Q. At what rate should I charge my LiFePO4 pack?
A. In general, don't exceed 3C, and best practice is charge at 1C.
Look, teaching you everything you need to know about battery charging
is beyond the scope of this website. We'll share some examples to try
and help guide you, but don't for a minute believe this is a
comprehensive explanation. Point being, it's on you to learn enough to
be safe. Further to this; the charger manufacturer will also have information for you to use in safely charging your battery pack.Take advantage of every resource available for learning if LiFePO4 (also known as A123 or LFP) are new to you.
Meanwhile, to begin, you need to be aware of a bit of math. Like how the 'm' in mAh
means mili, or 1/1000. Regarding the A, that means Amperes (or amps),
and h=hours. So the capacity of the pack is measured amps delivered in
one hour, or Ah (A*h or A x h), but since packs for models are often
measured in fractions of an ampere, then they're also often expressed in
terms of milliamps, or 1/1000th of amp. As for time, since the time
frame is always based one hour, then . . .
- 6000mAh = 6Ah - meaning it'll deliver 6A of current for one hour
- 3000mAh = 3Ah
- 1500mAh = 1.5Ah
. . . understand?
Further to this, C (capacity) of a 6Ah pack is 6A. And 3A for a 3Ah pack, get it?
So charging at 1C (meaning 1xC) it means a 6Ah pack can be charged at
6A, and if the same pack is being charged at 2C (meaning 2xC or 2x6=12)
it means it can be charged at 12A. Remember, since we're dealing with full hours, this means it'll charge in 1/2 that much time, or 1/2 an hour at a 2C rate of 12A. Confused? Don't be. Review as many times as needed until you achieve understanding before you continue
reading. And seek out other resources to learn because if you screw
this up you're possibly in for a world of hurt, OK? Bottom line? Incorrectly charging batteries can be dangerous!
Now let's switch to a 3Ah pack (same-same as a 3000mAh pack).
Charging a fully discharged 3Ah pack at 1C means hitting it with the
charger at 3A for one hour. Hit it at 2C (6A) means it takes half that
much time (1h/2=1/2h or 30-minutes) Ditto, charging at 3C takes 1/3 of
an hour or 20 minutes (60min/3=20minutes), or at 4C takes 1/4th of an
hour, or 15 minutes. Get it? If not, review and consider learning about
this material in other places until it clicks because if you screw up
you're putting your life at risk. Did I basically say the same thing in
this paragraph as in the previous one? Yes!
Charging can be dangerous, so please consider yourself warned!
Note; all this is in theory because it's never a good idea to discharge LiFePO4 packs
below 2.8V/cell.Some will push it going to 2.6V/cell but not me because
they fall off a cliff after that dropping to 2.0V/cell really quickly.
Since I view this as risky, I stop at 2.8V/cell. Did I already mention I stop at 2.8V/cell? This is important!
There's more to learn, and it's not really rocket science, but it's
on you to go learn about it before charging batteries. We're sharing some useful
rules of thumb that will for the most part keep you out of trouble - but
- you can burn your house down by being stupid so don't go trying to
blame us because a) we're telling you battery charging can be dangerous,
and b) that what we're sharing isn't everything you need to know. The
major point being, you should go learn how to do it safely before you
begin!
Q. My charger has a LiPo charge-cycle instead of LiFePO4. May I still use it?
A. No.
In general, chargers expressly made to charge LiPo packs are set to higher cut-off voltages than for LiFePO4-chemistry and thus, may damage the LiFePO4 pack and even result in fire. So always use a charger designed for the appropriate chemistry. Note; chargers are available to charge multiple chemistries.
Q. My charger has a LiFe charge-cycle instead LiFePO4. May I still use it?
A. In general, a LiFe-charge cycle is OK to run on a LiFePO4-battery.
But it's better to contact your charger-manufacturer for a
BIOS-update because a LiFePO4 charge-profile will optimize pack
performance.
Q. I'm Canadian and fly year around, sometimes in sub-zero temperatures. Is it OK to charge my LiFePO4 pack in these conditions?
A. Yes, but it depends because LiFePO4 packs may be safely
charged at temperatures ranging from 32° to - 131°F (0°C to – 55°C) but
for best results, we recommend
charging at temperatures above 32°F (0°C). In practice, what most folks
do is charge their packs whilst in their car!
That said, if you do charge in below
freezing temperatures, reduce the rate of charge to 0.1C . . . e.g. 10% of
the battery capacity.
Q. I'm confused, isn't LiFePO4 the same as LiFe? Also, why
don't you recommend LiFe packs? I like that they're cheap so what's
wrong with that?
A. Yes, LiFe and LiFePO4 are different. But one of the critical differences isn't so much in their chemistry (they're actually very
similar) but in their methods of construction. This is the key to understanding our recommendation for LiFePO4 versus LiFe.
This is because the LiFe is built in a poylmer
bag, like a LiPo (lithium-polymer). This gives it the characteristic brick shape as the
individual cells are flat-rectangles, which are overlaid upon one another. The shape is also the giveaway for
the LiFePO4, where these packs are built within cylindrical metal shells.
Note; the Po in LiPo refers to the polymer in it's construction (polymer
bags). Meanwhile, PO in the LiFePO4 refers to phosphate and oxygen
instead of
polymer (yes, it's a bit confusing). Anyway, the individual cylindrical
shells, because they're made of metal instead of thin polymer bags.
means they're more resistant to physical damage. By the way, this metal
shell is the same technology used in old school NiCds and
NiMH (and alkaline cells, for that matter). It's been around forever
because it works!
There are downsides to these metal shells. First, the metal is a bit
heavier than the plastic bag use in LiPos and LiFe cells. Second, simple
geometry dictates two cylinders contain less volume than flat cells
(capacity). Third, they're more expensive to produce.
Against these
disadvantages are upsides. Like metal is FAR more sturdy. This turns out to be a crucial advantage because metal
protects better against inadvertent damage (like a pack shifting during a
maneuver and bumping up against the hard edge of a former). If this
happens to polymer style packs, the dent may result in it puffing. Or in a fire. Need I mention our models are constructed of
flammable materials like balsa, foam, and fiberglass?
Bottom line? For an engineer, part of the remit is looking not at when
everything is
going right, but when things are going wrong. Look, nobody sets out to
install
their pack so it's dented due to shifting during a maneuver, but . . .
shit happens, right? So it's when things go pear shape that a good
engineer earns his pay.
Our deciding against continuing to offer 'Po' style packs for control
avionics is a direct result of data indicating it might sound good in
theory, but in practice, leaves something to be desired. This reminds of the immortal words of a wise wag of baseball.
Put another
way, when the data changes, we change our mind! This is why our control
avionic pack recommendation is to use durable LiFePO4 or LiIon instead of more fragile LiPo
or LiFe brick style packs.
Q. Where should I set LVC for my LiFePO4 pack?
A. 2.8V/cell
The
manufacturer goes down to 2.0V for testing purposes to determine
'capacity' but the amount of time between 2.8V and 2.0V happens in a
flash!
Q. How low can I take a LiFePO4 pack?
Q. How much capacity can I draw from my 6000mAh pack?
A.
These two questions are related so I'll answer them at the same time.
In general, 'I' am comfortable using 50% capacity before recharging. So
how do I determine this? By eyeballing how many milliamps my charger
puts back in!
So if my charger puts in 3200mA when recharging my
6000mAh, and this was over the course of 6 flights it tells me the
average consumption was 3200/6=533mA/flight. So this is safe enough - but - I'll also
make a mental note that I'm pushing things and consider recharging after
5 flights. Why not go further, to 80% of capacity (about 2.8V/cell), or about 9 flights? Why yes, I could - BUT - do I want to?
Me?
I'd rather hit the pack with the charger than risk my model. Some
people like to live dangerously . . . I'm not one of them. You, however,
are a big boy, so do as you please. Just also take into account the
risks not just to your model, but of an accident that hurts someone if
you loose control of it, capice?
Q. Can I store a LiFePO4 pack fully charged?
A. Yes. In fact you can store it discharged at 2.8V/cell, or fully charged, or anywhere in between. This is why I love the things.
In
fact, LiFePO4 cells don't seem to suffer the memory effects of older
technologies (like NiCds, which would also self-discharge such that in a
week, they were at about half of capacity). Neither do they suffer like
LiPos when being stored fully charged, e.g. delivering reduced cycle
times.
As an experiment, we charged a LiFePO4 and left it on the shelf for a year and when we tested it, we achieved 90% of the tested capacity. A freaking year! So I tend to charge them once I get home just so they're ready to go again at the drop of a hat . . . like if the opportunity presents itself to go flying I don't first have to mess about charging batteries - I can just go!
Q. My charger shows 3.6V/cell during the charge but I've seen these 2S packs rated 6.6V/cell.
A.
You're generally going to see about 3.3V/cell at a 1C load, and a bit
less, maybe 3.2V/cell at 3C. Nice thing about a LiFePO4 pack is they're
capable of being discharged at quite a high rate, e.g. 3C on a 6000mAh pack is 18A.
Q. How are LiFe different from LiFePO4 packs? I wonder because LiFe are a lot cheaper.
A. The chemistry is similar. Construction is
usually the big difference where the metal shells of the LiFePO4 is
considered an advantage due to being more rugged. However, the discharge rate is another difference. A
LiFePO4 pack is actually capable of being used for propulsion whilst a
LiFe simply doesn't have the stones to be discharged at a rate adequate
for propulsion.
Q. Can I use a 2S LiFePO4 pack with my DA ignition?
A.
Yes! In fact a 2S pack runs about 6.6V and this is virtually the same
as a 5-cell NiCd. For a single cylinder engine, a B21500 is going to
give you 4-5 flights between charges (if you're running it flat out, so
count on more flights if you're using throttle judiciously).
For a
twin cylinder ignition, just increase capacity. I'd recommend either a
B2S2500 or B2S300 to get 4-5 flight flights between charges (again, flat
out so you're basically going to fly the model all day on one charge).
With regard to 4-cylinder engines and 5-7 cylinder radials, a similar answer, opt for the B2S6000 and everything will be Jake.
Q. I use an IBEC to provide power to my receiver and my ignition, will a 2S LiFePO4 battery work with this?
A. Yes.
However, there's a 'but' in this response.
I've been at this for about 50 years. I've seen gizmos come and go to
do this very same thing. Typically the magic involves optical isolation
to keep noise out of the receiver.
Here's the thing. That's
all well and good when everything works as it's supposed to. What
happens when shit goes to hell in a handbasket? Usually you crash.
If
you want to play footsie with that risk, then that's on you. Me? I use a
dedicated ignition battery because first, I don't want to loose a $2000
model.
But even more so, I don't use them because I don't
want to take a chance on hurting one of my flying buddies, or a
spectator. Look, earlier I mentioned engineers earn their pay when
things go tango uniform. Consider yourself warned about using these
things by an engineer.
Note; I say this despite all the keyboard
experts on the forums swearing about how good they are. You're a big
boy, do as you please but please consider yourself warned against using
one of these things, OK?
Basically you're betting nothing ever goes wrong. Thing is, only God is perfect and suppliers of those things,
like everybody else in business, is doing the best they can. On the
one hand we have three components, a battery with two cells and a lead
against an electronic device with many more components. Statistics
informs me. What about you? What's guiding you to rely on one of these
things? Better not. Just don't. Trust me.
Note; for propulsion,
the weight and package volume (capacity) give an overwhelming advantage
to polymer bag construction. This is why LiPo packs are used for
powering RC models. But also know this, these packs are removed prior to
charging (or should be), so the risk profile is somewhat different.