I hear it repeated endlessly on boating forums that the only "good measure" of a flooded batteries health is to take a specific gravity reading (SG). A battery showing a 100% SOC reading will have a temp correct specific gravity of between 1.265 and 1.280. While SG can certainly help diagnose an inter-cell battery issue, all it does is tell you the state of charge of each individual cell, as related to the others, and create the potential for holes in your clothes.
The best use of SG testing is to determine if each cell in the battery is at the same state of charge (SOC). Obviously AGM and GEL batteries can't have their SG measured so an OCV test (open circuit voltage) is really the only predictor that can be easily used.
An OCV test is the sum of all cells in the battery. For example, in a healthy rested 12V battery, each cell would be delivering a 100% SOC voltage of approx 2.12V. There are 6 cells in a 12V battery so...
2.12V X 6 Cells = *12.72V or 100% SOC (*Trojan Data for OCV)
However, in a flooded battery that has become out of balance, due to partial state of charge use (PSOC), sulfation, age, over or under charging, or infrequent full charges with absorption voltage held long enough, the cells can get out of balance and the low cell causes the normal cells to begin getting over charged.
12V Battery Under Charge With 1 Bad Cell:
*2.51V + 2.51V + 2.51V + *2.05V + 2.51V + 2.51V = 14.6V
*A cell at 2.51V is the 12V equivalent of regularly charging the battery at over 15V.
*A cell at 2.05V is the 12V equivalent of being charged at just 12.3V. As once cell gets low the decline becomes more rapid due to chronic under charging of that cell.
For a 14.6V absorption charge, the voltage of each cell, in a healthy battery, should be: 2.43V X 6 = 14.6V.
With this example, taking an SG measurement can show you what is going on between cells.
SG = SOC of each cell in the battery - Can show you a cell imbalance
OCV = SOC of all six cells in the battery - Can't show you a cell imbalance
The Problem:
What OCV and SG can't tell you is the state of health of the battery as related to Ah capacity. Actual Ah capacity and SOC are entirely different measurements. A dead battery, by capacity standards, can still easily reach 100% full.
Example 100Ah Battery:
New Capacity & SG = 100Ah & 1.280 - 100% SOC
Used Capacity & SG = 55Ah & 1.280 - 100% SOC
In both cases the battery attained 100% SOC yet the stored capacity is telling us the battery is dead.
Even a severely unhealthy battery can deliver an SG reading of 1.265 - 1.280 or a 100% SOC resting voltage. It is often repeated, incorrectly, that an unhealthy battery will never attain 100% SOC or that SG will never be 1.265 - 1.280 with a battery that has diminished capacity. These points are simply not true.
Considering SG readings for a failed battery means a failed battery will have a cell imbalance of .050 SG difference between cells. With SG differences of .030 or less corrective equalization's can help but is not necessarily a cure. With a cell imbalance this is the only way SG can dictate SOH and it is really only a pass or fail..
Myths I've heard repeatedly:
"An unhealthy battery will never deliver a good SG reading." = FALSE
"A battery that has lost capacity will never reach 100% SOC." = FALSE
"Specific gravity can tell you the capacity of the battery." = FALSE
Truth:
"Specific gravity cell tell you the cell to cell balance within the battery." = TRUE
An example from two weeks ago where SG & OCV fails to show any SOH:
Trojan SCS-225 (Group 31) - May 2015 Date Code
Rated Ah capacity = 130Ah
In service Date June 17, 2015
Months of Actual use 8.5 Months
Actual Deep Cycles to 50% DOD = 45 (approx - owners data)
Lab Rated Cycles to 50% = 600
Boat Type = Sail / On Mooring
Ah Capacity Test Date = May 5, 2017
Tested Ah Capacity @ 76F = 81.77 Ah (6.5A constant load to 10.5V)
Percentage of Rated Capacity = 62.9% / FAIL
SG Readings = 1.280 - 1.283 (All six cells - using sight refractometer and hydrometer)
Factory Rated 100% SOC SG = 1.280
Resting Voltage Day 5 @ 72F Battery Temp = *12.803V
Factory Rated 100% SOC to OCV = 12.72V
* 24 hours rest to remove "surface charge" is often no where near enough to attain a "rested voltage", especially if the battery temp is below 80F. This one was at 72F. The 12.803V OCV was taken at day 5 with a NIST calibrated Fluke 289. This battery is considered at 100% SOC, as defined by Trojan Battery, at 12.72V. Even at 5 days after the last charge it was still sitting/resting +0.083V above "100% SOC" and this is with a DEAD battery confirmed by Ah capacity testing..
As can be seen above specific gravity tells us nothing about SOH in this case, even though the battery is testing at just 62.9% of its rating, and has failed. By industry standards a flooded deep cycle battery is considered dead when it can no longer deliver 80% of its factory rating. Even for coastal use 63% of rated capacity is simply not worth leaving the mooring with..
The only true way to know your deep cycle batteries actual SOH, as related to stored capacity, is to conduct a 20 hour capacity test. Specific gravity can be entirely misleading and in some cases does little more than ruin clothes.
The best use of SG testing is to determine if each cell in the battery is at the same state of charge (SOC). Obviously AGM and GEL batteries can't have their SG measured so an OCV test (open circuit voltage) is really the only predictor that can be easily used.
An OCV test is the sum of all cells in the battery. For example, in a healthy rested 12V battery, each cell would be delivering a 100% SOC voltage of approx 2.12V. There are 6 cells in a 12V battery so...
2.12V X 6 Cells = *12.72V or 100% SOC (*Trojan Data for OCV)
However, in a flooded battery that has become out of balance, due to partial state of charge use (PSOC), sulfation, age, over or under charging, or infrequent full charges with absorption voltage held long enough, the cells can get out of balance and the low cell causes the normal cells to begin getting over charged.
12V Battery Under Charge With 1 Bad Cell:
*2.51V + 2.51V + 2.51V + *2.05V + 2.51V + 2.51V = 14.6V
*A cell at 2.51V is the 12V equivalent of regularly charging the battery at over 15V.
*A cell at 2.05V is the 12V equivalent of being charged at just 12.3V. As once cell gets low the decline becomes more rapid due to chronic under charging of that cell.
For a 14.6V absorption charge, the voltage of each cell, in a healthy battery, should be: 2.43V X 6 = 14.6V.
With this example, taking an SG measurement can show you what is going on between cells.
SG = SOC of each cell in the battery - Can show you a cell imbalance
OCV = SOC of all six cells in the battery - Can't show you a cell imbalance
The Problem:
What OCV and SG can't tell you is the state of health of the battery as related to Ah capacity. Actual Ah capacity and SOC are entirely different measurements. A dead battery, by capacity standards, can still easily reach 100% full.
Example 100Ah Battery:
New Capacity & SG = 100Ah & 1.280 - 100% SOC
Used Capacity & SG = 55Ah & 1.280 - 100% SOC
In both cases the battery attained 100% SOC yet the stored capacity is telling us the battery is dead.
Even a severely unhealthy battery can deliver an SG reading of 1.265 - 1.280 or a 100% SOC resting voltage. It is often repeated, incorrectly, that an unhealthy battery will never attain 100% SOC or that SG will never be 1.265 - 1.280 with a battery that has diminished capacity. These points are simply not true.
Considering SG readings for a failed battery means a failed battery will have a cell imbalance of .050 SG difference between cells. With SG differences of .030 or less corrective equalization's can help but is not necessarily a cure. With a cell imbalance this is the only way SG can dictate SOH and it is really only a pass or fail..
Myths I've heard repeatedly:
"An unhealthy battery will never deliver a good SG reading." = FALSE
"A battery that has lost capacity will never reach 100% SOC." = FALSE
"Specific gravity can tell you the capacity of the battery." = FALSE
Truth:
"Specific gravity cell tell you the cell to cell balance within the battery." = TRUE
An example from two weeks ago where SG & OCV fails to show any SOH:
Trojan SCS-225 (Group 31) - May 2015 Date Code
Rated Ah capacity = 130Ah
In service Date June 17, 2015
Months of Actual use 8.5 Months
Actual Deep Cycles to 50% DOD = 45 (approx - owners data)
Lab Rated Cycles to 50% = 600
Boat Type = Sail / On Mooring
Ah Capacity Test Date = May 5, 2017
Tested Ah Capacity @ 76F = 81.77 Ah (6.5A constant load to 10.5V)
Percentage of Rated Capacity = 62.9% / FAIL
SG Readings = 1.280 - 1.283 (All six cells - using sight refractometer and hydrometer)
Factory Rated 100% SOC SG = 1.280
Resting Voltage Day 5 @ 72F Battery Temp = *12.803V
Factory Rated 100% SOC to OCV = 12.72V
* 24 hours rest to remove "surface charge" is often no where near enough to attain a "rested voltage", especially if the battery temp is below 80F. This one was at 72F. The 12.803V OCV was taken at day 5 with a NIST calibrated Fluke 289. This battery is considered at 100% SOC, as defined by Trojan Battery, at 12.72V. Even at 5 days after the last charge it was still sitting/resting +0.083V above "100% SOC" and this is with a DEAD battery confirmed by Ah capacity testing..
As can be seen above specific gravity tells us nothing about SOH in this case, even though the battery is testing at just 62.9% of its rating, and has failed. By industry standards a flooded deep cycle battery is considered dead when it can no longer deliver 80% of its factory rating. Even for coastal use 63% of rated capacity is simply not worth leaving the mooring with..
The only true way to know your deep cycle batteries actual SOH, as related to stored capacity, is to conduct a 20 hour capacity test. Specific gravity can be entirely misleading and in some cases does little more than ruin clothes.
