battery acid

[wotayottie]

Likely a daft a question but I want to be sure.

Batteries arrive with acid in them and you can measure their degree of charge with a hydrometer. Now I have installed an !advanced regulator" and the leaflet with it warns that thiss means more gassing and the need to top up the batteries. So I topped them up.

But if I have done this too generously, it seems to me that the specific gravity of the battery acid will fall, the hydrometer will then indicate that the batteries are discharged when in fact they are fully charged.

If this is not the case then there really is something odd going on. At the moment with no engine going my batteries are reading 13.6 v after a 2.5 hour run with the charger belting out 14.4 volts. So they should be charged but the hydrometer says they arent.

Any fault in this logic?

 

[pvb]

Fluid loss in batteries is just water, so you top them up with water (distilled, etc).

Reading the voltage soon after a charging period doesn't give an accurate picture of the state of charge. The battery needs to "rest" for some hours before you can get a sensible voltage measurement.

Bottom line is: make sure the batteries have adequate electrolyte, top up with water if required, but don't overfill them.

 

[VicS]

Normally the volume of water you add is small compared with the total volume of acid so it will have a relatively small effect on the density.

If you top them up to the correct level and measure the density when they are at the correct level you will get the "correct" density reading.
BUT
It takes quite a while for any water you add to fully mix with the acid so do not expect a "correct" reading on your hydrometer until this has happened.

No I dont know how long it will take to mix but a good charge that causes the battery to gas should help to mix it.
I seldom bother with a hydrometer.

 

[sarabande]

to get a useful handle on the figures you have given, please say what is the configuration, capacity, previous state of charge, output (amps) of charger.

 

[William_H]

Battery acid

I think you will find that battery acid (dilute sulphuric acid) becomes more acid with charging of the battery however...
The real action is the change of state of the lead material in the battery plates. The acid becoming stronger is a byproduct of that action. The acid is not "charged" but simply facilitates the charging. The battery itself can function with a range of acid Ph (strength). If the acid is too weak then on discharge the lack of acid will limit discharge capacity but an excessively strong acid has no advantage.
So if you start charging with a very weak acid low SG (specific gravity) then when fully charged the SG will still be low.
If you start with a very strong acid high SG when the battery is flat the hydrometer will indicate charged when it is flat. Charging the battery will increase the SG even further.
There is of course an ideal SG for the acid being about 1250 when charged (check on that) but variations of that number happen through the life of the battery. As said the boiling of with charge tends to remove water but not the acid unless it is a really vigorous boiling so mostly adding water is OK but does reduce SG.

So again like voltage SG is not a reliable measure of the charge of the battery. However by charting SG of all cells individually and charting voltage over a period of repeatable use you could get indication of impending/actual failure of battery or of failure of the charging.More like trend monitoring than actual useful numbers. Most people treat their batteries like their car battery and mostly leave it to the system to look after the battery and worry when there is a failure.
good luck olewill

 

[Plevier]

Fully charged specific gravity in most wet batteries is specified in the range 1.250-1.280. The higher figure tends to be used in small lightweight batteries and gives a bit more output at the expense of a slightly shorter life because of increased corrosion rate (particularly in high temperatures). Conversely the lower figure will tend to be specified for large upmarket long life batteries. Most common in the UK for vehicle type batteries is (probably) 1.260.
In all cases the s.g fully discharged will be about 1.100.
Yes when you top up you will lower both these figures but not enough to worry about.
The real point is that only a battery absolutely correctly commissioned, in perfect condition and in a fully charged state fresh from several days on a good quality charger is going to show that value of s.g.
2.5hrs engine on will certainly not get the battery fully charged.
Most alternator charged batteries on intermittently used vehicles tend to hover around 85% state of charge and show s.g of 1.230-1.240 rather than 1.260. On colour coded hydrometers (e.g Halfraud's £5 sort) this will be upper white or yellow rather than green.
More important than the actual number is whether all cells show the same. If there is much difference a boost charge is a good idea if you have the facility (only wet batteries that you can top up).

 

[Plevier]

The real action is the change of state of the lead material in the battery plates. The acid becoming stronger is a byproduct of that action. The acid is not "charged" but simply facilitates the charging.

Mmmmm a bit misleading I'm afraid.
It's more than a "change of state" of the lead and the acid is involved in the reactions.
A fully charged positive plate is lead peroxide. In discharging it combines with the sulphuric acid electrolyte to form lead sulphate and water.
A fully charged negative plate is metallic lead. In discharging it also combines with the sulphuric acid electrolyte to form lead sulphate and water.
So the acid is consumed in the discharge - hence the reduction in specific gravity - and reformed in the charge. It's an integral part of the storage of energy, not just a facilitator.
Batteries store energy in chemical, not electrical, form.

 

[wotayottie]

Interesting comments Troubadour. I topped up the batteries generously - from electrolyte just covering the plates to having maybe half an inch depth over them but I dont know what volume of electrolyte there was in there originally. My hydrometer is calibrated in SG numbers and working from these I would have had to have added 30% extra water to have taken the SG down from the 1300 the instrument shows as fully charged to the 1200 it shows as the start of discharged. I cant see that I have done that.

But then the whole thing depends on how much acid they put in in the first place. And presumably also on how much stuff has fallen out of the plates etc.

 

[Stu Jackson]

If this is not the case then there really is something odd going on. At the moment with no engine going my batteries are reading 13.6 v after a 2.5 hour run with the charger belting out 14.4 volts. So they should be charged but the hydrometer says they arent.

Any fault in this logic?

Not a daft ?, just let them sit for 24 hours before you test them again. It wouldn't hurt to go sailing for a day and mix up the new water with the old acid, either.

If your analysis continues and the hydrometer says they're dead but you still have good voltage (12.8 or so) after 24 hours, consider a new hydrometer.

If that fails, then tell us how old your batteries are.

 

[Plevier]

Interesting comments Troubadour. I topped up the batteries generously - from electrolyte just covering the plates to having maybe half an inch depth over them but I dont know what volume of electrolyte there was in there originally. My hydrometer is calibrated in SG numbers and working from these I would have had to have added 30% extra water to have taken the SG down from the 1300 the instrument shows as fully charged to the 1200 it shows as the start of discharged. I cant see that I have done that.

I wouldn't consider half an inch over the plates excessive in large-ish batteries.
1300 is too high; the only batteries I know of using sg that high are some SLA ones, they have to because of limited acid volume. If your batteries are normal truck or "leisure" you want 1250-1260. You haven't said what they are.
1200 is somewhat down however. As Stu J asks, has the acid been mixed up or are you getting a false reading from a dilute top layer? Stratification can be a problem. A boost charge should do it.
Have you got a good multi rate mains charger or do you rely on the alternator? If you have, give them a boost then 2-3 days float charge - ca 13.8-14.0V -and see what you get then. It is the only way of getting to full charge. Smart chargers get you to around 80-85% in a few hours but there is just no other way of getting the last 15-20% in. High voltage at this stage will just result in more gas not a significantly quicker charge.
If it's still 1200 after that they are not in good shape.

PS Above all assumes European style lead-low antimony construction. Lead-calcium - American style - tends to need higher voltages. Same sg though.