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Battery Kindergarten (part 1 of 3) Types, charging, maintenance

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I sent myself to Battery Kindergarten yesterday - spent the day with a few books in hopes of raising my boat-IQ a few points (Hey, it's worth a shot).

Parking a bunch of tidbits here mostly to bookmark what (I think) I've learned, so that I can find it when I need to... I recognize that 99% of you already know all of this, and more, but if it happens to prove to be helpful to anyone else, that's a bonus.

Also, any additions or corrections welcome, feel free to leave as comments and I'll integrate over time.

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Battery types

Conventional flooded flooded-cell
(eg, automotive) batteries are optimized for momentary output and immediate recharging (eg, starting a car)
Deliver high cranking amps
Deep discharge shortens life (damage can occur in as little as 20 cycles)
Will sustain damage if left in a discharged state
Good for a START battery, NOT good for HOUSE battery

"Maintenance free" batteries are a version of above, but in a sealed case. Not considered suitable for use on a boat.

"Deep cycle" flooded-cell batteries don't deliver as much instantaneous current as a conventional flooded battery, but have better deep-cycle life.
Makes them suitable for HOUSE use IF well charged and maintained
Can also be used for START
Heavier, thicker plates are more tolerant of deep discharge and overcharging
Best deep-cycle life ("thousands of cycles" if not discharged below 50%)
Slower to reach full charge when recharging
Can be equalized
Must be maintained (water)
Will sustain damage if left in a discharged state

Gel-cell batteries need the least attention, good if "completely unwilling to monitor your batteries"
Good for START use
Tolerant of deep discharge, but shorter life (~500 cycles) if used for HOUSE
Overcharging (charge voltage over 14.1V) causes damage
Expensive

AGM batteries have similar characteristics to Gel-cell
OK for either START or HOUSE
Most rugged
High cost
Tolerant of deep discharge but shorter life (~250 cycles)
Overcharging will destroy

A HOUSE battery can be used for START, but a START battery "won't survive more than a few deep discharge cycles"

A typical 12V boat battery has 6 cells. Each cell is nominally at 2.10V when fully charged (rested, open-circuit, 80F)
2.10 x 6 = 12.6V
Each cell is effectively "dead" at 1.75V
1.75 x 6 = 10.5V

NOTE: my notes from here are biased toward wet-cell deep-cycle lead-acid batteries (because that's what I have)
If you have AGM, Gel-cell, "maintenance free" or other, be sure you understand the differences/details

Battery Ratings

20-hour rating
is the # of Amp-hours the battery can deliver in 20 hours before cells drop below 1.75V (10.5V total)

Cold-Cranking Amps (CCA) is the number of Amps the battery can deliver and sustain at 0 degrees F (freezing) for 30 seconds without cells dropping below 1.2V (7.2V total)

Marine-Cranking Amps (MCA) is the same thing as CCA, but tested at 0 degrees C (32F) instead of 0 degrees F. Kind of a cheat - number will be artificially higher than CCA, but perhaps more reflective of marine use

Reserve indicates the number of minutes the battery can deliver 25A before dropping below 10.5V Mostly a car thing - how long will if keep going if alternator fails

Battery Charging

The optimal charging process is "as quick as possible without damage to the battery."

Modern chargers (and regulators) do 3-stage charging.

1: BULK charging sends a lot of current to the battery - up to 25% of Cb (battery's rated AH-capacity)
So, charger might be sending up to 25 amps to a 100-Ah battery; gel-cells may take up to 50% of Cb
Bulk stage continues until battery is about 75% charge
Bulk rate for Exide 27MDP = 14.5-14.9V, 25 A (from Exide spec sheet)

2: ABSORPTION charging is charging at 14.4V while allowing current to match what the battery can accept without GASSING
As state of charge increases above 75%, battery is able to absorb current at a decreasing rate
When current drops to 0.05 Cb, battery is about 85% charged
When current drops to 0.02 Cb, battery is about 90% charged
When current drops to 0.01 Cb, battery is "nearly 100% charged"
Absorption rate for Exide 27MDP = 14.5-14.9V, variable A (Exide spec sheet)

3: FLOAT charging is delivering current to keep the battery at a fully-charged state
"Fully charged" is generally 13.2V
Ideal float-voltage is 0.1V above "fully charged" voltage (so, 13.3-13.5V)
Float voltage for Exide 27MDP = 13.5-13.8V (Exide spec sheet)


Charging Problems

GASSING
occurs when the battery's internal chemical reaction can't keep up with the amount of current coming in.
Excess energy converts water in electrolyte to O2 gas and H2 gas
Gassing results in water-loss, and vents hydrogen gas into boat
Gassing begins at 14.4V, holding voltage at less than 14.4V avoids gassing
Max charging voltage should be 14.2-14.6V for wet-acid batteries, 13.8 for AGM, Gel-cell batteries

UNDERCHARGING is leaving the battery in a discharged state. This allows sulfate crystals to form on plates ("sulfation")
Impairs chemical reaction, reduces capacity over time (battery becomes difficult to fully charge)
"Equalization" may help re-convert sulfate crystals back into electrolyte solution.

OVER-CHARGING is delivering more voltage to the battery than the chemical reaction can absorb.
Damages battery by eroding the lead in the plates

OVERHEATING kills batteries. Never charge if the temp is over 120F (eg, a fast-charge in the tropics)

Battery Maintenance

Keep terminals clean

Keep top of battery clean and dry

Add distilled water when needed
Add water up to the BOTTOM of the tubes - don't over-fill
Distilled water only
Use gloves, eye protection against splashes of electrolyte (sulfuric acid!)
Replenish water AFTER charging - if filled when battery is discharged, excess electrolyte will boil out

KEEP RECORDS - can be great indicator of future problems
Amount of water added to each cell
Rested open-circuit voltage for each battery
Specific gravity (SG) of each cell, as measured with hydrometer, corrected for temp

(continued in part 2....)

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Updated 02-19-2018 at 02:09 PM by bgary

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