For example, this terminal was connected in our test vehicle to a 5 year old battery in poor but  functional condition.  We allowed the connection to corrode.  This battery started the vehicle just fine with no problems.  That is until one hot day during a sales trip.   The vehicle started and operated fine at the beginning of the trip.  But after the vehicle was warmed and the day became hot, we observed the following symptoms:

We parked the vehicle and made a short visit with a customer.  When we returned and attempted to start the vehicle, nothing happened; no lights, no cranking!   It was as if the battery was not there.   We needed to be at our next appointment and soon!   Fortunately, our test vehicle was equipped with a backup battery and and a BIC-75300A.  We were able to quickly switch "ON" the BIC to combine the batteries, start the vehicle, and make the remaining appointments throughout the day.  After starting with the backup battery, we returned the "Remote Switch" to the automatic position.   But, with the AC on, and the vehicle RPM reduced to idle, the engine suddenly died.  We concluded that the main battery had been electrically disconnected.  So anytime the vehicle load exceeded the alternator output, the system voltage dropped below the minimum required to operate the engine.  Therefore, we operated with the "Remote Switch" in the ON position for the remainder of the day.

When we returned, we were able to closely analyze the failure mode.  We observed that the battery had leaked acid through the top seal (not the vents!) and from the positive terminal.  We observed battery acid all around the battery and the battery tray was wet (with acid).  The corrosion on the clamp was so hardened that a special tool was needed to keep the clamp from moving while the screw was broken loose.  After the clamp was removed, we took it to the lab for analysis.  As you can see in the photo, we observed a black layer on the lead as well as the hardened yellowish crystels.  Measured with an ohm meter, the clamp showed no conductivity anywhere on the black layer.  This layer must be penetrated in order to achieve an electrical connection.  We determined that the leaking acid over time and hot conditions caused this insulation layer to crystalize to the point of breaking the electrical connection.

With this kind of problem, a simple jump start would not have helped!  Neither would a battery saver "disconnect" device.  The fact is, this battery was near full charge!  When we replaced the battery clamp, the battery performed as it always had.

2) Seal your cable wire exposure.  For a clamp like in the photo, use something like "Plasi Dip" from NAPA.  If you use a crimp type of clamp, then use a sealing type of heat shrink or "Plasti Dip".

3) Avoid cheap batteries that are prone to leakage.

4) Use corrosion inhibiting felt washers.

5) Ensure your alternator is regulated at the proper voltage (14.0 to 14.7 volts depending on temperature.)

6) Use of a sealed AGM type of battery will reduce the probability of corrosion problems.