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Batteries are the most neglected item in our fleet.  Fido worked in a US Army battery shop for a year at the US Army National Training Center and Fort Irwin, in the Mojave Desert.  He is quite knowledgeable on batteries.   

A battery is an electro-chemical device.
It creates a difference of electrical potential between the terminal posts, thru the chemical action of the electrolyte upon the active materials of the positive and negative plates, which are made of lead.
The electrolyte is a combination of sulfuric acid and water.  The chemistry of that mixture dictates the voltage of one cell.
Each battery cell is about 2.11 volts at full charge.
That's why all 12 volt batteries have 6 cells, totalling 12.66 volts at full charge.
Cold Cranking Amps is the number of amps a battery can deliver for 30 seconds at zero degrees, at 1.2 volts per cell.
There are 4 things to worry about in proper battery maintenance.
Deep cycling,  sulfation,  overcharging,  and vibration.
Deep cycling is when the battery is drained of all it's juice, then recharged to it's full charge.  This makes the battery hot and causes expansions and contractions which in turn causes the plates to warp. 
Sulfation is what happens when batteries are undercharged for a long time.
This is what makes a battery useless after it is pulled out and not put on a slow charger soon.  Anytime you pull out a battery, get it on a charger as soon as possible.  Sulfation grows on the plates and becomes thick and hard, and eventually the plates cannot create a chemical reaction with the electrolyte.
Overcharging is usually caused by too high a voltage regulator setting.
When the batteries are hotter than hell, or steaming, maybe even buldging in and out like lungs, they are overcharging. 
This happens when voltage higher than what the battery puts out is applied to the battery. 
Vibration is from the batteries not having any hold down attached.  The batteries just shake and shake all down the road, and the plates crack and crumble, the case cracks, the battery is toasted.  It takes about 5 seconds to put on a battery hold down bolt.
Batteries should always be replaced in sets.
As the bad battery was dying, the vehicles electrical system put extra stress on the other battery.  They both should be tested and put on a slow rate charger.
Most believe that if batteries were placed on concrete, they would discharge faster than batteries stored on wood.
The fact is, battery cases used to be made out of a more porous material, and small amounts of acid would leak out, removing the charge.
These days, we use polypropolylene plastic cases. Period.
The electrolyte of a new fully charged battery has a specific gravity of 1.26 to 1.28 at around 80 degrees.  A specific gravity of  1.19 is 50 percent charged.
1.12 is completely discharged.
Specific gravity can be checked with a hydrometer.  If you don't know what a hydrometer is, then how are you a mechanic ? 
A battery with 12.66 volts is at full charge.
12.24 volts is at half charge.
11.89 volts is discharged.
6 Volts is not half charged.
A batteries state of charge cannot be determined properly with a Digital Multimeter.  The measurement you get will not indicate if there is a weak or defective cell. 
When removing a battery,  always disconnect the negative terminal first.
And put it back on last.
After removing the cables and hold down, use a good battery cart to get the batteries out.  It is easier and avoids spillage of electrolyte. 
Once the batteries are out, inspect the box, clean all corrosion from the area, along with the hold down bolt and channel, replace as necessary.  HOLD DOWNS ARE IMPORTANT.
A solution of baking soda and water is good for cleaning battery and battery storage.
Clean all the battery connections in the battery compartment.  Use Aerosolve 2 from ZEP, and a wire brush.  Remove DDEC fuses before wire brushing the terminal strip at left of the battery box.
Clean the cable terminals good with a terminal cleaner.
Sweep out the battery box.  Spray paint any areas that may look bare. 
Clean off the batteries and terminals.  Use baking soda-water, then pressure wash if you have that luxury.  Check the electrolyte level in each battery.  It should be over the plates, do not overfill, leakage will result
Put the batteries back in the coach, reverse how you took them out, hook up the negative battery cable last.
Make sure you install the battery hold-down channel and bolt.  Otherwise the batteries will bounce and vibrate and be ruined.
There needs to be no battery movement during vehicle operation.
Vibration tends to shake the active materials out of a batteries plates, crack the plates, seperate plates from posts, loosen terminals, and even crack the battery case.
The battery should remain in a fully charged state  at all times.  This will prevent sulfation.  A discharged battery should always be charged as soon as possible. 
If the alternator is left to charge the drained batteries, it will probably be damaged from overheating and shorten the alternator life. 
Charge batteries with a low-rate slow charger.
In short....
Make sure the batteries are secured, cables tight and clean, caps on, not hot, not over flowing or steaming, no corrosion, and full of electrolyte.
Regulator should be set to 27.2volts on an MC12 and 27.4 on the rest.  Check regulator output with all lights on, AC running.    
Corrosion is the enemy, and is the cause of many, many problems and roadfailures.......

G45 AC problems
Corrosion here

Coach would not start
Caused by corrosion

Lights not working ???
It's usually corrosion.

Your source for maintenance communication. 

We keep 'em rollin'...

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