Specific Gravity Discrepancies

[MtB]

58 minutes ago, WotEver said:

 

2. A buck/boost converter such as https://m.ebay.co.uk/itm/DC-DC-Converter-10-12-15-20A-150-250-300-400-1200W-Step-up-Step-down-Buck-Boost/192298255711?hash=item2cc5de855f%3Am%3AmHy5fhZloIDZTHImuC1ILHQ&var=492234936334&_sacat=0&_nkw=buck+boost+converter&_from=R40&rt=nc&_trksid=p2380057.m4084.l1311.R1.TR6.TRC1.A0.H0.Xbuck+boost.TRS0

 

I have one of those and the output voltage is fiendishly unstable. Despite the specs the voltage drifts by volts at a time and needs constantly re-adjusting to keep it where you want it. It cannot be left to work unattended.

 

Also, if you turn it off then back on another day, it will not be outputting a voltage even vaguely similar to what it was putting out when you turned it off. 

[WotEver]

7 hours ago, Mike the Boilerman said:

 

I have one of those and the output voltage is fiendishly unstable. Despite the specs the voltage drifts by volts at a time and needs constantly re-adjusting to keep it where you want it. It cannot be left to work unattended.

 

Also, if you turn it off then back on another day, it will not be outputting a voltage even vaguely similar to what it was putting out when you turned it off. 

Ahhh, worth knowing.

 

 

I wonder if yours is faulty or if they’re all like that. After all, you do have a reputation for buying faulty goods

[MtB]

1 hour ago, WotEver said:

Ahhh, worth knowing.

 

 

I wonder if yours is faulty or if they’re all like that. After all, you do have a reputation for buying faulty goods

 

As you might imagine, I have several of these devices from various sellers on ebay, and they all exhibit broadly similar characteristics. 

 

Another problem I experience is a pot that needs turns and turns until you hit the spot when all the adjustment happens. Then the full range of adjustment happens over say 90 degrees of rotation of the screw, and beyond that 90 degrees you are back in 'no effect' territory.

 

 

[WotEver]

6 hours ago, Mike the Boilerman said:

Another problem I experience is a pot that needs turns and turns until you hit the spot when all the adjustment happens. Then the full range of adjustment happens over say 90 degrees of rotation of the screw, and beyond that 90 degrees you are back in 'no effect' territory.

If you’re bored one night, see what value that pot is then replace it with one of half the value with a couple of fixed resistors of quarter value either side of it. 

 

More accurate... remove the pot and measure the value over the range that works and then select the two fixed resistors and a new pot to put that new pot into the middle of the range

[Keajre]

On 22/07/2018 at 15:43, Mike the Boilerman said:

 

And capacity too, I wonder. 

 

Can't answer your question. The more I learn about batteries the less I find I know.

 

Another possible variation is the strength of the acid added when they are first sold. I'm suspect they are shipped dry and the acid added on the bench locally by the distributor. And I suspect sulphuric acid is a LOT more expensive than distilled water so there must be a temptation for an unscrupulous distributor to send them out with a bit of distilled mixed in.... who would notice!

I think they just add distilled water and the sulfuric acid is created when a charge is applied.  This thread made me finally dust off one of my lead acid books, which I bought with the best intentions over a year ago!  In summary it says that lead Sulphate (PbSO4) forms on both plates and is broken into Pb and SO4.  The water then disassociates with its 2 hydrogen atoms (H) and one oxygen atom (O).  Then the Sulphate (SO4) combines with the Hydrogen (H) to form the Sulphuric acid (H2SO4).  At the same time the Oxygen combines with lead to form lead dioxide (PbO2)

 

I didn't realise that the acid 'quantity' remains the same, I guess because its formed based on the Pb and SO4 on the plates.  So now makes sense to me why differing quantities of the electrolyte will change the concentration of acid.  That being said, what happens if you drained the electrolyte and filled with just new distilled water.  Would it have enough material on the plates to get back to the same amount of Sulphuric acid? I'm guessing it would . . .

16 hours ago, Mike the Boilerman said:

 

I have one of those and the output voltage is fiendishly unstable. Despite the specs the voltage drifts by volts at a time and needs constantly re-adjusting to keep it where you want it. It cannot be left to work unattended.

 

Also, if you turn it off then back on another day, it will not be outputting a voltage even vaguely similar to what it was putting out when you turned it off. 

I have one of these for setting down from 12v to 5v, works incredibly well, and is very stable.  Stepping up is much more complex than stepping down of course.  

 

I am going to try and trick my Victron into equalizing at 16.2v.  Will revert

[nicknorman]

1 hour ago, Keajre said:

I think they just add distilled water and the sulfuric acid is created when a charge is applied.  This thread made me finally dust off one of my lead acid books, which I bought with the best intentions over a year ago!  In summary it says that lead Sulphate (PbSO4) forms on both plates and is broken into Pb and SO4.  The water then disassociates with its 2 hydrogen atoms (H) and one oxygen atom (O).  Then the Sulphate (SO4) combines with the Hydrogen (H) to form the Sulphuric acid (H2SO4).  At the same time the Oxygen combines with lead to form lead dioxide (PbO2)

 

I didn't realise that the acid 'quantity' remains the same, I guess because its formed based on the Pb and SO4 on the plates.  So now makes sense to me why differing quantities of the electrolyte will change the concentration of acid.  That being said, what happens if you drained the electrolyte and filled with just new distilled water.  Would it have enough material on the plates to get back to the same amount of Sulphuric acid? I'm guessing it would . . .

I have one of these for setting down from 12v to 5v, works incredibly well, and is very stable.  Stepping up is much more complex than stepping down of course.  

 

I am going to try and trick my Victron into equalizing at 16.2v.  Will revert

No. The battery starts out as lead. Acid is added. When the battery is discharged the lead and acid react to form the lead sulphate.

[Keajre]

3 hours ago, nicknorman said:

No. The battery starts out as lead. Acid is added. When the battery is discharged the lead and acid react to form the lead sulphate.

Starts out as lead on the neg plate and lead dioxide on positive plate. When discharged then positive plate becomes just lead and no reaction can take place between the two metals since they have become the same.

 

so you say acid is added during production. What would happen if you fully charged a healthy battery, emptied the electrolyte, added distilled water and attempted to charge. Wouldnt the suphate from

the plates mix with the oxygen and hydroden atoms to create more acid?

Edited July 23, 2018 by Keajre

[nicknorman]

41 minutes ago, Keajre said:

Starts out as lead on the neg plate and lead dioxide on positive plate. When discharged then positive plate becomes just lead and no reaction can take place between the two metals since they have become the same.

 

so you say acid is added during production. What would happen if you fully charged a healthy battery, emptied the electrolyte, added distilled water and attempted to charge. Wouldnt the suphate from

the plates mix with the oxygen and hydroden atoms to create more acid?

There is no sulphate on the plates of a fully charged battery.

[Keajre]

1 minute ago, nicknorman said:

There is no sulphate on the plates of a fully charged battery.

So the sulphate comes from the added sulphuric acid

[nicknorman]

1 minute ago, Keajre said:

So the sulphate comes from the added sulphuric acid

Yes.

[WotEver]

37 minutes ago, Keajre said:

So the sulphate comes from the added sulphuric acid

... as the battery discharges.

[MtB]

So why is a fully charged battery 1.277 SG? Is there something special about this particular value? Is it standard neat acid as opposed to concentrated or diluted acid?

 

Chemistry defeats me. I blame the teachers.

[Keajre]

7 hours ago, WotEver said:

... as the battery discharges.

Makes perfect sense now, thanks for bearing with me

[Keajre]

6 hours ago, Mike the Boilerman said:

So why is a fully charged battery 1.277 SG? Is there something special about this particular value? Is it standard neat acid as opposed to concentrated or diluted acid?

 

Chemistry defeats me. I blame the teachers.

It’s because sulphuric acid is heavier than water, and in this case its 1.277 times heavier.  Water is just two Hydrogen atoms and one Oxygen atom.  Sulphuric acid is Sulphate (SO4) and Hydrogen (H).  So the molecules of acid are heavier than then molecules of water.  H2O vs H2SO4.  That gravitational weight is being measured.  Basically its almost 1/3 heavier. 

 

I think this doesnt answer your question.  Why is it that concentration exactly?  Not sure, I guess its just the net result of how much the two plates (lead and lead dioxide) can react between them before they become the same substance (lead sulphate when fully discharged).   Nick Norman would probably have a more concise and correct answer.  This thread has gotten me thinking too

[WotEver]

8 hours ago, Mike the Boilerman said:

 

So why is a fully charged battery 1.277 SG?

The acid strength is a balance between being high enough to give the required voltage and maximum capacity from the lead/antimony (or lead/calcium or whatever) on the plates while still being low enough to provide good ionic activity. @Dr Bob can probably provide a better description of the latter. I too had a rubbish chemistry teacher who could barely speak English. 

[Keajre]

Just when I thought I was starting to understand it, I watched this . . . 

 

[Keajre]

Getting back to basics though, the single battery in the bank of 4 thats showing a lower SG reading than the others actually has LESS electrolyte than the others in it, so on that basis alone the SG should be higher.  So where is the sulphate?  Must be on the plates right.  Or as mentioned before, it wasn’t topped up with the correct amount of acid at the time of production.  The load side negetive cable does connect to that battery, and I discovered yesterday that the crimp is faulty, the 70mm2 cable just slides freely inside the lug.  In fact I removed it and re-inserted it freely.  Need to re-crimp it when I get time.  Can’t see how that would cause over gassing, arcing and the potential for a nice explosion during charging sure, but increased sulphurificaiton of the plates?  Can’t see how.  

[Robbo]

All this talk of electrolytes reminds me of..

 

 

[Keajre]

2 minutes ago, Robbo said:

All this talk of electrolytes reminds me of..

 

 

Hahahaha

[nicknorman]

40 minutes ago, Keajre said:

Getting back to basics though, the single battery in the bank of 4 thats showing a lower SG reading than the others actually has LESS electrolyte than the others in it, so on that basis alone the SG should be higher.  So where is the sulphate?  Must be on the plates right.  Or as mentioned before, it wasn’t topped up with the correct amount of acid at the time of production.  The load side negetive cable does connect to that battery, and I discovered yesterday that the crimp is faulty, the 70mm2 cable just slides freely inside the lug.  In fact I removed it and re-inserted it freely.  Need to re-crimp it when I get time.  Can’t see how that would cause over gassing, arcing and the potential for a nice explosion during charging sure, but increased sulphurificaiton of the plates?  Can’t see how.  

Well obviously a bad connection could result in it not getting charged fully, however that would equally apply to the other battery in series.

 

Anyway, it is only a small sg discrepancy. Fix the crimp, move that battery to the other chain and see what happens!

[MtB]

Ok a brief goggle reveals the SG of H2SO4 is 1.840. And an acid/water mix of density 1.277 contains about 30% H2SO4. 

 

So I reckon there is every incentive for a battery supplier struggling to squeeze a profit from flogging batteries to price sensitive boaters to stretch the (probably) very expensive 1.277 battery acid in the carboy with a few litres of tap water!  

 

Thus might even explain the widely varying performance I’ve noticed from many different sources of apparently identical el cheapo leisure batteries I’ve bought over the decades. 

 

Next time I buy new batteries I’ll take my dvm and refractometer along with me. 

[MtB]

 

Ok this raises another question doesnt it? What will be the effect on a new battery from filling it with acid over-diluted to say 1.250?

[Keajre]

1 minute ago, Mike the Boilerman said:

 

Ok this raises another question doesnt it? What will be the effect on a new battery from filling it with acid over-diluted to say 1.250?

Reduce capacity and voltage readings as the ionic process would stop earlier 

[MtB]

Ok just goggled for the price of carboys of battery acid and now it gets interesting. First result is 20 litres of generic battery acid for $43. 

 

The really interesting thing is this cheapo generic battery acid in carboys is 1.265 SG so is already over-diluted!

[WotEver]

17 minutes ago, Mike the Boilerman said:

The really interesting thing is this cheapo generic battery acid in carboys is 1.265 SG so is already over-diluted!

Not necessarily. It depends on the battery construction. The acid strength also varies in different parts of the world according to climate.