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I have two questions about battery charging. I have tried to find the answer by searching the forums but can't find answers. If this has been covered before could you point me in the right direction please. 

1. I remember someone had posted a very detailed article that explaned battery charging and chemistry. I think it was pinned somewhere but can't find it. If someone can tell me where it is that would be great. 

2. If I use 100 AH from my batteries. I then want to recharge them back to 100% until the charging current is, let's say, 1% of battery size. Will it take (a) longer (b) the same or (c) less time to recharge a battery bank that has 4 x 100AH batteries compared to 5 x 100AH batteries. 

 

Battery chemistry is still a bit of a black art to me. 

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2b (same time) is most likely, but battery charging like this depends on rate and capacity and tail current and battery type so it's not guaranteed.

 

For example if you take 100AH out and recharge at 100A the 4x100AH bank will be charging at 0.25C from 75% SoC to 4A tail current (1A per battery), the 5x100Ah bank will be rechargingat 0.2C from 80% SoC to 5A tail current (1A per battery).

 

The bigger bank is charging from a higher SoC at a lower C rate, both of which are good for efficiency and time needed to get to 100% SoC (1% tail current), so it's possible that this will take less time than the smaller bank -- but it depends on the exact battery characteristics.

 

Any small difference in recharge time will probably matter much less to you than the 20% difference in capacity... 😉

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But - for discussion  :

 

1) You have taken 100Ah from a 400Ah battery bank it is now at 75% SoC.

2) You have taken 100Ah from a 1000Ah battery bank, it is now at 90% state of charge.

 

We know that it is very difficult and slow to get the 'last 10%' into the batteries, so in example 1) we will get (say) 15% back into the battery comparitively quickly but the last 10% will take several hours.

In example 2) the battery bank is at a much higher SoC (90%) so we are not going to get any charge in quickly and it will a slow slog and take several hours.

 

Will the only difference be the (say) 30 minutes it took to get the 15% into example 1) ?

Will the 'number of hours' taken to get from 90% to 99% be the same for both banks irrespective af the actual Ah (40Ah in example 1 and 100Ah in example 2) being put back into the battery bank ?

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26 minutes ago, Alan de Enfield said:

But - for discussion  :

 

1) You have taken 100Ah from a 400Ah battery bank it is now at 75% SoC.

2) You have taken 100Ah from a 1000Ah battery bank, it is now at 90% state of charge.

 

We know that it is very difficult and slow to get the 'last 10%' into the batteries, so in example 1) we will get (say) 15% back into the battery comparitively quickly but the last 10% will take several hours.

In example 2) the battery bank is at a much higher SoC (90%) so we are not going to get any charge in quickly and it will a slow slog and take several hours.

 

Will the only difference be the (say) 30 minutes it took to get the 15% into example 1) ?

Will the 'number of hours' taken to get from 90% to 99% be the same for both banks irrespective af the actual Ah (40Ah in example 1 and 100Ah in example 2) being put back into the battery bank ?

 

Will it take longer but need the same amount of input or more input? 

I presume all things being equal its about the chemistry. How far into the lead is the sulphation that has to be converted. If its all close to the surface then in the bigger battery bank it might use less energy. 

The smaller battery bank might use deeper lead which uses more energy to remove the sulphation? 

 

57 minutes ago, IanD said:

2b (same time) is most likely, but battery charging like this depends on rate and capacity and tail current and battery type so it's not guaranteed.

 

For example if you take 100AH out and recharge at 100A the 4x100AH bank will be charging at 0.25C from 75% SoC to 4A tail current (1A per battery), the 5x100Ah bank will be rechargingat 0.2C from 80% SoC to 5A tail current (1A per battery).

 

The bigger bank is charging from a higher SoC at a lower C rate, both of which are good for efficiency and time needed to get to 100% SoC (1% tail current), so it's possible that this will take less time than the smaller bank -- but it depends on the exact battery characteristics.

 

Any small difference in recharge time will probably matter much less to you than the 20% difference in capacity... 😉

 

What's the C rate about? 

 

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1 hour ago, Felshampo said:

 

Will it take longer but need the same amount of input or more input? 

I presume all things being equal its about the chemistry. How far into the lead is the sulphation that has to be converted. If its all close to the surface then in the bigger battery bank it might use less energy. 

The smaller battery bank might use deeper lead which uses more energy to remove the sulphation? 

 

 

What's the C rate about? 

 

The C rate is current divided by capacity, so of you charge or discharge a 100Ah battery at 50A the rate is 0.5C. It's this rate that determines how the battery behaves on both charge and discharge, especially lead-acid batteries which don't like rates much above 0.2C.

 

As both me and Alan said, there's no simple answer to your question, the time to get tail current down to 1% of capacity depends on SoC, C rate, battery type/model, phase of the moon...

 

What's much more important with LA cells is having enough capacity to keep SoC mostly above 50%, fully charging regularly to avoid sulphation, and not using high C rates because this reduces capacity and life. LA batteries are usually rated at 0.05C (the "20 hour") rate, if you run them at 0.5C ("2 hour rate") you lose anything up to half the capacity.

Edited by IanD
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14 minutes ago, IanD said:

The C rate is current divided by capacity, so of you charge or discharge a 100Ah battery at 50A the rate is 0.5C. It's this rate that determines how the battery behaves on both charge and discharge, especially lead-acid batteries which don't like rates much above 0.2C.

 

As both me and Alan said, there's no simple answer to your question, the time to get tail current down to 1% of capacity depends on SoC, C rate, battery type/model, phase of the moon...

 

What's much more important with LA cells is having enough capacity to keep SoC mostly above 50%, fully charging regularly to avoid sulphation, and not using high C rates because this reduces capacity and life. LA batteries are usually rated at 0.05C (the "20 hour") rate, if you run them at 0.5C ("2 hour rate") you lose anything up to half the capacity.

 

I have 160 AH AGM batteries and after inheriting some old knackered ones never let their expensive replacements go below 90%. 

The C rate is set by the battery charger? 

I have a Leece Neville 140 amp alternator which I always thought was a good thing. If it's giving off 70A that's 0.43 C. Is this a bad thing then. Although it usually falls down to 30A quite quickly. 

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42 minutes ago, Tony Brooks said:

How are you measuring that 90% and if on anything but a properly calibrated Smartguage have you set the meter up properly and regularly recalibrate it?

 

I know this is a can of worms. 

I have a mastervolt inverter with a mastervolt bm. It is supposed to be able to tell the amount of consumption and adjust as the batteries cycle (peukert numbers left me baffled). 

You can cycle through a plethora of menus which supposedly tell you all you need to know. This had to be set up when I got the new batteries but after then, according to the manual, was self calibrating? 

I know it probably tells lies but when ever I check the SOC with a multi meter it looks about right. 

We are in a marina for winter and so get to fully charge the batteries. Its when we are cruising that the BM gets looked at most. 

Edited by Felshampo
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24 minutes ago, Felshampo said:

 

I know this is a can of worms. 

I have a mastervolt inverter with a mastervolt bm. It is supposed to be able to tell the amount of consumption and adjust as the batteries cycle (peukert numbers left me baffled). 

You can cycle through a plethora of menus which supposedly tell you all you need to know. This had to be set up when I got the new batteries but after then, according to the manual, was self calibrating? 

I know it probably tells lies but when ever I check the SOC with a multi meter it looks about right. 

We are in a marina for winter and so get to fully charge the batteries. Its when we are cruising that the BM gets looked at most. 

The Mastervolt devices are "self calibrating" only to the extent of the Charge Efficiency Factor - the % of charge current that actually goes to increasing the SoC. Typically around 94%.

Provided you fully charge the batteries regularly and the battery capacity remains as entered into the meter, the SoC will be pretty good. The trouble with these meters is that the actual battey capacity starts to reduce with age and wear, and so the meter starts to over-read.

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11 minutes ago, nicknorman said:

The Mastervolt devices are "self calibrating" only to the extent of the Charge Efficiency Factor - the % of charge current that actually goes to increasing the SoC. Typically around 94%.

Provided you fully charge the batteries regularly and the battery capacity remains as entered into the meter, the SoC will be pretty good. The trouble with these meters is that the actual battey capacity starts to reduce with age and wear, and so the meter starts to over-read.

 

The manual isn't clear on this but it slowly lowers the CEF down to 70% as the batteries cycle. At this point it claims they are "dead". How accurate this is I don't know but that allows it to calculate SOC better I suppose. 

How does a smart guage differ that makes it more accurate? 

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11 minutes ago, Felshampo said:

 

The manual isn't clear on this but it slowly lowers the CEF down to 70% as the batteries cycle. At this point it claims they are "dead". How accurate this is I don't know but that allows it to calculate SOC better I suppose. 

How does a smart guage differ that makes it more accurate? 

The CEF doesn’t affect SoC during discharge. It only affects SoC during charge. It is pretty accurate because it simply compares how many Ah were taken out of a fully charged battery, with how many Ah it took to fully recharge the battery again. For a decent battery, CEF is above 90%. If it is anywhere near 70% the battery has major problems.

 

Smartgauge works on a different principle, relating no-load voltage to SoC. If therefore gives the SoC as a percentage of the actual capacity, not the new badged capacity. But it’s only of much use during discharge, during charge it isn’t very accurate because it “sees” the charger voltage, not the nominal battery voltage.

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10 minutes ago, Felshampo said:

 

The manual isn't clear on this but it slowly lowers the CEF down to 70% as the batteries cycle. At this point it claims they are "dead". How accurate this is I don't know but that allows it to calculate SOC better I suppose. 

How does a smart guage differ that makes it more accurate? 

 

What you seem to be doing is absolutely fine as long as you are using rested voltage on the multimeter.

 

The designer of Smartguage spent a great deal of time charging and discharging a load of lead acid batteries of different types and plotting how their state of charge and discharge related to voltage at discrete time intervals and from that hep produced a set of "maps" the electronics can use to ascertain the state of charge. It then gradually learns to give more and more accurate readings over the next few cycles  Even then although it is arguably the least prone to error as long as the factory set it up correctly it is   not good at giving an accurate state of charge during charging but gets much more accurate once charging stops and a little discharge has taken place.  I ver much doubt that you would benefit from one as you check your meter against rested voltage.

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43 minutes ago, nicknorman said:

The CEF doesn’t affect SoC during discharge. It only affects SoC during charge. It is pretty accurate because it simply compares how many Ah were taken out of a fully charged battery, with how many Ah it took to fully recharge the battery again. For a decent battery, CEF is above 90%. If it is anywhere near 70% the battery has major problems.

 

Smartgauge works on a different principle, relating no-load voltage to SoC. If therefore gives the SoC as a percentage of the actual capacity, not the new badged capacity. But it’s only of much use during discharge, during charge it isn’t very accurate because it “sees” the charger voltage, not the nominal battery voltage.

 

So the CEF determines how much "extra" needs to be put into the batteries to get them back to 100%. AGMs are supposed to have a CEF of 94%, which goes up and then down as the batteries age. 

The smart guage is more accurate at measuring % discharge as it uses voltage as well as AH to determine SOC?

Hope that's right. 

One last question please. 

As the battery ages and the lead becomes damaged I presume the capacity goes down. Do the guages take this into account, does it matter? Or is it just CEF that governs the life of the battery. 

In other words why does a knackered battery loose its charge so quickly? Is it less lead or too much sulphate or both or something else. 

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Just now, Felshampo said:

 

So the CEF determines how much "extra" needs to be put into the batteries to get them back to 100%. AGMs are supposed to have a CEF of 94%, which goes up and then down as the batteries age. 

The smart guage is more accurate at measuring % discharge as it uses voltage as well as AH to determine SOC?

Hope that's right. 

One last question please. 

As the battery ages and the lead becomes damaged I presume the capacity goes down. Do the guages take this into account, does it matter? Or is it just CEF that governs the life of the battery. 

In other words why does a knackered battery loose its charge so quickly? Is it less lead or too much sulphate or both or something else. 

 

No, the Smarguage is more accurate (during discharge) because it does NOT use Ah. think about what you say about CEF for AGMs for a moment. If it wanders about a bit and I suspect it also varies with temperature, any fudge factor built into the monitor firmware to account for this will be wrong a soften a sit will be correct, in fact probbably more often. This builds in an increasingly inaccurate state of charge that is not helped when the batteries sulphate and lose capacity.

 

You have to define what you mean  by a knackered battery.

 

A sulphated battery will go to a lower state of charge faster than one that is not sulphated under the same load, but that is just a function of actual capacity and load.

 

A battery that is developing cell shorts will discharge faster that one with a similar capacity and no shorts simply because it has a higher self discharge rate.

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PS there is/was a "son of Smartguage" that combined Smartguage technology with Ah counting so the Smartguage part updated the Ah counting part maintaining accuracy but I think it was aimed at critical installations so cost about £1500. However I think I read that something similar is available at a more consumer orientated price. It is even possible it is the gauge you have if it and rested voltage assessment always agree.

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22 minutes ago, Felshampo said:

 

So the CEF determines how much "extra" needs to be put into the batteries to get them back to 100%. AGMs are supposed to have a CEF of 94%, which goes up and then down as the batteries age. 

The smart guage is more accurate at measuring % discharge as it uses voltage as well as AH to determine SOC?

Hope that's right. 

One last question please. 

As the battery ages and the lead becomes damaged I presume the capacity goes down. Do the guages take this into account, does it matter? Or is it just CEF that governs the life of the battery. 

In other words why does a knackered battery loose its charge so quickly? Is it less lead or too much sulphate or both or something else. 

Smartgauge works only on voltage, it doesn’t measure current or AH.

 

As a battery ages and gets damaged, Smartgauge takes this into account because the displayed SoC is relative to the actual capacity, not the new capacity on the label.

 

Batteries have a couple of main ageing mechanisms - sulphation, caused by undercharging / not fully charging regularly, and loss of material from the plates. The plates are not just solid lead, the have “active material” bonded onto them, spongy lead and lead dioxide. Over time this active material becomes disbonded and slowly falls off. This gives two problems, a reduction in reactive material and also as the debris collects at the bottom of the cell and gets to the bottom of the plates, an electrical path to allow self-discharge.

 

Edited by nicknorman
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37 minutes ago, Tony Brooks said:

 

No, the Smarguage is more accurate (during discharge) because it does NOT use Ah. think about what you say about CEF for AGMs for a moment. If it wanders about a bit and I suspect it also varies with temperature, any fudge factor built into the monitor firmware to account for this will be wrong a soften a sit will be correct, in fact probbably more often. This builds in an increasingly inaccurate state of charge that is not helped when the batteries sulphate and lose capacity.

 

You have to define what you mean  by a knackered battery.

 

A sulphated battery will go to a lower state of charge faster than one that is not sulphated under the same load, but that is just a function of actual capacity and load.

 

A battery that is developing cell shorts will discharge faster that one with a similar capacity and no shorts simply because it has a higher self discharge rate.

 

The AGM's are supposed to have a CEF that rises for the first 5 cycles then goes down like others LA batteries according to the manual. This means the BM has to be calibrated to account for this. 

I suppose knackered is relative but my batteries could not fire up the central heating in the morning even though they had been charged up the day before. They had only about 10v having self discharged over night. So I got some new ones. 

Hadn't realised thy shorted. 

32 minutes ago, nicknorman said:

Smartgauge works only on voltage, it doesn’t measure current or AH.

 

As a battery ages and gets damaged, Smartgauge takes this into account because the displayed SoC is relative to the actual capacity, not the new capacity on the label.

 

Batteries have a couple of main ageing mechanisms - sulphation, caused by undercharging / not fully charging regularly, and loss of material from the plates. The plates are not just solid lead, the have “active material” bonded onto them, spongy lead and lead dioxide. Over time this active material becomes disbonded and slowly falls off. This gives two problems, a reduction in reactive material and also as the debris collects at the bottom of the cell and gets to the bottom of the plates, an electrical path to allow self-discharge.

 

 

I now understand that smart guage is measuring voltage to determine SOC. How does the smart guage know how much damage has been done and what the new lower capacity is? 

Let's say you you don't recharge the batteries properly and you permanently sulphate 20% of the plates. Does the smart guage know this purely on the voltage? 

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7 minutes ago, Felshampo said:

 

The AGM's are supposed to have a CEF that rises for the first 5 cycles then goes down like others LA batteries according to the manual. This means the BM has to be calibrated to account for this. 

I suppose knackered is relative but my batteries could not fire up the central heating in the morning even though they had been charged up the day before. They had only about 10v having self discharged over night. So I got some new ones. 

Hadn't realised thy shorted. 

 

I now understand that smart guage is measuring voltage to determine SOC. How does the smart guage know how much damage has been done and what the new lower capacity is? 

Let's say you you don't recharge the batteries properly and you permanently sulphate 20% of the plates. Does the smart guage know this purely on the voltage? 


Yes. Because the voltage profile of a battery depends on its actual SoC. So if the voltage of a badged 100Ah battery is 12.2v having taken out 50Ah, we can say the SoC is 50%. If that battery now loses capacity such that the voltage is 12.2 after taking out only 40Ah, the SoC is still 50% of actual capacity even though fewer Ah has been taken out. 12.2v (or whatever) represents 50% SoC regardless of what the actual capacity is.

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31 minutes ago, Felshampo said:

 

The AGM's are supposed to have a CEF that rises for the first 5 cycles then goes down like others LA batteries according to the manual. This means the BM has to be calibrated to account for this. 

I suppose knackered is relative but my batteries could not fire up the central heating in the morning even though they had been charged up the day before. They had only about 10v having self discharged over night. So I got some new ones. 

Hadn't realised thy shorted. 

 

I now understand that smart guage is measuring voltage to determine SOC. How does the smart guage know how much damage has been done and what the new lower capacity is? 

Let's say you you don't recharge the batteries properly and you permanently sulphate 20% of the plates. Does the smart guage know this purely on the voltage? 

 

The 10V in the morning could have three causes including they were not sufficiently charged the previous day, but I doubt that is the case here. We are left with loss of capacity caused by sulphation so although fully charged to their then capacity that capacity is not enough to last overnight or they have started to short circuit. Deciding which is difficult with sealed batteries like AGMs

 

On open cells a short once it is noticeable usually results or very soon will result in individual  cells getting hotter than the rest, gassing more or are much dryer than others when under charge. On all types of LA batteries you may be able to feel the local heat and eventually smell sulphur dioxide while charging but they are usually very bad by then.

 

Open cell batteries can be checked for the degree of sulphation by taking the state of charge from a hydrometer reading and comparing that with one from rested voltage. The bigger the difference the more sulphation you have. On all flat  plate battery types sulphation is likely to be shown by swollen battery ends.

 

Often shorting is caused by the plate shredding filling up the bottom of the battery until it touches both plates but an AGM should be less prone to that because the acid filled wadding between the plates will help hold the material on the plates.

 

As I said I think the CEF will also alter with temperature but anyhow how can the monitor compensate for a moving target. It can only guess and all too often it will get it wrong. That is unless you have a way of finiding to today's CEF and adjusting the monitor.

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