Something I don't understand about batteries

[NB Alnwick]

During recent times circumstances have dictated that we leave the boat for longer periods than we would like to. Our domestic battery bank (24v) is made up of four 115 amp/hour maintenance-free UPS batteries purchased second hand from 'vince1969' three years ago. We have always tried to keep the batteries fully charged and usually run the generator to charge them up to 100% before leaving the boat for any length of time. In our experience the state of charge, as shown on our SmartGauge, would drop to around 70% after a few weeks whenever we left the boat unoccupied and with everything switched off. Lately the opposite has happened - we had insufficient fuel in the generator to charge the batteries up to 100% so we left them at 87% - we switched everything off as usual and, when we returned after three weeks, the reading on the SmartGauge was still 87%! More recently we moved the boat up to Braunston (from Napton) and after running the engine for two and a half hours the reading on the SmartGauge was 84% but when we went back to check on the boat today (three days later) the reading had actually increased to 90%.

 

Is it possible that batteries lose their charge when not being used rather more in cold weather than they do it warm weather? And, if so is a loss of around 30% over three weeks acceptable? Also, why would the reading have actually increased when the batteries are neither being used or charged?

[paulcatchpole]

During recent times circumstances have dictated that we leave the boat for longer periods than we would like to. Our domestic battery bank (24v) is made up of four 115 amp/hour maintenance-free UPS batteries purchased second hand from 'vince1969' three years ago. We have always tried to keep the batteries fully charged and usually run the generator to charge them up to 100% before leaving the boat for any length of time. In our experience the state of charge, as shown on our SmartGauge, would drop to around 70% after a few weeks whenever we left the boat unoccupied and with everything switched off. Lately the opposite has happened - we had insufficient fuel in the generator to charge the batteries up to 100% so we left them at 87% - we switched everything off as usual and, when we returned after three weeks, the reading on the SmartGauge was still 87%! More recently we moved the boat up to Braunston (from Napton) and after running the engine for two and a half hours the reading on the SmartGauge was 84% but when we went back to check on the boat today (three days later) the reading had actually increased to 90%.

 

Is it possible that batteries lose their charge when not being used rather more in cold weather than they do it warm weather? And, if so is a loss of around 30% over three weeks acceptable? Also, why would the reading have actually increased when the batteries are neither being used or charged?

 

Two things spring to mind.

 

Firstly, the Smartgauge can be up to 10% out under charging or hard discharge. I've a similar bank, and under certain conditions will 'go up' an amount if left after I shut the gennie down, particularly if there's no immediate discharge.

 

Secondly, there's something odd going on with the self-discharge to 70% you mention early on. That's an awful lot. I've got a set of Vince's which I've abused and are a bit knackered, and they'll lose a couple of percent in a month. They do eventually collapse under load at about 40%, and I've since replaced them, with a view to testing the Vince bank to see if one is breaking down and the others okay. Anyhoo, you've surely got a drain somewhere? Alarm, bilge pumps?

 

PC

[alan_fincher]

Graham,

 

Unless there is something very different about these "Vince" batteries from a normal "leisure" one, then loosing 30% of charge in just a "few weeks" away from Alnwick really does sound an awful lot to me.

 

Googling gives a wide range of claimed self-discharged rates, for standard lead-acid technologies, but around 4% per week seems to be the ballpark that many sites are quoting. To me that says to loose 30%, you would need to be having around 7 to 8 weeks elapse.

 

That said, although I have never scientifically measured it, I don't think ours even self-discharge at anything approaching that rate.

 

Personally I'd want to check out SmartGauge's view of the world with the use of just an accurate multimeter. Broadly a loss of 10% of charge is equivalent to about a 0.1 volt drop, so if you have really had a 30% drop, you might expect to see around a 0.3 volt drop in the measured voltage.

 

(But you must take steps not measure immediately after generator or engine running, or surface charge will artificially increase measured voltages).

 

I can think of no reason in the world why your battery should be actually really be more charged after it has sat for a few weeks, so think it's an oddity of the measurement, and that an ordinary voltmeter might struggle to give the same view.

 

Wild thought, and I'm sure Gibbo will say I'm barking up the wrong tree..... Doesn't SmartGauge tell you the percentage charge, allowing for how much overall capacity it thinks your batteries have lost with age ? That is if they were originally 100Ah but now can't better 50Ah, I think SG says 100% charged if you managed to get 50Ah in. Any danger SG is actually revising it's view of the capacity of those batteries downwards between the two times you are studying it?

 

Probably gobbledygook, but if at one stage it thinks it has 70Ah out of 100Ah possible, but then revised it's view of max possible downwards to 70Ah, could it then think a battery it judged to be at 70% of max possible is now at 100% of max possible ?

 

Probably not! Let's see what Gibbo says!

[Nickhlx]

Could temperature be affecting readings ? I would guess nothing like this much, so the other possibility is something was left on, unless the batteries were isolated

 

We await the guru...

 

Nick

 

 

[nb Innisfree]

If your batteries have lost more than 50% of their capacity Smartgauge can be inaccurate.

[Chalky]

Could temperature be affecting readings ? I would guess nothing like this much, so the other possibility is something was left on, unless the batteries were isolated

 

We await the guru...

 

Nick

That's my thought. Self discharge does change with temperature. So does the rate of the chemical reaction within the cells. I've had batteries that report better SOC when warm compared to cold and its common to find that the cranking capabilities of batteries change with temperature.

There's also the issue of the accuracy of the electronics within a unit being affected by temperature. Thermal drift in analogue measuring circuits can affect accuracy in a number of different ways and that's one of the motivations to move to digital signal processing - digital accuracy is not temperature dependent. Any thermal drift would be small and might affect the accuracy by 1 to 2% at the most however this can be factored into the design which Gibbo will have done as part of the development work.

It would be interesting to know the terminal voltages of the batteries as well as the SG readings.

[Nickhlx]

That's my thought. Self discharge does change with temperature. So does the rate of the chemical reaction within the cells. I've had batteries that report better SOC when warm compared to cold and its common to find that the cranking capabilities of batteries change with temperature.

There's also the issue of the accuracy of the electronics within a unit being affected by temperature. Thermal drift in analogue measuring circuits can affect accuracy in a number of different ways and that's one of the motivations to move to digital signal processing - digital accuracy is not temperature dependent. Any thermal drift would be small and might affect the accuracy by 1 to 2% at the most however this can be factored into the design which Gibbo will have done as part of the development work.

It would be interesting to know the terminal voltages of the batteries as well as the SG readings.

 

 

Even ignoring the rate of self-discharge, the battery terminal voltage also changes with temperature... and we have had some significant changes in the last 8 weeks - near zero to maybe +40C in the hot spell....

 

Nick

[Gibbo]

This is an interesting one.

 

Temperature does affect battery self discharge. However the other way round from what you're hoping. In cold temperatures they self discharge less than they do in warm temperatures. So that's no explanation of what's going on here.

 

Following a partial recharge one is then left with two options. One can either start to discharge immediately or one can wait before discharging. If the one waits, the actual SoC shown by SmartGauge will actually increase for a short period. This seems counter intuitive (how can the SoC increase with no charger connected?) but it is in fact correct...

 

http://smartgauge.co.uk/sga_faq.html#why_count

 

Your batteries losing 30% over a few weeks is a lot. Much more than I would have expected. That really sounds more like a load was left on. Especially as it has now stopped.

[smileypete]

During recent times circumstances have dictated that we leave the boat for longer periods than we would like to. Our domestic battery bank (24v) is made up of four 115 amp/hour maintenance-free UPS batteries purchased second hand from 'vince1969' three years ago. We have always tried to keep the batteries fully charged and usually run the generator to charge them up to 100% before leaving the boat for any length of time. In our experience the state of charge, as shown on our SmartGauge, would drop to around 70% after a few weeks whenever we left the boat unoccupied and with everything switched off. Lately the opposite has happened - we had insufficient fuel in the generator to charge the batteries up to 100% so we left them at 87% - we switched everything off as usual and, when we returned after three weeks, the reading on the SmartGauge was still 87%! More recently we moved the boat up to Braunston (from Napton) and after running the engine for two and a half hours the reading on the SmartGauge was 84% but when we went back to check on the boat today (three days later) the reading had actually increased to 90%.

 

Is it possible that batteries lose their charge when not being used rather more in cold weather than they do it warm weather? And, if so is a loss of around 30% over three weeks acceptable? Also, why would the reading have actually increased when the batteries are neither being used or charged?

Temperature affects battery internal resistance, which in turn will affect the Smartgauge reading to some degree.

 

I'm sure Gibbo will correct me if I'm wrong

 

cheers,

Pete.

[by'eck]

Temperature affects battery internal resistance, which in turn will affect the Smartgauge reading to some degree.

 

I'm sure Gibbo will correct me if I'm wrong

 

cheers,

Pete.

 

Many factors affect battery internal resistance, most obviously their state of charge. Taking the extremes, a flat battery will have a low internal resistance allowing a high recharge current. As the battery charge is replenished the internal resistance rises & current falls. Near fully charged it rises to the point where only draw a very modest current will be drawn even with 14.5 volts presented to it.

 

Lead Calcium batteries have very high internal resistance when fully charged drawing as little as 1 amp for say a 500 a/h bank which is one of the reasons I prefer them. They also allow higher acceptance charge at up to 15 volts.

 

Ordinary lead acid batteries have an internal resistance that peaks at a lower plateau as they become fully charged. Continued charge current being dissipated in gassing & heat. This can get worse with age.

 

Battery monitors are useful but the information they give should be tempered with experience of the individual boat owner & the varied battery systems they are expected to monitor. To me knowing battery voltage & charge current are far more important than the vagaries of a state of charge percentage.

Edited May 19, 2011 by richardhula

[NB Alnwick]

I didn't keep accurate records so my observations are based on memory. We generally turn both battery isolation switches off when we are not aboard and the SmartGauge is the only piece of equipment that is permanently connected to each battery bank.

 

The batteries were already 'second-hand' when they were fitted three years ago so they are probably near the end of their useful life.

 

The highest rate of discharge occurred during the coldest weather (early February) and we noted that the SmartGauge reading would display a drop of between 10% and 15% within a few days, thereafter the rate of self discharge slowed gradually. On one occasion after leaving them isolated with no significant power drain for almost eight weeks, they dropped to 59% so I think that after the first three weeks the rate of self discharge is no more than may be expected of five year old batteries. This also followed a period of fairly heavy use - we lived aboard from November through to the end of January and used the portable generator to re-charge the batteries quite frequently.

 

During February and March, voltage readings after charging to 100% were in the region 25.4v (12.7v per battery) and the drop after three weeks or so with the SmartGauge reporting around 70% was usually around 25.2v (12.6v per battery) - I am not sure how relevant this is. The charging voltage varies between 28.6v and 29.6v and the 100% figures quoted in each case is a reading taken after the voltage has stabilised about an hour after charging had ceased.

 

Comparative voltage figures for the engine start batteries are 25.5v after charging and 25.4v after three weeks or more.

Edited May 19, 2011 by NB Alnwick

[Nickhlx]

Many factors affect battery internal resistance, most obviously their state of charge. Taking the extremes, a flat battery will have a low internal resistance allowing a high recharge current. As the battery charge is replenished the internal resistance rises & current falls. Near fully charged it rises to the point where only draw a very modest current will be drawn even with 14.5 volts presented to it.

 

Lead Calcium batteries have very high internal resistance when fully charged drawing as little as 1 amp for say a 500 a/h bank which is one of the reasons I prefer them. They also allow higher acceptance charge at up to 15 volts.

 

Ordinary lead acid batteries have an internal resistance that peaks at a lower plateau as they become fully charged. Continued charge current being dissipated in gassing & heat. This can get worse with age.

 

Battery monitors are useful but the information they give should be tempered with experience of the individual boat owner & the varied battery systems they are expected to monitor. To me knowing battery voltage & charge current are far more important than the vagaries of a state of charge percentage.

 

Well, maybe I am missing something, but I would have thought that a flat battery has a (relatively) high resistance to a fully charged battery. A flat battery can not turn an engine over

because the internal resitance is high and anything more than a few amps causes the internally generated voltage to be dropped across this high resistance and very little current can flow (through the starter motor) as a result.....

 

Experts will be along soon to shoot one of us down in flames

 

Nick

[WotEver]

Experts will be along soon to shoot one of us down in flames

 

Nick

I'm no expert, but curves that I've seen (if I'm remembering correctly) are far from a straight correlation either way. I seem to recall that internal resistance starts high with a discharged battery, steadily reduces and then rises again as the SOC slowly increases. Think of a cross section through a bowl, with the right-hand edge lower than the left-hand.

 

Or maybe that's entirely wrong. I'm sure Gibbo will enlighten us eventually

 

Tony

[by'eck]

Experts will be along soon to shoot one of us down in flames

 

Nick

 

I'm afraid they will if your arguing that Ohms law is wrong

 

Think of a flat battery as one hungry to be charged. It cannot take such unless its resistance to any charging voltage is low.

 

A flat battery at the end of its life may well not accept a useful charge. Sulphated plates for example will increase its resistance reducing acceptance of charge current.

Edited May 19, 2011 by richardhula

[Chalky]

I didn't keep accurate records so my observations are based on memory.

 

I've been thinking about getting one of these to help.

 

http://www.techsupplies.co.uk/epages/Store.sf/en_GB/?ObjectPath=/Shops/Store.TechSupplies/Products/AXE110P

 

It's a low cost data logger and I was thinking about using one to monitor the voltage on both banks along with the current into and out of the domestic bank. With a few mods (better memory chips and some compression algorithms) I should be able to record 3 channels at 2 minutes intervals for about 2 weeks. It'll take a lot of the guess work out.

[Nickhlx]

I'm afraid they will if your arguing that Ohms law is wrong

 

Think of a flat battery as one hungry to be charged. It cannot take such unless its resistance to any charging voltage is low.

 

A flat battery at the end of its life may well not accept a useful charge. Sulphated plates for example will increase its resistance reducing acceptance of charge current.

 

 

- Certainly not arguing that Ohm's law is wrong

- A battery though is not a pure resistance and has chemistry producing its own volts (current / power) which is "complicating" the issue, and I agree that the internal resistance will vary with its state of charge, but its not a linear transformation from (relatively) high resistance to low resistance

- I agree that the (poor/ non-conductive) sulphate will insulate the plates and prevent anything like the normal current being able to flow in/out of the cells / battery

 

Surely you agree a fully charged battery has a low internal resistance ? That's why it can deliver so much current .....

The main reason a battery is taking less current as it approaches full charge, is that the instantaneous difference in potential between the charger and the battery is diminishing to millivolts. If you remove the charger for even a short time, this (battery) voltage quickly drops off back towards the "more normal" 12.8 to 13 volts (full charge figure - depends on temp and chemistry in use) and re-attaching the charger will give an initially ( for a short time) high current, diminishing as the voltage comes back up to (nearly) equal the charging voltage again.

 

Nick

Edited May 19, 2011 by Nickhlx

[by'eck]

- Certainly not arguing that Ohm's law is wrong

- A battery though is not a pure resistance and has chemistry producing its own volts (current / power) which is "complicating" the issue, and I agree that the internal resistance will vary with its state of charge, but its not a linear transformation from (relatively) high resistance to low resistance

- I agree that the (poor/ non-conductive) sulphate will insulate the plates and prevent anything like the normal current being able to flow in/out of the cells / battery

 

Surely you agree a fully charged battery has a low internal resistance ? That's why it can deliver so much current .....

The main reason a battery is taking less current as it approaches full charge, is that the instantaneous difference in potential between the charger and the battery is diminishing to millivolts. If you remove the charger for even a short time, this (battery) voltage quickly drops off back towards the "more normal" 12.8 to 13 volts (full charge figure - depends on temp and chemistry in use) and re-attaching the charger will give an initially ( for a short time) high current, diminishing as the voltage comes back up to (nearly) equal the charging voltage again.

 

Nick

 

No matter how a battery works it offers a resistance to any electrically connected device. I agree that this internal resistance as perceived by a charging device does not necessarily change in a linear fashion as charge is absorbed, but if in good order a battery will present a higher resistance as full charge is reached. Anything other than this would mean that substantial current would continue to be drawn from the charging source (I = E/R) after a full charge has been accepted, resulting in excessive gassing & heat. This of course can happen with a knackered battery but definitely shouldn't with a good one.

Edited May 20, 2011 by richardhula

[Nickhlx]

No matter how a battery works it offers a resistance to any electrically connected device. I agree that this internal resistance as perceived by a charging device does not necessarily change in a linear fashion as charge is absorbed, but if in good order a battery will present a higher resistance as full charge is reached. Anything other than this would mean that substantial current would continue to be drawn from the charging source (I = E/R) after a full charge has been accepted, resulting in excessive gassing & heat. This of course can happen with a knackered battery but definitely shouldn't with a good one.

 

 

In you analogy I think it is is the "E" ( being difference in voltage between the Charger and the battery) that diminishes, not the "R" that increases....

 

Nick

[by'eck]

In you analogy I think it is is the "E" ( being difference in voltage between the Charger and the battery) that diminishes, not the "R" that increases....

 

Nick

 

With respect talking about voltage differences is just complicating the issue

 

When a charger is connected to a battery there is no voltage difference. There are electrically connected & by definition must be at the same voltage. A charger doesn't mysteriously know its connected to a battery it just sees a variable load & reacts accordingly.

 

Yes a smart charger will change its mode & reduce charge volts when dropping back to float mode, but for the sake of a simple explanation I was assuming a constant charge voltage such as when charger is in acceptance mode.

[Nickhlx]

With respect talking about voltage differences is just complicating the issue

 

When a charger is connected to a battery there is no voltage difference. There are electrically connected & by definition must be at the same voltage. A charger doesn't mysteriously know its connected to a battery it just sees a variable load & reacts accordingly.

 

Yes a smart charger will change its mode & reduce charge volts when dropping back to float mode, but for the sake of a simple explanation I was assuming a constant charge voltage such as when charger is in acceptance mode.

 

 

I think I can see what you are meaning...

 

 

... but I think that the value of the current flowing is as a result of, and dependent on, the (magnitude of the) voltage difference of that of the charger and that being produced by the state of the chemistry in the battery...

 

Nick

[Gibbo]

Oh my word....

 

The internal resistance of a battery is absolutely, 100%, definitely not higher when it fully charged. Absolutely not.

 

The highest internal resistance of a battery is when it is flat. There is absolutely no question whatsoever about this.

 

In you analogy I think it is is the "E" ( being difference in voltage between the Charger and the battery) that diminishes, not the "R" that increases....

 

Correct.

 

This isn't even open to debate!

[Nickhlx]

Oh my word....

 

The internal resistance of a battery is absolutely, 100%, definitely not higher when it fully charged. Absolutely not.

 

The highest internal resistance of a battery is when it is flat. There is absolutely no question whatsoever about this.

 

 

 

Correct.

 

This isn't even open to debate!

 

Thanks...

 

I was politely declining to discuss further as it was just plain wrong that a fully-charged battery bank, able to supply hundreds of amps to melt

spanners ( and start heavy diesel engines) could be anything other than extremely low internal resistance.

 

Nick

[WotEver]

The highest internal resistance of a battery is when it is flat. There is absolutely no question whatsoever about this.

Was I also correct in my post #13 that the lowest internal resistance isn't when it's fully charged. It drops down as the battery approaches fully charged then rises slightly again as it reaches 100%?

 

Tony

[Gibbo]

Was I also correct in my post #13 that the lowest internal resistance isn't when it's fully charged. It drops down as the battery approaches fully charged then rises slightly again as it reaches 100%?

 

I think this is where people get confused (including Mastervolt in one of their "explanations").

 

A fully charged battery, at rest, has the lowest internal resistance it will ever have. As the battery is discharged the internal resistance rises roughly as a function of the square of the depth of discharge.

 

The off load voltage falls as an almost perfectly straight line from the fully charged voltage to the flat voltage. However if one graphs the loaded voltage, it falls as an inverse square graph ie it gets steeper going down all the way. The difference between the straight line and the curved graph is due entirely to the square law increasing internal resistance.

 

The confusion seems to come from the fact that during charging, as the battery approaches a fully charged state, more of the incoming energy starts to simply electrolyse the water. This causes bubbles in the electrolyte which increases its bulk resistance (simple really, any path between two parts of the plates has less acid between them and more bubbles which are crap at conducting electricity). As soon as charging stops the bubbles go, and the internal resistance falls to the proper value ie low.

 

This can give the impression that the internal resistance rises as the battery approaches full charge. But that's all it is, an impression.

[WotEver]

This can give the impression that the internal resistance rises as the battery approaches full charge. But that's all it is, an impression.

Perfect explanation, thank you

 

Tony