Battery Charging Graphs

[WotEver]

It would be interesting to see what happens (obviously on a different occasion) if you try dropping the voltage to say 14.2V once the batteries are in the mid-90s and see what the current does. Wouldn't surprise me if it slowly rose.

 

Tony

[nicknorman]

Maybe the very fact they have recently been left on float for an extended period may have caused minor sulphation in excess of that they would have received from regular daily charge/discharge use.

Possibly, but that was why I discharged them somewhat the previous day and then recharged before starting the test. However perhaps only one modest cycle isn't enough. There is absolutely no doubt that the batteries get "lazy" when on prolonged float charge, but they seem to snap out of it fairly quickly.

[cuthound]

1) the regulator regulates voltage and the data shows the voltage at the peak current, of 14v. The final regulated voltage is at least 14.4v so it has not overshot on voltage.

2) I suppose the battery will have slight capacitance, not much though due to the plates being fairly far apart. Whereas the peak of current lasts nearly a minute and is perhaps 40A over the settled value. That would take a massive capacitor.

Sorry to be negative, keep coming up with the ideas!

Batteries have a lot of capacitance. Early DC power plants relied on them for the final smoothing. Modern plants have an output smooth enough to supply the load without a battery, but obviously need the battery for its standby energy during mains failures.

 

I suspect that the initial curve is sîmply the plates losing light sulphation as charging commences. Not sure how it can be proved, but I was told that sulphation begins the second the batteries begin to discharge, hence the importance of recharging ASAP after a dischsrge finishes.

 

Edited to add the last paragraph.

Edited February 15, 2016 by cuthound

[nicknorman]

Batteries have a lot of capacitance. Early DC power plants relied on them for the final smoothing.

No I'm going to have to disagree on that one. In a capacitor, no current flows through the dielectric (which would be the electrolyte in this case), it is based solely on electric field strength and follows the equation I = CdV/dt. A battery has no insulating dielectric nor follows the equation mentioned. It does however have a rapidly reversible chemical reaction that allows it to give and receive charge at a relatively high frequency and thus is able to, for example, smooth out a fluctuating supply such as you mention.

 

So you might want to consider it having some pseudo-capacitance for the purposes of power smoothing, but it is not real capacitance and thus would ultimately be misleading.

Edited February 15, 2016 by nicknorman

[nicknorman]

It could be nothing more than an overshoot on the MS logging software. Do you have any other ammeter to compare with? Maybe your clamp meter?

Tony

So today I started it with the Clampmeter on as well. The initial current was 200A whilst the MS was catching up, they both settled on 165 or so for a short while, but then I saw exactly the same pattern again on both the Clampmeter and the MS, ie the current fell off rapidly to around 125A and then slowly started rising after a couple of minutes, with accompanying slight drop in terminal voltage. I'll post the graph shortly once I've done the test with the kettle on.

[WotEver]

That discounts overshoot then

 

I suspect that Cuthound's probably on the right track but the only person I know that would know is Gibbo, and he ain't here

 

Tony

[nicknorman]

OK so here is the data. I took the MS' current in/out of the battery and added to the current in/out of the Combi. The Combi has no smoothing on its current measurement, the MS has lots, hence the apparent spikes when the kettle is turned on and off. They are simply a facet of the measurement process.

 

I note that the alternator had been working reasonably hard for 15 minutes and so would be quite hot. However it still managed 165A, although some drift-down can be seen once it is flat out, it is not that much. I did notice that when I increased the revs slightly (it had been 1200) the current went up slightly which is not something I've noticed before because when it is cold it can put out 175A just above idle (say 950rpm). So clearly it is less efficient when hot and needs more revs to get max output, but the loss of max current is not too bad. That said of course I saw 200A on the clampmeter briefly when cold, but the rating is 175A and that seems reasonable.

 

 

Oh just to mention that I forgot to turn off the Empirbus master so current used by the fridge, engine room lights etc wasn't measured, but in the context of these sorts of currents I don't think that is significant and probably the fridge was off on its thermostat anyway.

Edited February 15, 2016 by nicknorman

[cuthound]

No I'm going to have to disagree on that one. In a capacitor, no current flows through the dielectric (which would be the electrolyte in this case), it is based solely on electric field strength and follows the equation I = CdV/dt. A battery has no insulating dielectric nor follows the equation mentioned. It does however have a rapidly reversible chemical reaction that allows it to give and receive charge at a relatively high frequency and thus is able to, for example, smooth out a fluctuating supply such as you mention.

So you might want to consider it having some pseudo-capacitance for the purposes of power smoothing, but it is not real capacitance and thus would ultimately be misleading.

OK, I concede that a battery behaves like a capacitor, but isn't one.

 

Love the term pseudo-capacitance.

[Graham.m]

OK so here is the data. I took the MS' current in/out of the battery and added to the current in/out of the Combi. The Combi has no smoothing on its current measurement, the MS has lots, hence the apparent spikes when the kettle is turned on and off. They are simply a facet of the measurement process.

 

I note that the alternator had been working reasonably hard for 15 minutes and so would be quite hot. However it still managed 165A, although some drift-down can be seen once it is flat out, it is not that much. I did notice that when I increased the revs slightly (it had been 1200) the current went up slightly which is not something I've noticed before because when it is cold it can put out 175A just above idle (say 950rpm). So clearly it is less efficient when hot and needs more revs to get max output, but the loss of max current is not too bad. That said of course I saw 200A on the clampmeter briefly when cold, but the rating is 175A and that seems reasonable.

 

graph4.JPG

 

Oh just to mention that I forgot to turn off the Empirbus master so current used by the fridge, engine room lights etc wasn't measured, but in the context of these sorts of currents I don't think that is significant and probably the fridge was off on its thermostat anyway.

Wasn't going to bother, but whatever way you look at the two graphs the alternator is power limiting. Hence on the first graph as the battery voltage came up the current goes down. On this graph the current level drain is greater than can be sustained so the voltage level drops.

 

Also regarding the comment about higher charge rate when you restarted the charge on shoreline. Think about surface charge and the voltage tension across the plate forcing the reaction deep into the plates so that it can revert lead sulphate back to lead & lead dioxide. Also the opposite effect when the available surface charge has been used and the time taken for the charge to dissipate from the plate core to the surface to rebalance the voltage tension across the plate.

[smileypete]

.

Edited February 15, 2016 by smileypete

[dmr]

OK so here is the data. I took the MS' current in/out of the battery and added to the current in/out of the Combi. The Combi has no smoothing on its current measurement, the MS has lots, hence the apparent spikes when the kettle is turned on and off. They are simply a facet of the measurement process.

 

I note that the alternator had been working reasonably hard for 15 minutes and so would be quite hot. However it still managed 165A, although some drift-down can be seen once it is flat out, it is not that much. I did notice that when I increased the revs slightly (it had been 1200) the current went up slightly which is not something I've noticed before because when it is cold it can put out 175A just above idle (say 950rpm). So clearly it is less efficient when hot and needs more revs to get max output, but the loss of max current is not too bad. That said of course I saw 200A on the clampmeter briefly when cold, but the rating is 175A and that seems reasonable.

 

graph4.JPG

 

Oh just to mention that I forgot to turn off the Empirbus master so current used by the fridge, engine room lights etc wasn't measured, but in the context of these sorts of currents I don't think that is significant and probably the fridge was off on its thermostat anyway.

 

I am confused (it happens lots with electricity, but even more with the Windows API but that another story).

Are you measuring current into the batteries or out of the alternator?? Why does the current go UP when you turn the kettle on? You must be measuring current out of the alternator.....

so..................

The regulator in the alternator not only controls voltage but also has a current limiter built in (have a half recollection of reading this in some Iskra data?????). This current limiter has a slow response hence the current peak?????

 

...............Dave

[nicknorman]

I am confused (it happens lots with electricity, but even more with the Windows API but that another story).

Are you measuring current into the batteries or out of the alternator?? Why does the current go UP when you turn the kettle on? You must be measuring current out of the alternator.....

so..................

The regulator in the alternator not only controls voltage but also has a current limiter built in (have a half recollection of reading this in some Iskra data?????). This current limiter has a slow response hence the current peak?????

 

...............Dave

I did explain this Dave but never mind, I'll try again! I have a Mastershunt that measures current in/out of the battery. I have a Combi that measures current in/out of the Combi (in, for the purposes of this test since there is no shore power/genny). I don't have a means of logging alternator output directly, however what comes out of the alternator goes to the batteries and to the Combi and therefore by summing these I can derive the alternator output. As I mentioned, I forgot to allow for other DC current going into the normal distribution system but that would just have been the fridge (if the thermostat happened to be on) and the engine room lights (2A) which is pretty inconsequential in this context. So with the kettle going on the Combi demand about 185A and since this is more than the alternator can put out, around 20A comes out of the batteries. However the sum mentioned above calculates the output from the alternator which is the trace I have shown, along with the voltage which obviously drops as the kettle goes in since the batteries go from being briskly charged, to having to deliver around 20A.

 

Does that now make sense? In a minute I'll post the xls which has quite a few other parameters a well but if I show too much on the graph it just gets confusing.

 

The peaks around where the kettle goes on and off are factets of the circa 5 second time constant on the MS measurement vs the virtually zero time constant on the Combi's measurement. In reality it doesn't exist.

Edited February 15, 2016 by nicknorman

[nicknorman]

OK so here is the xls. The data is all as recorded by the logger with the exception of the right-most column which is calculated from the MS and Combi currents.

 

Data4.xls

 

ps the telly and HD recording Freesat box (which is a bit thirsty) were on, hence the 0.6A mains output from the Combi from the start.

[dmr]

Sorry, one of my bad habits is to look at graphical data first and only read the words some time later.

Its quite a good approach in some ways as it has allowed me to form my own conclusions from data before getting influenced by somebody else's interpretation.

 

Will battery current be in your xls????

 

............Dave

[Loafer]

All very interesting chaps, thanks.

[nicknorman]

Sorry, one of my bad habits is to look at graphical data first and only read the words some time later.

Its quite a good approach in some ways as it has allowed me to form my own conclusions from data before getting influenced by somebody else's interpretation.

 

Will battery current be in your xls????

 

............Dave

Yes it is the H column "Battery A"

[dmr]

Yes, thanks, I am plotting and studying this right now.

 

Ive had a hard couple of weeks with the ones and zeros, have to do a bit of real work to pay for this boaty life, could do with some beer rather than doing work type stuff as a hobby, but its very interesting. I hope some of the following rambling makes some sense....

 

You are maybe a bit unusual in that you have a typical 450Ah bank but a huge alternator, I have more Ah but a smaller alternator, maybe more typical?.

You start at 62% charge but have no bulk phase at all, you go directly into absorption (of sorts) because the alternator is never at capacity (140amp against 165 ish). Maybe this is why you see this lazy battery start up oddity? I have a good period on true bulk so would never normally get to see this bit at all.

But, although its absorption with a (more or less) fixed voltage and reducing current, the voltage is only 13.96 (up till approx kettle time). So it looks like we are seeing the Iskra soft regulator characteristic here? so its a sort of pseudo absorption mode really.

 

Gibbo had a rule of thumb that max current into battery in amps = Amp hours to go (I think)???

with 62% state of charge on a 450Ah bank that would give 170 amps, you only get 120amp which is a bit low but not far off.

Again this fits in with the soft regulator.

 

It would be really really interesting to put an Adverc controller (or equivalent) on your alternator and see what would happen.

 

To go back to a previous bit of this thread, I have an Adverc so I no longer see the soft Iskra controller, bud sadly I do have to under-rev my alternator a fair bit which is maybe why I see a bigger thermal effect???

 

Ive been trying to do a few crude measurements here too but we're running the engine everyday to keep the back cabin warm so not getting the batteries much below 80% so I cant do long enough bulk to get the alternator fully warm!.

 

.............Dave

[WotEver]

You start at 62% charge but have no bulk phase at all, you go directly into absorption (of sorts) because the alternator is never at capacity (140amp against 165 ish). Maybe this is why you see this lazy battery start up oddity? I have a good period on true bulk so would never normally get to see this bit at all.

But, although its absorption with a (more or less) fixed voltage and reducing current, the voltage is only 13.96 (up till approx kettle time). So it looks like we are seeing the Iskra soft regulator characteristic here? so its a sort of pseudo absorption mode really.

Nick has mentioned before that it's a somewhat soft knee from Bulk to Absorption but his setup does appear to make it excessively so.

 

Tony

Edited February 15, 2016 by WotEver

[nicknorman]

Hi Dave

 

Perhaps a bit of an unusual setup in some ways, but on the other hand anyone with a Beta 43 made in the last 5-10 years or so will be in the same boat (pun intended), and it is a fairly ubiquitous engine.

 

But sure, no bulk. But then "bulk" is a facet of the alternator, not the batteries. I did contemplate doing a charging test with just the shore power / 100A charger but then didn't see the point since the charge would be capped at 100A (bulk) for quite a long time and any such effect would therefore not be manifest.

 

The whole thing of the soft regulation is interesting. Tony (Wotever) recently pointed me at a very old Gibbo thread where he was talking about the relationship between gassing voltage and SoC (gassing voltage gets higher with increasing SoC as the electrolyte gets stronger), and how if the charge voltage was too high too early, the charge current (not necessarily total current since that is charging current + gassing current) was actually lower due to all the (insulating) bubbles sticking to the plates and reducing the conductivity of the electrolyte. Which makes sense.

 

So I have a vague theory that the regulation is deliberately soft in order to prevent the voltage getting too high too soon, which would be counterproductive. It was interesting to note that, with the regulation voltage of a hot Iskra seeming to be 14.4v, and the Combi's charing voltages set to 14.4v, when I switched from alternator to shore power towards the end of Sunday's full recharge, the voltage and current dropped noticeably. ie the Combi's regulation is even softer than the Iskras.

 

This also explains why, if I change the Combi's charging voltages from say 14.4v to 15v mid-charge, the actual voltage doesn't change at all, it only changes quite near the end of the charge when the current is pretty low, whence it will wend its way up to 15v or whatever. So I think all the manufacturers realise that too much voltage too soon is a bad idea.

 

So (Tony) this is why I wasn't able to test the theory about reducing the voltage to get more current - the Combi simply doesn't allow an unreasonably high voltage too soon in the charge process.

Edited February 15, 2016 by nicknorman

[Keeping Up]

It seems to be a characteristic of the Iskra alternator to have a very soft regulator curve. It's even specified as such in the Beta manual. We have a Beta 43 and the 175 amp Iskra is the standard; we have 440 Ah but even if we had only 110 Ah it would still be being charged by a 175 amp alternator.

[WotEver]

(gassing voltage gets higher with increasing SoC as the electrolyte gets stronger)

Lower.

 

I know you know that, but others may not

 

Tony

[nicknorman]

Lower.

 

I know you know that, but others may not

 

Tony

No actually I didn't know that! But having checked back I see you are right and my thoughts were wrong. Bxxxxx! Back to the drawing board to think up an idea as to why the regulation might be deliberately so soft! And discount my BS in post #43. <sulking>

[WotEver]

 

So (Tony) this is why I wasn't able to test the theory about reducing the voltage to get more current - the Combi simply doesn't allow an unreasonably high voltage too soon in the charge process.

But... it's the tail end where you need to try the lower voltage. To get those last few percent in more quickly. Not the early stage where it goes in fast anyway.

 

Tony

[smileypete]

Out of interest I did a logarithmic plot of Nicks data, comes out fairly near to a flat line towards the end before the charge voltage is increased - but not quite:

 

 

If I was a mathmetician I'd be able to derive a formula to model tail current - but I'm not. It looks to me like the tail current starts to level off at around 3A which may be a baseline due to the current at that level going into gassing.

 

 

But I think the most notable point is that having started at 91% SoC, the batteries were still not fully charged 5.5 hours later, as can be seen by the fact that the current is still falling slowly. So (bearing in mind this a favourite point of mine) at what point should one consider the batteries to be fully charged?

 

Having a look at your original chart, it would appear that after the tail current drops by 4A (C/100 or 1% of C) over one hour, there is little remaining charge to go in, if 3A of the tail current is due to gassing.

 

So for a rough stab at a guess for the type and condition of batts that Nick has, when the charge voltage has reached the correct level*, and the tail current has dropped by less than 1% of batt capacity over an hour, AND the batt is gassing nicely, then it's fully charged enough.

 

*I expect that most batts in wintertime, even Nicks, aren't charged at quite a high enough voltage for the batts temperature, so getting this right first would do far more good than chasing the last 1% of charge.

 

Monthly eq. charging should mop up any light sulphation, some industrial chargers even do a 'mini eq' at the end of charging, and this is one of the methods of charging that US Battery recommend.

Edited February 15, 2016 by smileypete

[nicknorman]

But... it's the tail end where you need to try the lower voltage. To get those last few percent in more quickly. Not the early stage where it goes in fast anyway.

 

Tony

Well I did try it a bit, current was 40 - odd A at around 14.1v. I reduced it to 13.8v but the current reduced, picked up slightly but remained nearly 10A less than it had been, with a slow downward trend. I think in order to show this effect one has to start off with a much higher voltage but my system simply won't give it. Or as you say, do it very near the end but the trouble is one can only measure the total current going in, being gassing current + charging current. If you lower the voltage you lower the gassing current a lot, total current goes down a lot but there is no means to measure the effect on the charging current since the change in gassing current cannot be known.

If I was a mathmetician I'd be able to derive a formula to model tail current - but I'm not. It looks to me like the tail current levels off at around 3A which may be a baseline due to the current at that level going into gassing.

In case you didn't know, XL can do a trend line (approximation to the curve) in which you can specify the order of the curve (ie number of powers of x you want it to use) and you can get it to display the trend line's equation.