Trying to decide what replacement batteries to buy Nimh , Li-Po or stay
with trusty NiCads, heard that Li=Po are much better for their size and
weight but also they need special charging and are likely to burn if
overcharged so what are the benefits and the risks of Li-Po?.
The benefits are as advertised..the risks are overstated.
The key to using LIPOS is to use them with commoon sense. Run within
their limits they are safe. Run outside and you are asking for trouble,
possibly explosive trouble.
- don't ever let them get over about 70C...one person left a pack on the
front seat of his car in a hot carpark in full californian sunshine. No
- Never let the voltage go above 4.2v per cell, or much below about 3.3v
per cell..yes you CAN exceed these limits in SOME cases..but play safe
for now. The charger SHOULD take care of the first, and the LVC the
second point, but mistakes in setting cell counts are probably the most
likely cause of LIPO fires when charging. Even autosensing chargers like
the Astro 109 - my definite choice - can be fooled into wrong detection
if the current setting, or state of charge are wildly different from
whet it expects.
- de-rate the cells to about 70% of manufacturers and vendors claims,
unless you have the test equipment to verify them and an application
that is trying to squeeze the ultimate out of a pack. I would typically
operate a '30A rated' pack at 20A *burst* peak, and much much lower for
general flying..and if it says its 2000mAh capacity, try to use never
more than 1300-1400mAh. There is an accumulation (sic!) of evidence that
packs treated this way last far longer than packs that are pushed..
Using all of the above techniques works really well..if you stop flying
well before the packs are flat, and run them cool, they don't overheat,.
don't get out of balance and last many cycle. My only three pack
failures are due to being stuck in trees..and in one case left switched
on for two days in a test rig. In tow of the cases the pack was pretty
much showing zero volts but very careful and slow charging was able to
restore about half the capacity. I still fly these packs, but they ain't
what they used to be. The third did the puffy thing after 2 months up a
tree.. and no attempt to revive it worked.
I have momentarily shorted cells and packs when making them up and
fitting connectors, with no problems.
All these techniques will reduce the chances of an accident to almost
zero. However the consequences of an accident are more severe than for
Nickel chemistry, so it pays to treat the things with care. I would say
that I trreat my LIPOS about the same as I treat a can of fuel, or a
firework. Basically they are safe enough, unless you let them catch
fire..then you have a serious prioblem.
Storage in *cool* conditions requires nothing special..lets face it
LIPOS are in every laptop and mobile phone, and they do not
The three ever present dangers are due to accidental shorting of them
while being transported, getting too hot, and being pushed too hard -
either when charging or discharging.
For that reason one should never charge then in a flammable situation -
inside a fuselage..or on a wooden desk etc - just get a simple sort of
fireproof container with a loose lid - a flower pot, or a cooking pot of
ceramic is ideal - and put them in that to be safe. I admit I am fairly
cavalier about charging, but always keep an eye on them while I do..I
watch TV in the same room for example when charging up to go flying..and
I know where the window is to toss the lot out if the worst should happen.
If they burn up in the model while flying you probably have a writeoff
pure and simple.
That's the risk. However I've only ever had a model fire with a shorted
Likewise be careful when field charging in dry conditions..if you fly at
a club, a fire extinguisher is a sueful thing, as is a fireproof area to
Contrary to myth, there is no lithium *metal* in Lithium batteries...its
actually a lithium salt, typically lithium carbonate, and whilst some
lithium metal is formed during charging and discharging (I believe) its
quantity is very small. The issues with self ignition are far more to do
with the possibility of a cell internally shorting die to a localised
hot spot under high discharge rates, or breaking down and shorting due
to over charging. That so to speak provides the energy to ignite, and
what burns is a complex mix of organic chemicals that form part of the
electrolyte, and, finally the aluminium electrodes themselves.
In short a lithium battery is about as good at burning as a firework of
comparable size. And has enough oxidant inside to do it in the absence
of air too. Which means that once its going, it won't go out till its
finished. You use the extinguishers to stop collateral damage to
property and the environment.
Your focus should be - as with a firework - to prevent it happening at
all by correct treatment, and isolate its effect in the very unlikely
event it ever does it.
Strangely enough, the frequency of accidents does not seem to have
increased with the massive quantities of flight packs sold, so either
manufacturers are getting better or people are just learning what to
do..or what NOT to do.
So far in the 5-7 years I have been back in this hobby, I have seen one
death a year reported on average from an IC model, and probably around 2
fingers a year lost to IC prop's. Yes. electric priops are just as
dangerous..but..you don;t need to fiddle with needle valves or flick
start them. Although they can occasionally burst into life unexpectedly ;-)
I can recall reports of one burnt out car, and one workshop fairly well
gutted, due to LIPO fires, and no personal injuries to date. In the case
of the workshop the owner freely admitted he had made a mistake in
setting his charger..several planes have burn out though..due to
crashing/shorting or being charged as a whole..and I think one small
You should put these things in perspective.
Yes LIPOS can be dangerous. So can firearms, eating raw fish, driving a
car and fishing with a steel aluminium or carbon pole in a thunderstorm.
Understanding the risk is the key to reducing it.
As far as the advantages go, they simply have turned electric models
from short duration overweight underpowered eccentricities, to be in
almost every way comparable, and in some ways better performance than IC
engines. My models can swing bigger props at less power and often less
weight than IC engines can..I get typically on a lightweight sport or
scale model more duration than the average tank of fuel..I don't get
deadsticks, and I don't have starting problems. I carry NO field
equipment except a selection of charged packs - there is no need to even
take a charger for a short flying session. LIPS hold charge for
typically a year or so..though some of the 'hotter' ones lose their edge
after a couple of weeks charged. Those will die if not charged even
when not in use..the lower battery weight has implications beyond just a
simple weight reduction..less weight means you need less power to stay
up, and that means more duration..less battery weight means no massive
structure to contain the batteries, so structures get lighter..no big
vibration means lighter engine mounts..less weigh means a smaller motor
can be used..which means less weight..
When you see a 60" span model flying off a specific power that is about
equal to a rather poor 049, at less than 2lb all up weight, you realise
that electrics with Lipos is indeed something rather special.
If you use care and common sense, LiPos are a big improvement. With a good
LiPo charger it's close to impossible to overcharge them, although nothing
is totally idiot proof. Charge them on a fireproof surface or in a
fireproof container and you should be safe. Dispose of them if they get
damaged(you'll see literature on how to do that safely)
Start here for info, but you can find a ton of sites with google....
I fly strictly LiPo now except for some powered gliders that require NiCd,
NiMh to compete...
| Start here for info, but you can find a ton of sites with google....
To be fair, Red's site barely touches on LiPo cells. It's got good
information for NiCd, NiMH and even Pb batteries -- but almost nothing
on LiIon or LiPo cells. You'll need to look eslewhere.
| I fly strictly LiPo now except for some powered gliders that require NiCd,
| NiMh to compete...
Ditto. (Except that I don't really compete.)
| The benefits are as advertised..the risks are overstated.
| - Never let the voltage go above 4.2v per cell, or much below about 3.3v
| per cell..yes you CAN exceed these limits in SOME cases..but play safe
| for now.
Don't exceed the 4.2 volt/cell limit, ever.
3.0 volts/cell is a reasonable cutoff for a high performance plane, as
the voltage will go down somewhat while the motor is on. But once the
cutoff cuts off once, don't just throttle back and keep flying -- it's
time to plan your landing. (As TNP said, don't push it.)
Going from 4.2 volts to 3.0 volts gives you about half your original
power, so it should be pretty obvious when the battery is this low,
and you want to land before you reach this point.
| - de-rate the cells to about 70% of manufacturers and vendors claims,
| unless you have the test equipment to verify them and an application
| that is trying to squeeze the ultimate out of a pack.
Absolutely! If you get 20C `rated' cells, don't actually push them at
20C. 10C is much more reasonable, and your cells will last much
longer. (And really, 10C still means only a six minute flight.)
The only time you'll really want to push cells as hard as they're
rated at is in some sort of serious competition, where having to
replace your pack after every few flights is acceptable. And even
then it's dangerous if you can't really test things first.
| I have momentarily shorted cells and packs when making them up and
| fitting connectors, with no problems.
Ditto, as long as it's quick. But this isn't really any different
than NiCd/NiMH cells.
Shorting out cells is bad, be they NiCd, NiMH or LiPo.
| Storage in *cool* conditions requires nothing special..lets face it
| LIPOS are in every laptop and mobile phone, and they do not
| spontaneously combust.
Actually, they do. But it's rare. And when it happens, it tends to
make the news, so we hear about it.
| The three ever present dangers are due to accidental shorting of them
| while being transported, getting too hot, and being pushed too hard -
| either when charging or discharging.
Actually, TNP forgot one danger of LiPo packs -- physical damage. If
you physically damage a LiPo pack, like by dropping it or in a crash,
that can cause it to immediately short out with incendiary results --
or sometimes the incendiary results won't happen for a few minutes.
So any time you crash your plane, or just drop your pack, take the
pack out and put it somewhere safe from fire for a while (an hour or
so) just to make sure nothing is going to happen.
In any event, most accidents with LiPo packs ruin the pack themselves
and don't start a fire. They are much less tolerant of mistakes than
NiCd or NiMH cells, however -- one mistake can easily ruin a LiPo pack
entirely, when a similar mistake on a NiCd pack will just cost you
like 1% of the capacity (warning: made up figure!)
from The Natural Philosopher contains these words:
well, asked for a bit of info and got the grand tour thank you
very much and when you have been out of the game for so many years
every bit is welcome, for the time being I am only planning on building
and flying power assisted gliders of 4 to 6 foot span and building as
light as possible because as you point out the lighter the airframe and
less wind resistance you generate translates into endurance.
The one I am experimenting with is a 74'' span approx 1970's design
that used mainly Balsa sheet for the fuse so i have swapped this in some
areas for 1/32 ply which is lighter and much less bulky so has allowed
me to slim the fuse down somewhat saving more weight, when I come to
paint it I will use an airbrush which on a big model can save the weight
of a can of paint and so on and with this one if I can have an all-up
weight of 40 ounces I will be happy.
Thirty years ago free flight gliders used dethermalisers and I see they
are still used but now its a bit of string in an ally tube which burns
through a little elastic band etc but the ones we used were little
clockwork timers that released a pin allowing the tailplane to cock up
at 45 degrees and bringing the a/c down, looking at modified kitchen egg
timers for that one.
Looks like once the decent weather gets here I will be ready to have a
bit of fun again, right now though it is a dirty grey day with odd heavy
showers and blustery wind so not good flying weather I am afraid,
English weather for you.
That whole "six minute flight" thing is a bit misleading. It's only a
six-minute flight if you run at a continuous 10C draw from start to
finish. The 20C bashers also say, "Well it only gives you a 3-minute
Well, okay, IF full throttle current is equal to the 20C rating of the
pack, AND IF you fly full throttle for the entire flight. Reality is
that the left stick does go up and down, and most people already know
how to use it, or will quickly learn when their flights only last 3
The truth of the matter is, that it is perfectly safe to match your
full-throttle current to the pack's rated capacity, provided that you
actually use the throttle! Planes should be set up so that cruise power
is around half-stick, give or take. If you're going to go roaring
around at full throttle all the time, then yes, I would agree that
cutting the packs' ratings in half is a prudent move.
Yup, I've had NiMHs catch fire in the past.
No, they don't spontaneously combust. Some prior damage or abuse has to
be present for a pack to combust. An undamaged, properly-built pack
will not burst into flames just sitting on the shelf. Drive a nail
through it, okay. Set the charger for one cell more than the pack has,
okay. Set the charger for more than 1C, okay. Just not sitting on a
shelf doing nothing.
The "exploding" packs that made the news were caused by defective
construction. Remember the cell phone battery scare a while back?
Shoddy workmanship created a situation where the packs were easily
damaged or overcharged. Same thing with the Dell batterys of late.
| That whole "six minute flight" thing is a bit misleading. It's only a
| six-minute flight if you run at a continuous 10C draw from start to
Of course. But when you get a 10C rated pack, that generally means
10C *continuous* -- you can go higher for short periods of time.
But if you push it at 10C *continuous*, that's a six minute flight.
| The 20C bashers also say, "Well it only gives you a 3-minute
I've never heard of `20C bashers'. In any event, 20C packs generally
have lower internal resistances, so they tend to perform better than
10C packs, even at lower currents. (But often that lower internal
resistance comes at the cost of reduced capacity or more weight and
usually at higher cost, so it's all a trade-off.)
| Well, okay, IF full throttle current is equal to the 20C rating of the
| pack, AND IF you fly full throttle for the entire flight.
Yes. That's one possible meaning.
It could also mean a full throttle current of 30C (which the pack
might tolerate for 15 seconds) and 20C being throttled back a bit.
Either way, my point is that if you take a pack rated at whateverC,
and you run it at whateverC for the entire flight, it's not likely to
last long. Unless you're doing a pylon race or something, I'd instead
suggest going for (whatever/2)C -- your pack will last a lot longer.
| Reality is that the left stick does go up and down, and most people
| already know how to use it, or will quickly learn when their flights
| only last 3 minutes.
Reality is that if you really do push your 20C pack at 20C (or more!)
for the entire flight, you'll be replacing it soon. This could be a
result of never throttling back, *or* of just not putting a big enough
pack in your plane.
| The truth of the matter is, that it is perfectly safe to match your
| full-throttle current to the pack's rated capacity, provided that you
| actually use the throttle!
Yes, that's reasonable. It's also safe to put your full-throttle
current well above your pack's rated maximum continuous capacity --
but you'd better make *sure* you use that throttle appropriately.
(Also, note that the prop will unload in the air, so unless you
actually measure the current draw in flight and instead only look at
on-ground current draws, you might have over-estimated it.
Fortunately, the Eagle Tree logger isn't very expensive, and neither
are tools like Motocalc.)
| Planes should be set up so that cruise power is around half-stick,
| give or take. If you're going to go roaring around at full throttle
| all the time, then yes, I would agree that cutting the packs'
| ratings in half is a prudent move.
I don't care what your throttle is set at -- I'm talking about the
current you actually use, no matter what throttle setting is needed to
achieve that current.
Your pack may be rated at 10C or 20C continuous, but if you actually
use 10C or 20C continuous, your pack won't last long. Get a bigger
pack, or another pack, or throttle back, or a smaller prop -- or just
get used to the idea that your packs won't last long.
| > | LIPOS are in every laptop and mobile phone, and they do not
| > | spontaneously combust.
| > Actually, they do. But it's rare. And when it happens, it tends to
| > make the news, so we hear about it.
| No, they don't spontaneously combust. Some prior damage or abuse has to
| be present for a pack to combust. An undamaged, properly-built pack
| will not burst into flames just sitting on the shelf.
Ok, fine, there's a reason they combust -- it's not spontaneous. But
my point is that even laptop and cell phone batteries DO occasionally
combust, either due to abuse, or accidents or whatever else. Just
like R/C batteries.
| Drive a nail through it, okay. Set the charger for one cell more
| than the pack has, okay. Set the charger for more than 1C,
| okay. Just not sitting on a shelf doing nothing.
Not very useful sitting on a shelf.
| The "exploding" packs that made the news were caused by defective
| construction. Remember the cell phone battery scare a while back?
| Shoddy workmanship created a situation where the packs were easily
| damaged or overcharged. Same thing with the Dell batteries of late.
Yes, I recall the cell phone companies saying that. It was an OEM
pack -- our genuine packs would never do that! (Of course, all the
evidence was burned up, so we never got to really *prove* where the
battery came from.)
Either way, cell phone and laptop batteries sometimes burn, just like
R/C batteries do. But not very often, just like R/C batteries. The
only reason you hear about it so often is that it's considered
Benefits: Low weight/small size compared to same mAh capacity Nimh or Nicad.
I fly strictly LiPo in all my electrics these days. My Slow Stick is
completely stock other than a 2 cell 2100mAh LiPo pack. Weighs about the
same as the 700mAh nicad pack that is the stock pack for this plane. The
plane will hang on the prop thruout most of its flight time and flies longer
than my neck can take looking up at it. I have an MAE Nemesis running a
CDRom brushless motor powered by a 700mAh LiPo pack. Wouldnt fly using the
same capacity nicad or Nimh pack due to weight.
Risks are overdraining the packs which can ruin them or overcharging which
can explode them.
You do need a special charger but many multi chargers will also do LiPos
now. I use the original version Triton. Charges nicads, nimhs, LiPos, etc.
Also most LiPo packs now come with a "balancing plug". This allows a device
to be added which balances the cells while charging or discharging. It keeps
some cells from overcharging while allowing the other cells to reach proper
Another advantage is that LiPos dont lose much of a charge sitting on the
bench the way Nicads and Nimhs do.
I think that is probably achievable, but it will not be overpowered at
that weight..the contest gliders I have seen run ridiculously high
powers to essentially get a 1000 ft of altitude out of less than 30
seconds 'burn'..my humbler efforts will get about 3-400 feet out of that
sort of motor run.
Are you not intending radio control at all? This is possible, but most
people strap some servos in..and fly RC....the modern equivalent to a
D/T is crow braking..flaps down ailerons up..which gives a very smooth
and rapid rate of descent at very little forward airspeed at all.
Good luck anyway..its all different these days but there is still fun to
> regards, Terry
It will if the shelf is in direct summer sunlight. IF the internal
pressure and temperature in the pack is high enough, mechanical
distortion and internal shorting are distinctly possible.
Drive a nail
Depends where you live mate. I spent some years designing stuff in
Johannesburg..I had temp gauges and all the gear. One day I came in to
test my latest design, and was astounded to see that a black heatsink in
the sun was at 65C before I even switched the circuit ON..IIRC air
temperatures in e.g. the Mojave desert are 130F...over 50C..it doesn't
take much direct exposure to sunlight to push an object in those
conditions well over 100C. Yes, people can and do fry eggs on exposed
sheets of metal ;-)
Care to put a LIPO in a frying pan? Or an oven? And expect it WON'T
Agreed such conditions are rare in temperate climates..but the
possibility exists allright.
from The Natural Philosopher contains these words:
the first one off the stocks is the largest of the two I am
building and I am fitting servos to rudder and tailplane with an
auxiliary motor and folding prop for the time being but once I am
proficient enough to fly without using it i will probably take the motor
out and fit a normal nose end which will save weight and drag, one idea
I had which is a variation on something I saw years ago is a stop block
on the aileron push rod so that when you have 50% up aileron a contact
switch connects the motor, a weak compression spring behind the contact
allows full travel so you can still go to 100%.
The other glider I have an idea for is a 53'' span slope soarer but is
so lightly built there is nowhere to fit r/c gear so I was thinking of
stepping back 40 years anf building it as it was meant to be with a
dethermo unit but finding a clockwork version is not very likely unless
I can engineer one from a kitchen timer os something like that.
I am working on two or three projects at the same time at present
finishing off the last of the commission models for a L.A client
building the first of my gliders and also building something for my new
grandson to play with when he is older, I bought a life-sized Mallard
decoy duck then cut an access panel in its back so that I could fit a
prop shaft and motor and the steering servo and receiver out of one of
these cheapo radio controlled cars I bought as a wreck for a few
dollars, should keep him amused down the local pond chasing the swans
and maybe get him interested in model making and r/c too.
It really made that ducks eyes water when I rammed a 3/8'' drill up his
rear orifice to make a hole for the prop shaft,
| Are you not intending radio control at all? This is possible, but most
| people strap some servos in..and fly RC....the modern equivalent to a
| D/T is crow braking..
Well, that's one of many.
- There's also spoilers, typically seen on gliders.
- Spoilerons, where you raise your two ailerons.
- Simply doing a `slip', where you turn your rudder one way and use the
ailerons to bank the other way, so your plane flies sideways.
- Or just put your plane into a `spin' -- loses altitude fast and
normally doesn't stress the airframe too much.
I've seen dethermalizers on R/C planes (gliders) before -- if you
moved the elevator too far, the entire horizontal stab would come lose
and go up and rotate to a 90 degree angle. I wouldn't want it on my
own plane -- I'd rather land it myself normally.
Personally, I would have though that free flight would be dead by
today, or only done by kids with toys, but the Model Airplane magazine
tells me differently. (I'm guessing that what's really going on is
that FF is *almost* dead (at least compared to R/C) and that MA is
just showing most of what's going on.)
There is a small but extremely active bunch of people who do indeed
chuck models up and hope..I must say I am tempted too..
I fly a lot of 'old timer' stuff..basically old FF or S/C designs
converted to electric with 3 channel radio..takes a lot of the sting out
of trimming them if you have an elevator..and mostly all they do is not
fly *away*, and do a lot of climbing and gliding.
It's not ALL I fly, but it has a relaxing and nostalgic quality to it I
from The Natural Philosopher contains these words:
as I remember it was great fun and the gliders were pretty easy
to build too with no worries about battery charging and so on, presently
playing with a Chinese made kitchen timer clockwork movement that is
just 2'' diam and 3/4'' thick settable from 3 min to 60 min which may be
adaptable quite easily as a dethermaliser and as it is all nylon it only
weighs about 20 grams,