Lithium Iron Phosphate (LiFePO4)
LiFePO4 is a relatively safe type of Lithium battery. It has good energy density (available power per pound), but the maximum power output is typically limited to around 2C. Therefore a higher total capacity is needed to keep the C-rate low enough (e.g. with a 25A controller, get at least 15 Ah). A LiFePo4 battery configured with the required BMS is easy very easy to charge. This is as close as we get to a "plug and play" battery.
As with all lithium batteries, individual cells are wired in series and parallel to reach the desired voltage and Ah. Nominal cell working voltages are between 3.3V and 3.0V. Max charging voltage is 3.6V. Minimum voltage is 2.8V. Most LiFePO4 batteries include a Battery Maintenance System (BMS) to keep the cells within a safe working range. While charging, the BMS balances the cells and prevents overcharging. While discharging, the BMS limits the total current, and prevents individual cells from being discharged below their minimum voltage. Typical BMS behavior is to shut off power when system over-current voltage under-current are detected. Turning the battery off and back on resets the BMS.
A123 batteries have higher discharge rates than other LiFePo4 batteries. These cells uses nano technology to allow a high discharge rate of up to 17C. A123 batteries are also correspondingly more expensive.
Compared with LiPo, LiFePo4 has a higher upfront cost, however the lifespan is usually longer (generally rated at 3 times the cycles of LiPo). LiFePO4 has a pretty flat discharge curve that makes a battery meter hard to use.
There are two common types of LiFePO4 batteries:
- Lithium Iron Liquid Electrolyte - cylindrical cells, such as A123M1 and Headway. More fault-tolerant than LiPo. Voltage range: 3.3V nominal, 3.65V to 3.7V maximum, no lower than 2.7V on deep discharge.
- Lithium Iron Polymer Electrolyte - pouch batteries,  such as Ping and A123.
- Lithium Iron - the commonly used term to describe LiFePo4.
- Ah - see Amp hours
- C - see C-rate
- BMS - see Battery Management System
- A123 - A type of LiFePO4 battery which uses nanotechnology in the electrode design and is often found in power tools.
- Prismatic - A hard sided rectangular package with screw terminals. Typically for 10ah and bigger batteries.
- Pouch - A flat mylar bag with flat electrodes. The lightest, least expensive and least robust format of cell.
- Round cell - Cylindrical cell with screw terminal on the ends
- 18650 - A cell slightly long and fatter than a AA battery. A123 batteries are commonly available in this format.
- 26650 - An A123 cell, 26 mm in diameter, 65 mm in length.
Recommended Vendors of complete battery packs for E-Bikes
Note:! There are many suppliers of LiFePO4 batteries on ebay, etc who will offer an LiFePO4 battery packs for great prices. These are often lower quality battery packs with substandard cells, wiring, BMS, and packaging. Buyer beware.
Why would one choose the Turnigy over the Zippy (LiFePO4) or the other way around?
It sounds as if you are under the impression that Turnigy is one chemistry, and Zippy is LiFePO4--that's not the case. Turnigy and Zippy are just two "brands", as it were.
In case you're actually asking about which one of those brands has better LiFePO4 cells, AFAICT neither has any good ones. You would want Ping, A123, or *maybe* the Sunthing (IIRC) packs for LiFePO4. *Don't* get Vpower/Volgood/etc packs--they do work (usually) but they suck to have to fix whenever tab welds break or cells die and take the string with them, etc. :( (see my threads on repairing them for examples).
How does one actually purchase genuine A123 cells?
Depends on the kind you want--becuase A123 does not sell directly to the public, (and they don't actually really exist as such anymore, either), you can't actually know *for certain* that any cell you buy as A123 actually will be. Some places, like EM3EV (cell_man here on ES) almost certainly have the real cells, in good quality, and some other places *might* have good ones, whether genuine A123 or not (like OSN)--but it is a crapshoot with most of those places in China, other than EM3EV.
As for a consensus that it's top-of-the-line, well, that entirely depends on your needs. If you don't need super high currents out of it, NMC like LG Chem and EIG would do just as well, and be lighter, too. And at least for EIG, you can buy them directly from the manufacturer, guaranteeing genuineness. (it'll cost ya, though)