LiFe Batteries as Flight Packs
Lithium Iron Phosphate, LiFePO4, or LiFe for short, are widely used to power receivers and servos in RC airplanes. Their use as flight pack batteries is not so common, but I have found them to be preferable in many applications.
Benefits of LiFe Batteries
The main benefit is safety. Where LiPo batteries can spontaneously combust while charging, or after suffering damage, the LiFe battery will not. They can be charged unattended without fear of burning down your house. Even if abused, they will not burn. I leave the batteries in the airplane while charging at the field.
The second benefit is longevity and durability. They may be charged and discharged for thousands of cycles without suffering capacity loss. Batteries left on the shelf for years will still be fully functional with little loss of charge. They suffer abuse with little effect. I had an airplane go down in a tree due to a servo problem. It was there for three weeks, and so the battery was completely flat when finally recovered. After bringing it back to life with a slow NiMh trickle charge, I gave it a 1C charge to 3.6 volts/cell and 1C discharge to 2.0 volts. I got 3100 mah out of it, and it is labeled 3000 mah! So, it had no apparent ill effects from the long discharge.
These are not bullet proof, however. I have another battery that was inadvertently charged on the LiPo setting. Before I discovered my error, one of the cells was way over charged, over 4 volts instead of the recommended 3.6 volt cutoff for charging. This one didn’t survive so well. After discharging to a safe voltage, and charging properly, a discharge at 1C to 2.0 volts gave me 2678 mah out of a 3800 mah pack.
In all, this means that you will wind up spending less for LiFe than LiPo batteries in the long run, (especially if you avoid charging on the LiPo setting).
LiFe batteries have no cobalt as do LiPos. Cobalt is mined in the Congo by hand, often with child labor. Perhaps this should be reason number one to use Life batteries where possible.
Disadvantages of LiFe Batteries
A big one (pun intended) is the size and weight for the energy in watt-hours stored compared to LiPo. Related to that is the lower voltage per cell. The nominal voltage is 3.3 volts, compared to 3.7 for LiPo. Also, the voltage under load can sag to 2.8 volts. As a result, you need four LiFe cells in series for every three LiPos. So, a plane that would normally use a 6S LiPo would need an 8S LiFe pack.
When volume is limited, or weight is critical, LiPo may be the only practical solution. But in sport or scale airplanes, LiFe batteries should serve well. I have an 81 inch span scale Bonanza with an 8S LiFe pack. I use the same pack in my 78 inch span Sig Kadet Senior. I use a 4S pack in a 50 inch span, scratch-built foam-board Synapse.
It is important to know that the discharge curve is flat. That is, the voltage is constant until the battery is nearly dead, then it falls off rapidly. For that reason, you cannot use the pack voltage to tell you what percentage of charge you have remaining.
Since the LiFe voltage is lower, you must disable any low-voltage cutoff (LVC) you may have set on your ESC or you will have a premature power loss.
To determine a safe flight time, I recommend that you use a watt meter to see the current draw of your system at full power. Use that to calculate the time to use 2/3 to 3/4 of the mah in your pack, and set your timer accordingly. After flying, take note of the mah returned to your battery during charge. If you are using a lot less than the capacity of your pack, then you can extend your flight time.
For example, say you have a 3000 mah pack (verified by charging and discharging to 2 volts), and a power system that draws 30 amps at full power. For reserve, lets say you want to leave 1000 mah in the pack. So, to use 2000 mah (2.0 amp-hours), 2 ah divided by 30 amps is 0.067 hours, or four minutes. Set your timer for that, and you should have plenty of juice in reserve, especially if you don’t fly at full power the whole time. If the battery takes only 1000 mah during recharge, you could double your flight time and still have 1000 mah in reserve.
Where do I Get Them?
I have two main sources. Radical RC sells the cylindrical form cells formerly known as A123 but now labeled LithiumWerks. I use the ones labeled as 2500 mah capacity. You can buy individual cells, or Radical RC can make packs of any size and configuration that you can think of and with any kind of connector. Here is a 2S pack that I wrapped in electrical tape:
According to the Radical RC web site, these cells can be charged at 10 amps, discharged at 50 amps continuous or 120 amps for 10 seconds, and discharged to 2.0 volts minimum.
Innov8tive Designs sells the BadAss line of LiFe packs. These are in the prismatic (flat) form factor, and are available in 2S or 3S packs up to 3800 mah. They recommend charging at 1C and discharging at a maximum of 25C continuous. So the 3800 mah packs could be discharged at up to 95 amps. They come with XT60 connectors and a balance plug.
Innov8tive Designs also sells a line of ESCs (Cobra) that can be programmed with a low-voltage cutoff for LiFe batteries at 2.8, 2.5 or 2.2 Volts/cell.