WHAT DAMAGES NiMh CELLS
The First reason why batteries fail has to do with the manufacturing and matching process.  However, most R/C batteries have been tested and meet the required specifications to operate under the conditions that we subject them to. 
AGEING
NiMH Battery performance gradually deteriorates with time due to unwanted chemical reactions and physical changes to the active chemicals. This process is generally not reversible and eventually results in battery failure.

UNCONTROLLED OPERATING CONDITIONS
NiMh Batteries are not immune to failure which can be provoked by the way they are used or abused. 
HIGH CELL TEMPERATURE is the main killer of NiMh batteries... this can be brought by the following:
Overcharging, Dead Shorting, Discharging at high rates, cell reversal, and wrong SOLDERING TECHNIQUES.
Physical abuse also damages the cells.  Avoid dropping your batteries.  When being transported, keep them in a battery case that isolates them from vibration and falling objects.  Make sure the battery terminals NEVER touch a conductive material as they will conduct and can short out.  This are just a few of the reasons why batteries fail.

HOW MANY TIMES CAN A NiMh BATTERY BE CHARGED IN A DAY
Tests have shown that Intellect IB3800 cells can be charged up to 4 times per day with 2 cycles being optimal.  The main thing to take into consideration is to allow the battery pack to get back to room temperature before recharging.  It is important that after each run the battery pack be properly discharged as per our instructions, then allow it to cool down.  Once it is cool you can recharge your pack again.  You should avoid recharging your pack while it is still warm.
Most of the times you will notice a slight increase in power during the second charge.  Thereafter, the battery pack will loose a few seconds running time and the voltage will drop around 60mv.  If cycled properly, the pack will regain its capacity and power within a few days.
Care should be taken to make sure that the batteries never get charged while HOT.

DEAD SHORTING FACTS
It is claimed that dead shorting serves to increase the power output of a battery pack by slightly increasing the on-load operating voltage as well as decreasing the internal resistance of the cells. However, short circuiting a battery pack is definitely not recommended as the destructive energy unleashed by the cells is seriously under estimated.

Here is an example:
An IB3800 Battery stores approximately 5,900 Joules of energy (5,900 watt seconds). A 6 cell - 3800mah battery pack stores around 35,400 Joules!!!!

Now, what does that mean? Well, it only takes 15 Joules to punch a hole through aluminum foil creating lots of sparks. 35 Joules is enough to weld a small wire to a surface. This is because the discharge period is very short, almost instantaneous, resulting in a power transfer of hundreds of watts. A 3800mah - 6 cell battery pack has 1000 times the power necessary to weld a small wire...

Short circuiting these cells can cause extremely high currents and temperatures within the cells resulting in the breakdown of the chemical compounds from which they are made. This in turn can cause the rapid build up of pressure within the cell resulting in its catastrophic failure with unpredictable consequences including the uncontrolled rupture of the cell or even fire (The cells can actually explode if you dead short with too much energy left inside them). Severe damage, burns, or other serious injuries can occur.

Dead shorting a charged battery pack is OUT OF THE QUESTION unless you know exactly what you are doing.
It is not only bad for the batteries; it is very dangerous to yourself and the people around you.

I have read on other battery matching websites recommendations to discharge a pack to 5.4 volts (0.9 volts/cell) and then dead shorting it. This is extremely dangerous and the only thing you will achieve is the speedy destruction of your pack and possible severe injuries or a fire. The battery pack can still store over 25,000 Joules or 800 times the energy necessary to weld a small wire. Do you see the potential danger involved with this?

If you plan to dead short a battery pack in order to gain more power, you must make sure that the pack is completely discharged down to 0 volts over a long period of time. Discharging with a battery discharger like the one on your charger is out of the question. In order to do this, you must discharge your pack in battery equalization board (like the Axxis EQ series or Novak Flatline) that will discharge the batteries down to 0 volts over an 18-24hour period. Once a battery is completely discharged then you could attempt to dead short your pack. First, and for your own safety, you must double check with a voltmeter that each cell is at 0 volts before attempting it.
Once EACH cell in your battery pack has been discharged to zero volts, attach a jumper wire from the positive to the negative terminals of the pack.

You SHOULD NOT see a spark when attaching the jumper. If you do, STOP, because the cells still contain energy and it will be DANGEROUS and DAMAGING to continue. The jumper should be made out of 12 or 10AWG wire, smaller jumpers should not be used.

After attaching the jumper wire, leave your battery shorted until you plan to use it again (not less than 5 days). The small amount of energy (Joules) that is left in each cell will slowly break down any crystalline formations that impede the flow of energy into the electrodes. This will lower runtime since a portion of the

electrolyte is consumed during this process. You can typically expect your cells to loose 15-35 seconds of runtime, as well as the pack longevity. Once your cells have been dead shorted, the damage to runtime and cell life has been done and is irreversible. Generally, only professional racers who run in either stock or 19t classes use this process. Racers who run modified or 1/12 scale rarely dead short their packs, as they do not want to loose runtime in favor of a few millivolts.

Should you decide to dead short your packs, you must continue the process for the life of the pack. If you choose not to, the overall performance of your pack will rapidly decrease.

AXXIS RACING WROTE THIS AS A TECHNICAL EXPLANATION OF BATTERY DEAD SHORTING. THIS ARE NOT INSTRUCTIONS. WE ARE NOT ENCOURAGING YOU TO DO THIS. SHOULD YOU DECIDE TO DEAD SHORT YOUR PACKS, YOU ARE DOING IT WITH THE KNOWLEDGE THAT IT IS A DANGEROUS AND POSSIBLY DEADLY PROCEDURE. WE DO NOT ENCOURAGE THE USE OF THIS PROCEDURE. USE IT AT YOUR OWN RISK, AND IF YOU DO, PLEASE BE CAREFUL AND FOLLOW ALL THE SAFETY PRECAUTIONS INDICATED.