Before we crash course this battery subject lets identify and realize the different types of battery technology we are using today in the many wonderful tools and toys in our American housholds. 

Here are a few:
Li Ion
Lithium Iron Phosphate (LiFePo4)
lithium polymer
alkaline
lithium rechargeable
mercury cel
zinc air 
Nickel Cadmium (NiCd)
nickel metal hydride Ni-MH

An attempt to explain why we use LiPo batteries and some theory

LiPo are the batteries used in Drones!
        Long run times and High power!  Lithium Polymer Batteries.     LiPo

LiPo battery advantages are as below:

• High voltage.
• High energy density
• Long cycle life
• Light weight
• Smaller size
• No memory effect
• Environmental friendly
• Good big current discharge performance
• Flexible shapes
• Great high low temperature endurance

They come in All shapes and sizes from really small mp3 player & iphone

Usage Note: After one year or longer, the lithium-ion polymer battery will have a loss of battery capacity, but a well-maintained lithium battery will be used for a longer time and the loss capacity can be much less

In the drone application these battery slabs are stacked together and rated depending on their capacity

2s 2 cells in Series same for the others, 3s,4s,5s,6s . attaching the cells in Series increases the voltage each time by 3.7

2s = 7.4 volts
3s = 11.1 volts
4s =14.8 volts

a LiPo battery cell, 3.7v is mentioned because that is the nominal voltage of the cells. However, when fully charged, a LiPo cell should reach 4.2v. So for a 4s LiPo, 14.8v would be considered the nominal voltage but when fully charged, a 4s LiPo can reach 16.8v.

6s = 22 volts, put two 6s battery packs together in series for 44volts.   Make some Smoke!

disadvantages of LiPo Battery:
➨It supports less recharge life which is about 300 to 400 cycles.
➨Its chemistry leads to fire when LiPo battery is punctured.
➨It requires special care during charging, discharging and storage.
➨It is expensive. The cost is almost double than Li-Ion battery.

Maintenance:
LiPo batteries are fully charged when they reach 4.2v/cell
Minimum safe charge is 3.0v/cell

It's important to use a LiPo compatible charger for LiPos.

we use voltage as the primary measure of how full a LiPo battery is.

Lithium Iron Phosphate (LiFePo4)

Twice the power, half the weight, and lasts 4 times longer than a sealed lead acid battery  The #1 battery for fishing electronics, outdoor use

This is a Li-ion
single cell is 3.7 volts. standard most common battery, rechargeable flashlights, portable stuff

 These get stacked together to get high voltage, these are in power tools, just wire them together to get the desired voltage, I think my 20v dewalt has 6 cells to get the 20 volt

https://forumla.us/the-lithium-ion-battery-is-here-to-stay-the-18650-the-most-popular-style/

maintenance

optimal maintenance of lithium batteries: the best maintenance method for lithium-ion batteries is light use, shallow discharge and shallow charge,

avoid deep discharge of the lithium battery, 

avoid frequent full charge and discharge.

NiMh.
Nickel hydroxide positive electrode plate, a hydrogen ion negative electrode plate, a separator, and an alkaline electrolyte such as potassium hydroxide

advantages of going with a NIMH battery:

• Quite a high capacity compared to other rechargeable batteries
• Resists both over-charging and over-discharging
• Extremely lightweight construction
• Friendly to the environment: no hazardous chemicals like cadmium, mercury, or lead

Drawbacks: And here are some of the limitations:

• More expensive than other rechargeable models
• Self-discharge rapidly while in storage
• Cut power suddenly rather than a slow trickle down
• Some only work with manufacturer’s charger

NiCad

advantages of NiCAD batteries:

• Relatively inexpensive
• Charge very quickly, simple to charge
• Easy to store, easy to ship
• Take a high number of charges
• Functional in low temperatures

Drawbacks: And here are some of their shortcomings:

• Not as powerful as some other rechargeable batteries
• Self-discharge while in storage
• Contain toxic metals that are harmful to the environment