Battery guide

[GiantShark]

The first is batteries... what's left out?
What's unclear?
What's wrong?

Thanks-

Simple Terminogy

To start off we’ll use the old analogy. Think of electricity like water. Amps = flow
Volts = pressure.

Amps are like water flow, like liters per hour. There can be flow at low pressure and
you can have flow at high pressure.
Volts is like pounds per square inch, psi. This says nothing about quantity or how much is flowing, just how hard it is being pushed. You can have 100 psi
with zero water flow.

Amp hours is how much flow can be sustained for how long. It is used as a
way of measuring how much electricity is in the battery. Like how many
gallons of petrol in your tank. It is a capacity number. It means nothing about
flow or pressure, it is about capacity.

1 amp = 1000 miliamps (mili means 1/1000 amps)

Examples

So a 3 cell lipo provides 11.1V (pressure).

The motor will draw electricity from the pack at a certain flow rate, or
amps.

If you have a have a 1000 mili amp hour pack (mAh), it can deliver a flow of 1
amp (1000 miliamps) for one hour. If you draw it out faster, it
doesn't last as long. So your motor might pull 10 amps for 1/10 of an
hour, or about 6 minutes. (10C= 6 minutes)

A 2000 mah pack has twice the capacity of the 1000 mah pack, so it should
last twice as long.


What is C rating?

C ratings are simply a way of talking about charge and discharge rates for
batteries.

1C, = 1 times the rated mah capacity of the battery. So if you charge or discharge your
650 mah pack at 1C, you charge it a 650 miliamps, or 0.65 amps.

1C on a 1300 pack is 1.3 amps. Duration = 1 Hour (60/1)

2C on a 1300 pack is 2.6 amps. Duration = 30 minutes (60/2)


If you have a 500 mah pack rated at 15C it knocks out
7.5 amps. Duration = 4 minutes (60/150

If you have a 1000 mah rated at 12C it knocks out
12 amps. Duration = 5 minutes (60/12)

If you have a 1000 mah pack rated at 7C that means
it can deliver 7 amps. Duration = 8.5 minutes

If you have a 1500 mah rated at 8C it knocks out
can deliver 12 amps. Duration = 7.5 minutes

If you have a 2000 mah pack rated at 20 C that means
it can deliver 40 amps. Lasting only 3 minutes.

If you have a 2000 mah pack rated at 30 C that means
it can deliver 60 amps! Lasts 2 minutes!

So, if you need 60 amps you can use a pack with a higher C rating (2000mAh 30C) or a pack
with a higher mAh (4000mAh 15C) rating to get to needed amp delivery level.

Burst Rating

Back to the 2000mah 11.1v 3S battery. Lets say the rating of this battery is 20C constant and 30C max. This would be a high performance battery

So as we know the maximum constant discharge rate would be 20 x battery capacity (2000mah or 2amps),so 20 x 2amps = 40 amps constant. Using a 2000mah battery at 40amps will make it flat pretty quick – about 3 mins flight time only. You will see batteries marked as 20C constant 30C burst.
The battery cannot discharge at this rate for very long – maybe just a few seconds, but during this time its knocking out max power. So in this instance

2000mah (2amps) x 30 = 60amps!

This means a 30C 2000mah Battery is actually capable of delivering a whopping 60amps @ 11.1volts. This translates to 11.1 x 60 = 666watts power. (Watts = Volts x Amps).

However - It is best to size your battery packs so they run below their
max C rating. You will stress them less and they will last longer. Eg if your motor needs a battery that can deliver 10 amps, getting a 1000
mah pack that is rated for 10C ( 10 amps ) will meet the spec, but it is
running at its limit. A 15 C rated 1000 mah pack would be better, or
perhaps a 1300 mah 10 C pack. In either of these cases, the pack will be
less stressed and should handle the load much better over the long term giving you a long lasting, better performing lipo.
I’m often asked what effect does voltage have on the duration of a lipo.
Duration = number of minutes / C rating. There’s nothing about volts in there.
Voltage doesn’t enter into it. However, if you have a 1200rpm/v (kv) motor running on 11.1V (3 cells). 11.1v x 1200 = 13320 rpm.

Imagine you put a 4 cell on there. That’s 14.8v. Look what happens to the rpm and the power (VxI). Things should be falling into place now...

Why Are The "C" Ratings Important?

If you over-discharge the Lipo, it will get VERY hot and probably catch fire. If you want to give your plane a good sending off, put a battery rated for 20 amps burst into a plane that will demand 40 amps - November 5th would be good choice for this sort of stunt.
Check your motor’s manufacturers specs out for how many amps it needs. Every motor has them so it shouldn’t be too challenging. As a safety net if a motor needs 20 amps get a lipo that can give it 40amps. The battery can’t force feed it amps unless you over prop it. If that happens and you’ve factored in the extra amps you’ll lose your motor, but you won’t lose your lipo, esc and probably your plane and motor to the Balrog.


In Series and Parallel

When you put cells in series, their voltage is added together. For example:
-----> [- 3.7V +] [- 3.7V +] [- 3.7V +] ----->

This battery has 3 cells in series, so this is called a "3S" battery or “3S1P”. The total voltage of this battery is 3*3.7V = 11.1V.

When you put cells in parallel, their capacity is added together. For example:
.--> [- 1.2Ah +] --.
---|--> [- 1.2Ah +] --|--->
'--> [- 1.2Ah +] --'

This battery has 3 cells in parallel, so this is called a "3P" battery or “1S3P”. The total capacitance of this battery is 3*1.2Ah = 3.6Ah.

You can use combinations of cells in series and in parallel to achieve different levels of Voltage and Capacitance. For example:
.--> [- 1.2Ah, 3.7V +] [- 1.2Ah, 3.7V +] [- 1.2Ah, 3.7V +] [- 1.2Ah, 3.7V +]--.
---|--> [- 1.2Ah, 3.7V +] [- 1.2Ah, 3.7V +] [- 1.2Ah, 3.7V +] [- 1.2Ah, 3.7V +]--|--->
'--> [- 1.2Ah, 3.7V +] [- 1.2Ah, 3.7V +] [- 1.2Ah, 3.7V +] [- 1.2Ah, 3.7V +]--'

This battery has 3 parallel groups of 4 cells in series, so this would be called a "4S3P" battery.
The total voltage of this battery is 4*3.7V = 14.8V, and the capacitance of the battery is 3*1.2Ah = 3.6Ah

Motor batteries vs. receiver batteries

Some batteries can sustain high discharge rates. Others can not.

Those used as transmitter/receiver packs are made for low amp
rates while those made for motor packs can discharge higher rates.

Having a 500 milli amp hour pack does not tell you if it is a motor pack that can put
out 6 amps, or if it is a transmitter/receiver pack that would be damaged if
you tried to pull power at 6 amps. It is enough to say that they are
different.

Clearly a motor pack could be used for a transmitter/receiver job, but a
transmitter/receiver pack should not generally be used as a motor pack.

Each Lipo cell is 3.7 volts. The capacity (storage) of a battery pack is determined by the mAh (milli amp hours) rating of the cell itself, regardless of the voltage.

[Andy2No]

More about connecting packs in series and in parallel

It's probably worth pointing out that although adding battery packs in parallel adds the capacities together, connecting them in series doesn't - it just adds the voltages together. Two 1000mAh 2 cell lipos in series is just a 1000mAh 4 cell lipo. Two of them in parallel, is a 2000mAh 2 cell lipo, and so on.

When connecting packs together in series or in paralled, the C rating doesn't change - at least if they're both the same C rating. If you connect two 1000mAh 10C 2 cell lipos in parallel you get a 2000mAh 10C 2 cell lipo - the amount of current you can draw is doubled, because there's twice the capacity, but it's still 10C, not 20C.

Much more importantly:

Looking after lipos

- don't over discharge

People who are used to NiCd and NiMH packs are used to squeezing the last bit of useful power out of them before they recharge them. For NiCds that's definitely the best thing to do, and for NiMH packs it's good to at least do it once in a while, and doesn't hurt to do it every time.

If you do the same thing with lipos you will kill them very quickly. Lipos hate being over discharged. You may not notice the difference straight away, and they'll still charge up, but the capacity of the pack goes down every time you do it, and the C rating starts to fall too.

- keeping them healthy for longer

So, if you want your lipos to last, it's far better to just use 40%-50% of the capacity at a time, then land and put in a fresh pack. Relying on the low voltage cut out in your ESC is generally not a good idea with lipos, for that reason. Arguably the best way is to check your lipos after each flight to read the voltage, and time your initial flights - start with a few minutes, check what's left in the lipo and decide whether you can stretch it a bit more next time. Allow for windier weather by reducing the length of the flights. A plane that can happily fly for 15 minutes on a calm day without wearing out the battery pack might only manage 5 minutes in a stiff breeze.

After a while, you get to know how long you can fly a particular plane. Personally, I think it's still good practice to check the pack voltage after every flight so you don't get complacent, and so you notice when a pack is starting to wear out.

I aim for no less than 3.8V per cell after a flight. A lipo checker that plugs onto the balance lead to read the cell voltages is very cheap to buy and will save you a lot of money in knackered lipos. The voltage you measure after a flight is a lot higher than when the motor is running. It's important not to let lipos fall below about 3V per cell under load. That will look more like 3.7V per cell when you measure it afterwards.

- charging more slowly is better, and safer

If you keep charging a worn out lipo assuming it's the same capacity and C rating as when it was new, you may well have a fire. It's safe to charge any lipo at 1C (e.g. 1000mA for a 1000mAh lipo) but as the capacity and C rating falls with wear and tear, the current you used to charge it at becomes too much and it starts to get hot when you charge it. If it gets too hot, it'll either explode or do a good imitation of a marine flare. It won't burn for very long, but it will burn very hot and you won't be able to put it out. All you can do is stop the fire spreading to things around it til it's finished.

While charging at 1C is safe, charging at 0.5C or lower is safer. Lipos like being charged slowly. You ought to be there while they're charging, of course, so you don't want it to take all week, but there's no point trying to do it as fast as you can - it'll just mean you'll be replacing them sooner, or possibly even risking a fire. If you need to leave the room, unplug it and finish charging it another time. That won't do any harm.

- charging to less voltage is better

The maximum you can charge a lipo cell to, without damaging it, is 4.2V. Above that voltage, the anode starts to build up a layer of plating that it will never lose, and the capacity goes down. If your charger allows you to charge to a lower voltage, it's better to do that. For one thing, there's always a margin for error whenever you measure something, so if your meter says it's 4.2V it might be 4.22V - which is too much. A lipo that's only ever charged to 4.15V per cell, or lower, will live a lot longer than one that's always charged to 4.2V per cell. At 4.15V, it's still holding 95% of the same charge but you'll get many more useful cycles out of the battery. At 4.1V per cell it's still holding about 90%. A slightly shorter flight duration, with a pack that you can use over and over again, has to be better than pushing it to the limit and constantly having to replace them.

Storing lipos

Lipos are great for holding their voltage for weeks, so if you fly often there's no harm in charging them up as soon as you get home and putting them away ready for next time. However, if you know you're not going to use them for a few weeks, or longer, it's best to put them away without recharging them. A lipo with around 3.85V per cell or a bit less (usually after a fairly short flight) can be stored for a long time without losing capacity. A lipo that's stored for months with a full charge will lose quite a lot of its capacity, which is why they're never fully charged when you buy them new.

[whitemice]

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[Andy2No]

I use a timer to give me some idea, but I find the time taken to discharge to the same level depends a lot on the weather. I find I can fly for three times as long on a very still day as I can in a wind that's just within the comfortable limit for the plane.

I wouldn't want to regularly run packs down to 20%, but just for reference, what would that show for the cell voltages on, say, a 20C pack?

[whitemice]

edit

[Andy2No]

Thanks for the example.

I mostly fly slightly souped up park flyers, and generally stay below about 15A on a 2s lipo. They need far less power on a calm day, so for example my brushless CC Lee Piper Cub, set up for 13A max, flies for 15 mins on a calm day with a 20C 2s 1300mAh lipo or 5-6 mins in 10-12 mph winds. AUW is about a pound, so I don't like to use batteries much heavier than that in it.

In calm weather, a park flyer can glide down wind, with some up elevator, and doesn't need full throttle much unless you feel like trying aerobatics. In windy weather, mine tend to need at least half throttle most of the time, and more to fly into the wind.

I can see how the wind effect is less on something as powerful as the planes you fly. You basically have an electric equivalent of an i/c powered plane so wind speed is less of an issue.

[Stu Phedd]

A question???

now that some of my lipos are showing their age , and some I have split for other reasons , does anybody out there know of a soldering flux so I can rebuild some of my lipos ??
Rob may be able to ask the lipo suppliers.
years ago when you bought lipos separately they gave you some soldering flux to make up your own packs so it does exist somewhere !

cheers

Stu

[whitemice]

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[mkdave]

I refer to Andy2No post dated 24 February 2008 where he stated the following -

"When connecting packs together in series or in parallel, the C rating doesn't change - at least if they're both the same C rating. If you connect two 1000mAh 10C 2 cell lipos in parallel you get a 2000mAh 10C 2 cell lipo - the amount of current you can draw is doubled, because there's twice the capacity, but it's still 10C, not 20C."

This does not follow my experience when I bench tested a 200W motor through a Hobbywing 30 amp speed controller using a multiplex watt meter and either one 11.1 volt 1300mAh HiModel 20C constant Lipo or two of the same connected in parallel. The batteries were bought at the same time earlier this year and they have had more or less the same use.

The first test conducted was with the single Lipo, which was capable of pulling 20 amps (10 volts giving 200 watts) but which became very hot in the process. I know I really should not pull 20 amps from this Lipo but it was not done for very long and I usually use the battery in my GWS Bf109 with a 130 watt motor for which it is ideal.

The second test conducted was with the two Lipos in parallel again pulling about 20 amps. The batteries hardly got warm.

So I reckon each battery must have been sharing the load of say 10 amps a piece.

I agree that my 20C Lipos are still 20C Lipos no matter how they are connected but it seems to me that by connecting in parallel they share the current and thus a relatively small 1300 mAh capacity Lipo can be used in a relatively large motor.

What do you guys think?

[DH106]

I agree - connecting in parallel should increase the 'C' and the capacity levels, whilst connecting in series increases the voltage, but not the C and capacity values. This parallel connecting is how some manufacturers get higher C ratings - I have a 3S2P pack containing 6 cells.