it straight away? Confusing. 

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As for the '50% rule'; most people's usage is reasonably predictable so 'charge to 80% every 2nd morning  (or whatever interval you need) - when most people charge their batteries whether by cruising or generator - is much more useful than discharge up 50% - which could be in the middle of the night. 

Sound advice is to supplement with a weekly long charge and a monthly equalise, but i can never be arsed with that, solar seems to fill the gaps nicely. 

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45 minutes ago, hounddog said:

As for the '50% rule'...

... it's widely misunderstood. 

The 'rule' such as it is, states that for best battery life with 'standard' LA batteries (whatever they are) you should try to avoid discharging much below 50%. Discharging to 65% is better. Discharging to 70% is better still. 

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5 hours ago, Richard10002 said:

I think it needs a combination of real life experience, (watching the dials), and explanation from others, (why reinvent the wheel?).

If you put someone brand new to boating and batteries, and sat them in front of the dials 24/7, (or even once an hour for a few days, other than sleep), they would probably know as much about batteries and charging at the end of any period as they did at the start - nothing :)

I've been destroying batteries since 2006. I've watched dials, monitors, (NASA BM2 and Smartguage), read loads of stuff on this forum and ybw, and by various experts, and have only really just become aware of what might be the secret... it might even have been yourself and MtB who helped me to what might have been my lightbulb moment :)

OK just a little test for you and anyone else interested, tell me what happens to the volts and amps while a batt is being charged....?

And how long does it take to form this observation? :)

Edited by smileypete
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1 hour ago, WotEver said:

... it's widely misunderstood. 

The 'rule' such as it is, states that for best battery life with 'standard' LA batteries (whatever they are) you should try to avoid discharging much below 50%. Discharging to 65% is better. Discharging to 70% is better still. 

It depends, it's not that simple, which is why the '50% rule' emerged. 

For instance, if you discharge to 75% and charge every day you may have, for instance, 350 cycles before your battery needs replacing, if you discharge to 50% then maybe 170 cycles. But need to charge every two days so the life is the same. By looking at the graph you may find discharge to 30% gives only 100 cycles but you only charge every 3 days. Assuming your charging has no cost obviously  charging every day from 75% is cheapest but really each charge has a cost and the added inconvenience may have a 'cost' too. 

Personally I'm a 30% 3 day kind of person. As i can count on one hand the sets of batteries i've been through in 30 years it works for me. 

An interesting fact is that the time spent charging in my examples will be roughly the same because charging from 75% the current drops much faster. 

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3 hours ago, hounddog said:

Sound advice is to supplement with a weekly long charge and a monthly equalise, but i can never be arsed with that, solar seems to fill the gaps nicely. 

That's the advice that MtB and I have been following, and destroying our batteries... weekly isn't often enough. With enough solar, it's possible/likely that they are being topped up enough all the time.

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On 02/07/2017 at 18:35, hounddog said:

 

Sound advice is to supplement with a weekly long charge and a monthly equalise, but i can never be arsed with that, solar seems to fill the gaps nicely. 

 

Part time liveaboard presumably then.

In the three winter months when battery use is highest, solar is effectively useless in my experience. 

Or is your experience otherwise? If so can you describe your solar installation in full detail please as I'd like one the same. Thanks!

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14 hours ago, WotEver said:

Well yes, I didn't mean to only discharge to 70% SoC every time. If that's all you're doing then your bank is over-sized. 

I would have to disagree with this even as a non leccy person. Reasons are my bank is bigger than I could get away with but as I rarely take mine down below 70 percent surely they last longer than if I had less batteries and took them down to 50 percent each day? Or is that not the case? The older they get the lower I have to take them.

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1 hour ago, mrsmelly said:

I would have to disagree with this even as a non leccy person. Reasons are my bank is bigger than I could get away with but as I rarely take mine down below 70 percent surely they last longer than if I had less batteries and took them down to 50 percent each day? Or is that not the case? The older they get the lower I have to take them.

Battery life depends upon how low you discharge them and how long they are like that before a recharge.  So if you recharge most days (for example solar) and only go down to 70% DoD then I think your batteries will last a lot longer than some one that either discharges significantly deeper, or fails to recharge within a day or so.  Of course if you do both - deeper discharge and only recharge once per week - then as others have reported, batteries will not last long, maybe a year, and if really abused, just weeks.

Edited by Chewbacka
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20 hours ago, Richard10002 said:

My own rules are now:

Try not to discharge below 50%, (but it's not critical if you do). It's OK to rely on Smartguage to monitor this percentage.

Charge up regularly after any use, to as full as you can, (daily if possible, and no longer than 2 or 3 days). It is not OK to rely on Smartguage to monitor this percentage.

At least every 3 days of use, charge up fully, such that the Amps drawn are 2% of capacity, or less. 

(I probably do other things subconciously but cant think what they are right now).

Sounds good, I'd also include correct charge voltage, according to the batt mfr ideally, and compensated for temperature (higher in winter).

In deep winter the correct voltage may be some way above the output of a typical charger, which may partly explain performance problems.

So summing up, for lead acids, proper charging is all about correct charge voltage and sufficiently low tail current. :)

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9 minutes ago, Chewbacka said:

Battery life depends upon how low you discharge them and how long they are like that before a recharge.  So if you recharge most days (for example solar) and only go down to 70% DoD then I think your batteries will last a lot longer than some one that either discharges significantly deeper, or fails to recharge within a day or so.  Of course if you do both - deeper discharge and only recharge once per week - then as others have reported, batteries will not last long, maybe a year, and if really abused, just weeks.

Yes I think this is spot on and its why Mike tb kills his decent batteries yet my cheapos last quite well. Of course there is no winner as I spend more on diesel and engine wear as nowt is for nowt.

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7 hours ago, Chewbacka said:

Battery life depends upon how low you discharge them and how long they are like that before a recharge.  So if you recharge most days (for example solar) and only go down to 70% DoD SoC then I think your batteries will last a lot longer than some one that either discharges significantly deeper, or fails to recharge within a day or so.  Of course if you do both - deeper discharge and only recharge once per week - then as others have reported, batteries will not last long, maybe a year, and if really abused, just weeks.

Fixed that ;)

I'd agree with the above with one caveat - you've possibly spent £100 too much on getting a big enough bank to achieve your 30% DoD. There's a reason that the 50% 'rule' exists :)

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