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Battery Kindergarten (Part 2 of 3) Testing, problems, equalization

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(continued from Part 1)


Battery Testing

Five kinds of testing

1: Open-Circuit Voltage

Open circuit means no current in, no current out
Battery switch to OFF
Battery charger not sending any current (shore-power disconnected?)
Anything directly wired to battery (eg, bilge pump) disconnected
Important to let the battery "rest" (become stable)
Nominally 24 hours, in practice 2 hours is probably enough

SOC for various Open-Circuit voltages:
Voltage
12.6V = 100% SOC
12.4V = 75% SOC
12.2V = 50% SOC
12.0V = 25% SOC
11.8V = 0% SOC

(1) Battery is considered "fully discharged" at 11.8V
Battery plates will start to sustain damage if cells go below 1.95V (11.7V total)
Best practice is to not let voltage go below 50% (12.2V, rested, open-circuit, 80F)

2: Electrolyte / Specific Gravity (SG)
The electrolyte in a lead-acid battery is made of water and sulfuric acid
pure water has a specific gravity of 1.000
pure sulfuric acid has a specific gravity of 1.830
When charged, electrolyte (sulfuric acid and water) as at its most dense (~1.265-1.280)
When discharged, electrolyte is less dense (specific gravity decreases)t

The relationship between charge and specific gravity is linear
This means that measuring specific gravity gives an indication of stored energy at a moment in time
NOTE - This is at a moment in time - it does NOT tell you the capacity, just the current charge-state
SG
1.265 = 2.10V (12.6V) = 100% SOC
1.225 = 2.07V (12.4V) = 75%
1.190 = 2.03V (12.2V) = 50%
1.155 = 2.00V (12.0V) = 25%
1.120 = 1.97V (11.8V) = 0%

(1) Because the relationship is linear, Specific Gravity can tell you state of charge
SG + 0.84 = voltage of cell: e.g., 1.225 + 0.84 = 2.065V\
In theory, a cell could read as much as 1.360, which indicates 2.2V (1.360 + 0.84)
That's where the notion that "fully charged = 13.2V" comes from

(2) Specific gravity readings have to be corrected for temperature
Add .004 for each 10F above 80F
Subtract .004 for each 10F below 80F
So, a reading of 1.240 at 50F is corrected to 1.228 (temp is 30 below 80F, so subtract 3 x .004)

To measure the specific gravity of the electrolyte, let the battery rest for 24 hours with no current in, no current out
(in practice, 2 hours is probably enough to let the battery stabilize)
Electrolyte must be homogenous
Don't measure just after adding water, let the reaction mix the solution for a while
Measure the SG of each cell with a hydrometer
Be sure electrolyte goes back into the same cell it came from
Adjust readings for temperature and keep a record

NOTE: Neither of the above tests indicates battery CAPACITY, only the STATE OF CHARGE.
They just tell you"whatever capacity the battery has, it is fully charged"

3: Load Testing is applying a significant load for 15 seconds, while monitoring voltage
Voltage should drop to around 10.5V and be stable. If it drops below 9.5V, indicates reduced capacity

Can do this test by cranking starter (without starting engine) for 15 seconds, while measuring voltage
Can also do this with a "battery load tester" (eg from Harbor Freight)

4: Full Capacity Testing (eg, "20-hour test") is applying a load of 5% Cb (5A for a 100Ah battery) for up to 20 hours
Leave the load on until voltage drops to 10.5V (1.75V per cell, adjusted for 80F)
Load x Time = actual capacity (eg, 5A x 20 hours = 100Ah)
Note that battery is considered "essentially dead" at 11.8V / 1.97V per cell

If test comes back at less than 80% of rated capacity, try again
Do a full charge cycle (equalize, if needed)
Let the battery rest
Re-do the test
If it still comes back at less than 80% rated capacity, replace it
"once below this point, the battery is likely to fail more quickly"

Battery shop can do this test with specialized equipment (eg, ability to vary the load)
May also be able to do this with a home-built array
12V inverter with enough 120V load (eg, 60W incandescent light) to produce the appropriate load
60W appliance at 12V uses 5A of current, but inverter probably has overhead inefficiency
Can use battery monitor or multimeter to view current usage to get to a good load
May be able to get a valid result with a different load (e.g., 7%) and then adjusting)

5: Conductance Testing
measures the internal conductance of the battery (its ability to transmit current through internal structure)
Can do with a "battery analyzer"
Note that capacity is a required input, the test may not be meaningful without knowing actual (tested) capacity


Problem indicators

Uniformly low electrolyte specific-gravity readings
If the voltage (readings) can't be brought up, probably indicates battery is dying

Electrolyte readings more than 0.015 different between cells
Indicates cells are out of balance, need to be EQUALIZED (deep-cycle wet-cell batteries ONLY)

Electrolyte readings more than 0.050 different between cells
Indicates a bad cell, battery should be replaced

Battery shows full voltage at rest with no load, but voltage drops steadily when load is applied
Charging is okay, but battery capacity has decreased

Wet cells NEVER need water
Indicates undercharging
"A wet-cell battery in sustained cycling may use a cup a month"

Wet cells ALWAYS need water
May indicate overcharging
Should never need more than 2oz per cell in 30-50 hours of charging time

Battery loses significant voltage in short period of time (25% in 2-3 weeks with no load)
Indicates internal short

Poor performance in cold temps
May or may not indicate a problem. Performance declines with temperature
Can expect performance to be degraded as much as 30% at 32F


Battery equalization

Equalization
is "controlled over-charging" to bring all cells back to a fully-charged state
Equalize if specific gravity of cells is more than 0.030 different
Out-of-balance cells can indicate sulfation on plates.
Battery can only discharge as deep as weakest cell, so overall capacity is impaired if out of balance
In an equalization cycle, charger sends 4% of Cb (4A for a 100Ah battery) for 4 hours, or until battery gets to 16.2V
Manufacturer should specify their "conditioning voltage"
Exide flooded batt = 17.5 (see Wing, P37)
Conditioning voltage varies by temp (see Calder, P123)
Equalization is ONLY for heavy-duty deep-cycle batteries

Make sure all electronics are turned off and/or disconnected during equalization
16V can damage them
Equalization will produce lots of gassing
Make sure there's adequate ventilation
Be prepared for electrolyte to boil out of cells
Replace water after equalization

Don't let battery get above 125F (or up more than 35F from ambient temp?)

Battery Monitoring

"monitoring batteries is the key to their performance"


Battery voltage is a good indicator of current state of charge *IF* not loaded, not charging, and at rest
If charging, voltage will read high
If under load, voltage will read low
If recently charged or loaded, battery needs to "rest" to deliver a meaningful reading

Hard-wired monitor
Shows real-time voltage as well as current amount of load or input
NOTE: Casey says amp-hour function of a battery monitor can't be trusted as a way to keep track of available Ah
Batteries use Ah at different rates
A 100Ah battery can deliver 5A for 10 hours before getting to 50% SOC
A 100Ah delivering 50A can take you below 50% SOC in significantly LESS than an hour
So.... it may not be as simple as keeping track of Ah-in and Ah-out, the rate of discharge also has to be taken into account
Monitors have to be calibrated to the battery
Monitors have to be "synchronized" with a resetting charge in order to stay calibrated

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Updated 02-19-2018 at 02:08 PM by bgary

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