Lithium Polymer (LiPo)
Lipo batteries, also known as Lithium Cobalt Polymer Electrolyte, are pouch batteries which use the LiCoO2 chemistry.  They are used RC-type LiPo packs, smartphones and PDAs. LiPo's voltage range is: 3.7V nominal, 4.2V maximum, no lower than 3V on deep discharge (preferably no lower than 3.5 to 3.6V for best life). remaining capacity by resting voltage has been measured: 
Advantages of LiPo:
- High power output for the weight and size. The higher C-rating means that a large battery is not needed to achieve required maximum current.
- High capacity for the weight and size (25% advantage over LiFePo4).
- Price (although cost of charger and monitoring equipment reduces this advantage)
- Due to the discharge slope, the voltage of a LiPo pack can be used as a "fuel gauge" more accurately than LiFePo4.
Disadvantages of LiPo:
- Risk of fire if charged or discharged incorrectly, or if punctured
- Expensive charger needed, and care in balancing cells
- Often the voltage and capacity of a single pack is not sufficient for e-bikes, so you need to build up a pack using series or parallel connections.
Photos of various LiPo setups are shown here: http://endless-sphere.com/forums/viewtopic.php?f=14&t=17970
- Continuous Rating (C) - Sustained maximum current without damaging battery
- Sustained Rating (C) - 30 second maximum current per charge cycle
- Peak/Burst Rating (C) - 10 second maximum current per charge cycle DANGEROUS
From 20% to 80% capacity, the voltage drops at a slow and constant rate, which allows the voltage to be used as a "fuel gauge".
When the battery is nearly empty, the voltage falls rapidly, so voltage must be closely monitored to ensure it does not drop below a safe level. There is not much usable capacity below 3.5v. Also, the deeper a battery is discharged (eg using the full capacity of every charge), the more its lifespan will be decreased.
While the battery is under load (eg. powering a motor), the voltage temporarily drops. This is known as "voltage sag". While LiPo has less voltage sag than other battery types, it is important to consider the effect of voltage sag if you are deeply discharging batteries.
- Charging current (C) - Maximum charging current e.g. 2C on a 4Ah battery = 2*4 = 8 Amps
- NEVER discharge a Lithium Polymer battery below 3V
- NEVER charge a Lithium Polymer battery above 4.2V
- NEVER exceed manufacturers discharge and charge rates (C-ratings)
- NEVER exceed temperature parameters (0-60 Celsius)
- NEVER puncture a Lithium Polymer battery
- Beware of puffy cells, it indicates the cell is damaged
- Charge only in a safe place- away from flammable materials such as carpet, cardboard, wood, drapes. The charging process be monitored, and done in a fire resistant area.
Failure to follow these rules can result in a chemical fire, smoke, even explosions.
Overvoltage occurs when the battery is under charge so typically it's built into the charging circuit of the batteries. The charging circuit monitors the voltage of the cells and gradually reduces current until 0A when the battery has reached the target voltage of 4.2V. As an additional safety measure, most LiPo-specific chargers automatically disconnect power after a pre-determined charge time, typically 2 hours.
Undervoltage occurs when the battery is under demand so typically it's built into the electronics on the vehicle. The safest option is to disconnect the batteries from the motor in the event that the voltage reaches below 3.1V. Alternatively an audible or visual alarm can alert the driver to stop using the electric motor of the vehicle.
LiPo batteries are particularly vulnerable to short circuits due to their extremely low internal resistance, that induces very high short circuit currents. Extreme care should be taken to minimise the possibility of terminals being in close proximity to one another. This can be achieved with insulation, fuses and only using high voltage by connecting in series when absolutely necessary.
Optimising Battery performance
Depending on conditions, cells can start to gradually go out of sync with each other in terms of what voltage they're running at and their subsequent capacity. In order to minimise this and ensure your battery set-up is at peak performance, you should aim to balance cells on at least every other charge cycle. Cell balancing is performed using an output positive from each serial cell allowing them to be connected to individually.
LiPo batteries should be kept between extremes of 0 Celsius and 60 Celsius. Optimum performance can be achieved at 38 Celsius. In order to achieve this you can cool your battery enclosure using passive cooling (heatsinks, vents, ducts) or active cooling (fan).
- LiPo specific balancing charger that is compatible with the batteries you will be using (eg 4S, 5S, 6S).
- 12 volt mains power supply for charger (many LiPo chargers use 12v power, not mains power)
- Low voltage alarm (to alert you while you are riding if any of the cells drop below a safe voltage)
To speed up charging of multiple packs, the main options are
- Purchase a multi-charger, such as an IMAX.
- Connect multiple chargers to a power supply (as long as the power supply has enough Watts).
- Connect the batteries to the charger in parallel (be careful to not exceed the charger's maximum current). However, this means that cells are not individually balanced.
Building a LiPo battery pack
RC LiPo Batteries require respect. It's a lot of power in a little package. In the past treating them badly could result in flames, but the newer LiPos are a bit more forgiving. Luke on endless sphere notes: Never exceed 4.3v per cell and never discharge below 2.7v per cell. To extend the battery life and improve reliability, charge the cells up to 4.15V and don't discharge below 3.0V If you abide by these guideline they should behave. RC LiPo Batteries are nominally 3.7V per cell, but generally run closer to 4 volts each. When you buy RC battery packs they are rated by:
12S pack = 50V hot off the charger.
To build a pack that is 10Ah and 48V requires four 6s 5000Mah (5ah) Lipo packs. This battery pack will supply 48V at 10 amps for one hour. This weighs about 7 lbs (3,2 Kg).
Be very careful to not touch the red (+) and black (-) wires together. This will make a huge spark which can burn fingers, damage eyesight, and damage the expensive batteries and electronics. Your goal is to keep the magic smoke inside the batteries and electronics!
Lipo discharge C-rate
For LiPo, it is recommended to size the battery based on half the manufacturer's claimed C-rating, to get a good lifespan out of the battery. This assumes you are not riding at full throttle for extended periods- if you are regularly drawing max current for over 10 seconds, you should size the battery to draw 1/4 of its C rating.
Combining RC packs (series/parallel connections)
The individual high power discharge and lower power charge wires in each parallel group of cells should be connected either by soldering them or using a custom wiring harness.
- Charging Lithium Polymer batteries
- A detailed write-up of using LiPo
- LiPo wikipedia article
- Discussion about cell voltage and state of charge
- Lecture from Robotics Institute on Lithium-ion battery chemistry and performance (beware that the focus is LiFePo4, but many of the principles also apply to LiPo