Avoiding Internal Shorts During Battery Charge
We recently had a question from someone about a battery failure and charging issue in their RV.
They had an internal short while attempting to charge their battery overnight and were looking to find out how this can be avoided in the future. 

A short can develop when a battery is older and is exposed to vibration and/or heat. A short that develops below the electrolyte level will only result in a dead cell thus creating a 5 cell 10.4-10.7 volt battery.

In this case, the battery most likely got warm during charging and enough fluid evaporated out the vent, lowering the level inside, which likely exposed a short while producing hydrogen gas.  If the short sparks, the hydrogen gas can ignite causing the battery to burst.  Fluid levels on these batteries can and should be monitored by the built in hydrometer.

Please note the following explanation, guidelines and recommendations are high level and it is always recommended that proper training or technical advice be sought while working on any electrical component within a vehicle. 
What condition could cause a battery to fail and short?
A battery can fail because of the ignition of gases inside the battery. An ignition can occur because: 

1. An electrical short developed inside the battery.
2. The cell with a short had lost enough water to expose the short.
3. Combustible gases built up inside the battery due to overcharging.

Removing any one of the 3 conditions would prevent this failure.

A battery with an electrical short will exhibit a voltage of 10.7 volts, often only 10.5 volts.  If only tested with a voltmeter one may be inclined to recharge a 10.5 volt battery in an attempt to raise the voltage to 12.5+ volts.  However, if a short is present then charging this battery could lead to the generation of dangerous gases.  Testing with a battery analyzer will give the technician a much better opportunity of finding battery shorts.
Water loss inside the battery is caused by two conditions, high temperatures leading to evaporation and overcharging leading to the conversion of liquid to gas (oxygen and hydrogen). 

Water loss due to evaporation depends on battery temperature and length of time in service.  Because these RV batteries are most likely mounted away from the engine, they may or may not be exposed to temperatures that would cause significant water loss.
New battery designs are able to handle overcharge conditions much better, but if overcharging does occur, gas generation will result.  System overcharging can be recognized if the battery voltage is monitored.  Charging batteries that are already at 12.5 volts is not recommended; a battery at 12.9 volts is a clear sign of overcharge, especially after 8 hours of resting.

Most of today’s batteries are designed to not need water, however, in some applications if no water is added, the battery will fail early.  If done with care, the addition of water to a battery that has access to the cells, can return the battery to a much healthier condition and provide many more months of service.

• Monitor the health of the battery using the hydrometer.  The built in hydrometer can be used to determine the batteries charge level and state.  Any battery with a clear eye has low electrolyte level and should be replaced (never charge a battery with a clear eye).

• You can use length of time in service to determine when a battery should be removed from use, however, this is a costly method of control if records of time to failure have not been kept.  Many batteries could be discarded while they are still in good condition.  A preventive maintenance (PM) program works best when inspection records are kept starting from the date of installation.

• Purchase a quality battery analyzer (ex. Midtronics, Argus) and test batteries every 6 months after being in service for 18 months (for better results, disconnect cables before testing).

• Discontinue use of batteries that are tested and indicate that they have failed

• Use a battery analyzer to determine if the continuous charging cycle is leading to overcharge conditions, do not charge batteries that are at 12.5 volts or greater.

• If possible, adjust the trickle charge system to shut off earlier or not produce any current until the voltage is below 12.5.

• Never leave a battery on a charger overnight or for extended periods of time without monitoring charge progress.

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