Basics consider electricity as water flowing in a hose, in this analogy amps represent the volume of the water going through the hose and volts represent the pressure of the water. Amps times volts equals watts (P=V*I) and 746 watts equals one horsepower. A battery, in the simplest terms, can be described by two parameters: voltage and capacity. Each cell will on average produce 1.2 volts at full charge and will have a fixed capacity. Total voltage is determined by the number of cells in your pack, 1.2 volts per cell is the nominal value so a 6-cell pack would produce 7.2 volts. The capacity or how much charge a pack can hold is measured in Mah and tells you how long the pack can produce a fixed voltage above a cutoff value. A 1000 Mah battery can supply a current of 1.0 amp for one hour. A 1200 Mah battery could supply 18amps for 4 mins.  Packs are generally made up of 4, 6, 7 or 8 cells, although larger ones are available and you can always assemble as many as you would like. Cell Labels  Quality battery packs come with cell labels which indicate various battery values including run time, average voltage, and internal resistance.
Run time - Amount of time it takes to discharge a cell to the cutoff voltage. The cutoff voltage on this cell is 0.90V on this cell, some manufactures use 0.85V. Charge rate - Amperage rate the cell is charged at. Discharge rate - Amperage load used to discharge the cell. Make sure when comparing cells that they have the same discharge rate, the run time at 30 amps will be lower that one at 20 amps.  Discharge cutoff voltage - Voltage level used to stop the voltage averaging. The average voltage is calculated by sampling the voltage 10 times per second, starting at the peak voltage and terminating the sampling when either the discharge cutoff voltage is reached.
Internal resistance - The resistance of the cell's internal structure. The lower the internal resistance, the more voltage the cell can deliver during at a given amp draw. Discharge cutoff time - Number of seconds before terminating the average voltage calculation. Pack Quality Pre-built stick packs are by no means top quality, the construction is made with metal tabs soldered on four and sometimes eight points. Although these packs work well for many tasks, when you begin to use high performance, high amp draw motors then you can run into problems. I have had pack temperatures get so hot (not all the packs fault!) that the shrink wrap fogged up (batteries outgas at high temperatures), the end caps melted and the shrink split.  Why is this? Well it has to do with those metal connecting tabs. Those tabs get very hot because they are both welded on tiny spots and are thin, both represent high resistance. Pull enough amps and they will get red hot just like the elements in your toaster. You should take the time to rebuild these packs and use copper lugs and solder, I like deans parts for this. Look at the picture to see the difference between the two. If you like to experiment, buy two identical packs, rebuild one and leave the other stock. You will notice the difference, more speed and cooler packs.
 When I build my packs I insert them in the tube of shrink wrap, I like CS Shrink. Some folks glue the cells together but I prefer using shrink wrap since its easy to take the pack apart should I need to. I also like to make a small slice in the shrink on each end of the cell. When you shrink the wrap the slices will open up into small holes. These holes allow the pack to cool faster and if water gets in you can easily get it out.
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