Hydrometers and batteries - and happy Christmas

[stuhaynes]

I know that a hydrometer is probably best used to compare individual cells to identify duff cells, but out of interest, what SG would you expect from a battery cell if charged to full and in the best of order??

Happy Christmas everyone.

 

[David2452]

1.275 or slightly above.

 

[nigel1]

Will depend on type of battery, Lead Acid/Gel/AGM, and what the battery has been designed for, deep cycle, starter or traction.
Temperature has a big effect, 10C temp difference produces a 0.007 difference in SG
Bit more info here
http://batteryuniversity.com/learn/article/how_to_measure_state_of_charge

Remember you need to fully charge the battery, allow it to rest, and remove surface charge before testing

 

[Dave100456]

Happy Christmas to you too

This may be of use.

http://www.batteryfaq.org/ follow to FAQ then index 4.4.1

I have an A4 excel version of this table that contains other useful battery info which I have laminted and keep on the boat for reference. pm me if with your email and I'll send a copy to you if you want it.
Happy to share it if someone can tell me how to attach it to a post.
Cheers
Dave

 

[David2452]

Will depend on type of battery, Lead Acid/Gel/AGM, and what the battery has been designed for, deep cycle, starter or traction.
Temperature has a big effect, 10C temp difference produces a 0.007 difference in SG

Anybody tried drawing off electrolite from a gel or AGM with a hydrometer?

 

[pvb]

Anybody tried drawing off electrolite from a gel or AGM with a hydrometer?

Detail, merely detail!

 

[William_H]

Hydrometer SG reading

A battery works by converting lead sulphate into lead to produce current and the reverse happens on charge. During this process the electrolyte of sulphuric acid becomes more concentrated with charge and less with discharge. However much depends on the concentration of acid at full charge or full discharge.
So a battery will function with quite diluted acid or quite strong acid. Indeed over the life of the battery the acid will become more dilute as water is added. which does mean that the actual SG of the acid is not in itself a reliable measure of charge of a battery.
As said it can be used as comparison between cells provided one cell has not had more water added than any other, but mostly between discharge and charged state.
So to really asses a battery you need to compare voltages and recent charge history. (battery monitor) good luck olewill

 

[Plevier]

Indeed over the life of the battery the acid will become more dilute as water is added. which does mean that the actual SG of the acid is not in itself a reliable measure of charge of a battery.

No no and thrice no!*
Water is added only to replace water lost through electrolysis (gassing). No acid is lost in this process and it has no effect on the s.g.
Where acid is lost is in the formation of permanent sulphation in the battery - when lead + acid irreversibly becomes water + lead sulphate, rather than reversibly in the charge/discharge cycle. This will result in some reduction of the s.g. over the lifetime. That (apart from spillage of course) is the only mechanism reducing s.g.
In a healthy battery, s.g. (or open circuit voltage) is a good indicator of state of charge.
Where it goes wrong is when people assume that a good s.g. (or OCV) indicates a healthy battery. It doesn't. You can have a battery with negligible capacity left and it will still show full s.g. and OCV within normal tolerances.
There is a very simple relationship between s.g. and OCV over the normal working range of s.g from about 1.100 to 1.300 at temps of 20-25 deg C.
OCV=(s.g. + 0.85)

* note for olewill - not being over-emphatic, just plagiarising catch phrase "nay, nay and thrice nay" of deceased comedian Frankie Howerd who you may be unaware of!

 

[sailinglegend420]

No no and thrice no!......

Thanks for that very good explanation of why SG is not reliable for anything except comparing unbalanced cells.

 

[stuhaynes]

Thanks for that very good explanation of why SG is not reliable for anything except comparing unbalanced cells.

Thanks everyone, very helpful. I now know an approx acceptable SG and that the comparison of cells is important in deciding whether a battery is kn&ckered or not. Thanks!

 

[William_H]

No no and thrice no!*
Water is added only to replace water lost through electrolysis (gassing). No acid is lost in this process and it has no effect on the s.g.
Where acid is lost is in the formation of permanent sulphation in the battery - when lead + acid irreversibly becomes water + lead sulphate, rather than reversibly in the charge/discharge cycle. This will result in some reduction of the s.g. over the lifetime. That (apart from spillage of course) is the only mechanism reducing s.g.
In a healthy battery, s.g. (or open circuit voltage) is a good indicator of state of charge.
Where it goes wrong is when people assume that a good s.g. (or OCV) indicates a healthy battery. It doesn't. You can have a battery with negligible capacity left and it will still show full s.g. and OCV within normal tolerances.
There is a very simple relationship between s.g. and OCV over the normal working range of s.g from about 1.100 to 1.300 at temps of 20-25 deg C.
OCV=(s.g. + 0.85)


* note for olewill - not being over-emphatic, just plagiarising catch phrase "nay, nay and thrice nay" of deceased comedian Frankie Howerd who you may be unaware of!

Thanks Troubadour. Yes i was aware of chemical reaction which should retain actual acid but was considering the way a battery if enthusiastically charged will always seem to spit out a bit of acid ultimately resulting in loss of acid and SG.
Frankie Howerd? yes I am old enough to remember him, in those days we had a lot of UK TV comedy. I don't rememeber his saying s though. What you really mean from Vicar of Dibley is No No No No ...Yes
Happy New year to you olewill

 

[Plevier]

Thanks Troubadour. Yes i was aware of chemical reaction which should retain actual acid but was considering the way a battery if enthusiastically charged will always seem to spit out a bit of acid ultimately resulting in loss of acid and SG.
Frankie Howerd? yes I am old enough to remember him, in those days we had a lot of UK TV comedy. I don't rememeber his saying s though. What you really mean from Vicar of Dibley is No No No No ...Yes
Happy New year to you olewill

Keep the vent plugs in when boost charging and you won't lose any acid with any decent modern battery.
It used to be the practice to remove them when battery alloys contained around 10% antimony and really did fizz if the typically manually regulated charger put too many volts on. Bad practice now with low antimony or calcium and automatic chargers.
Vents normally contain a porous ceramic or sintered plastic disk avoiding flashback, another good reason to keep them in.