Battery - Checking Specific Gravity.

[larryinnewyork]

Several years ago, I bought a Refractometer for checking the Coolant strength.
Works good and very accurate.

Yesterday (for the 1st time) I used it to check the Batteries Specific Gravity.
If anyone isn't familiar with Refractometers, here's a LINK

https://www.amazon.com/Fendior-Specific-...tometer+coolant

Note: Some measure temp. in 'C' ... Mine is 'F'

Checking all 6 Cells (compensating for temp), I got readings of 1.228 thru 1.238 (Fair)
------------------------
1.250 - 1.400 Good
1.200 - 1.250 Fair
1.100 - 1.200 Recharge
------------------------
Although I plan on replacing the Battery every 4-5 years, I think checking the Specific Gravity
will be good PM work and may show me early a defective battery (by comparing cells).

Your thoughts and comments are welcomed.

 

[WobblyElvis]

I used to have one of those tools but I eventually stopped using it. A simple volt meter and a visual examination of the plates tells me all I need to know. If after charging slowly for days, a battery is still no good, I just throw it away.

 

[JHZR2]

How does fhis inform you of capacity loss or impedance growth? It cant really. You only get that by looking at conduction voltage under reasonable loads, and doing a reasonable constant power or constant current discharge.

With a high enough over potential, the cells may equalize and then have more reasonable SG.

 

[George7941]

Originally Posted By: JHZR2
How does fhis inform you of capacity loss or impedance growth? It cant really. You only get that by looking at conduction voltage under reasonable loads, and doing a reasonable constant power or constant current discharge.

With a high enough over potential, the cells may equalize and then have more reasonable SG.


Capacity loss is usually due to sulfation. Doesn't sulfation tie up sulfuric acid and thus weaken the strength of the electrolyte? Correct me if I am wrong but a sulfated battery will show low specific gravity even when fully charged.

 

[funflyer]

Originally Posted By: JHZR2
How does fhis inform you of capacity loss or impedance growth? It cant really. You only get that by looking at conduction voltage under reasonable loads, and doing a reasonable constant power or constant current discharge.

With a high enough over potential, the cells may equalize and then have more reasonable SG.



No load necessary, all one needs is a capacitance tester and to stop messing with the greasy kid's stuff. SOLAR BA5

 

[kb27]

I use this on occasions when I need to ensure the battery is not sulfated
https://www.amazon.com/gp/product/B000JFHMRU/
Very simple test and accurate.

 

[SHOZ]

If the battery gets acid on the top or around the post and you add water then you lower the gravity. A larger than normal battery with a lower specific gravity (around the 'fair' numbers) will last longer.

 

[JHZR2]

Originally Posted By: funflyer
Originally Posted By: JHZR2
How does fhis inform you of capacity loss or impedance growth? It cant really. You only get that by looking at conduction voltage under reasonable loads, and doing a reasonable constant power or constant current discharge.

With a high enough over potential, the cells may equalize and then have more reasonable SG.



No load necessary, all one needs is a capacitance tester and to stop messing with the greasy kid's stuff. SOLAR BA5


loading is the only definite. That Solar BA-5 doesnt even report in mOhms, so is entirely useless to me. Vedors report new battery impedance in mOhms.

 

[JHZR2]

Originally Posted By: George7941
Originally Posted By: JHZR2
How does fhis inform you of capacity loss or impedance growth? It cant really. You only get that by looking at conduction voltage under reasonable loads, and doing a reasonable constant power or constant current discharge.

With a high enough over potential, the cells may equalize and then have more reasonable SG.


Capacity loss is usually due to sulfation. Doesn't sulfation tie up sulfuric acid and thus weaken the strength of the electrolyte? Correct me if I am wrong but a sulfated battery will show low specific gravity even when fully charged.


Hard irreversible sulfation takes some lead out of the mix. Soft surface sulfation can be reversed, but the yields on reversing (or attempting to reverse) either arent 100% understood.

Sulfation does obviously remove available sulfate from the electrolyte. The greater concern though is the formation of high resistance hard sulfate. For things like starter batteries, the capacity is barely utilized, but impedance (which shows as voltage drop and increased current) is critical. That's not necessarily the case for DC and UPS batteries, of course, but that's not really the discussion here.

 

[wrcsixeight]

as an Example.

I have a group31 marine battery made by US battery. It is 3 years old, and has accumulated over 500 deep cycles in that time, before being relegated to much shallower, much less frequent cycling.

Recently a hydrometer read 1.275+ or higher on 5 cells with the last cell, closest to (-) post topping out at 1.260.

It recently sat fully charged, for 16 days, and read 12.78 volts on day 17. Disconnected from all possible loads and charging sources.

The specific gravity on 5 of the 6 cells, and resting voltage after 2 weeks would indicate this battery is still relatively healthy.

But if I load it with 18 amps of 12v air compressor, voltage drops quickly to under 12 volts and kind of levels off at 11.7v, and will rebound fairly quickly to 12.65v+ a minute after load removal.

Specific gravity can determine the level of charge of a cell, If a low reading never exceeds a certain number despite being held at a high cell voltage, it can indicate a weak cell. But good readings and good resting fully charged voltage, does not necessarily indicate a healthy battery which can start the car when temperatures fall, or with a minimal amount of the charge removed from its compromised capacity.

The hydrometer is a battery polygraph, indicating how well the battery is charged, but it does not necessarily indicate good health of the battery if the SG readings are indicating 1.275+.

Nor does a high resting voltage indicate a healthy battery, as so many seem to think, but a quickly falling fully charged resting voltage does indicate an unhealthy battery.

 

[JHZR2]

Originally Posted By: wrcsixeight


It recently sat fully charged, for 16 days, and read 12.78 volts on day 17. Disconnected from all possible loads and charging sources.

The specific gravity on 5 of the 6 cells, and resting voltage after 2 weeks would indicate this battery is still relatively healthy.

But if I load it with 18 amps of 12v air compressor, voltage drops quickly to under 12 volts and kind of levels off at 11.7v, and will rebound fairly quickly to 12.65v+ a minute after load removal.


That's conduction voltage which is based upon impedance.

In an electrochemical cell, at equilibrium, youll get a voltage that is the sum of the two half-cell's reduction potentials. That's HS chemistry. Its called Voc or Open-Circuit voltage.

To discharge, the conduction voltage that the battery outputs an amount of current at is calculated as Vt=Voc-I*R, where Vt is the voltage at the terminals, I is the current flowing and R is the impedance of the battery.

To charge, you need to overcome some thermodynamic penalties, so the calculation is then Vt=Voc+IR.

The open circuit voltage, Voc is state of charge-dependent. The lower the state of charge, the lower the open circuit voltage will be.

Conversely, for a semi-discarged battery, if a charge voltage is set, you can calculate how much current the battery can accept by running the calculation:

I=(Vt-Voc)/R

A lead acid battery shuttles back and forth between lead oxide and an unstable form of lead sulfate. If hard, unrecoverable hard sulfate is produced, it takes sulfate out of solution (fixable), but increases impedance.

Notionally, a drop or three of sulfuric acid would fix the lost sulfate ions. Nothing will fix the conduction voltage drop due to a high impedance.