soloboss

I’m sure there are a lot of us who use LiPO packs and wonder why the big difference between the 2S and 3S packs (or even 5S and 6S packs). I thought that, since we have a bunch of new helicopter folks and spring is coming, I'd toss this out for anyone who cares about such things. If I messed up somewhere let me know and I'll edit it.

Power is measured in watts (or volt-amps if you're mad at Mr. Watt). Watts = VOLTS X AMPS. Ya got 100 volts and 1 amp, ya got a hundred watts - like a light bulb. Your 100 watt light bulb at home draws 1 amp. If you want the same brightness outa your car light at 13.8 volts you'd need a bulb drawing 7 1/4 amps. easy breezy. So lets look at your helicopter LiPO pack.

You get 3.7 volts per cell. You don't have to like it, but that's the way it is.
That's 2 X 3.7 = 7.4 volts out of a 2S pack.
And 3 X 3.7 = 11.1 out of a 3S pack.

A Lithium Polymer pack will deliver 2800 w/kg. That's 2,800 Watts per 1000 grams of battery weight. Put your pack on a scale and weigh it. You'll be close, but the connectors and wires don't count toward power. Suppose we have a 100 gram battery. If you get a 2S pack that weighs 100 grams it will deliver 207 watts. It's a good thing they use an electronc speed control (ECS) or you'd have blade pieces over two counties. If you get a 3S pack that weighs the same 100 grams, you'll have 310 watts of power available. That would spread blade bits over 3 counties. Of course those big numbers are like flash bulb numbers. The pack will put out that power for a split second and then explode. So we regulate the power used back to something reasonable. As cool as it sounds, you really don’t want 300 Watts all at once unless you are making a bomb. And I'm not gonna talk about that part. We use electronic regulators to control the output of the LiPO packs because it’s a really good idea.

Enough battery stuff – lets look at the helicopter. The bottom line is that it takes some amount of power to fly your helicopter - and that power needed is measured in Watts. Maybe it takes 10 amps from a 2S battery to hover your heli. Multiply 7.4 volts X 10 amps and you'll get 74 Watts of power required to fly that helicopter at that weight. If you pull out that 2S pack and insert a 3S pack of the same weight it will still require 74 Watts to fly, but because the 3S has more voltage it needs less amps (current). You'll only be pulling 6.66 Amps. Hmmm. That's way better! That 3S pack doesn't work nearly so hard to hover the helicopter. So really, you can get a smaller 3S pack that weighs less to do the same work as the 2S pack. Or go ahead and use the 100 gram 3S pack and go faster!

This is just stuff about charging and discharging and technical stuff the new guys may have missed, so I’m tossing it in here.
The voltage of a Li-poly cell varies from about 2.7 V (discharged) to about 4.23 V (fully charged), and Li-poly cells have to be protected from overcharge by limiting the applied voltage to no more than 4.235 V per cell used in a series combination. Overcharging a Li-poly battery will likely result in explosion and/or fire. During discharge on load, the load has to be removed as soon as the voltage drops below approximately 3.0 V per cell (used in a series combination), or else the battery will subsequently no longer accept a full charge and may experience problems holding voltage under load.

RMG2

Good stuff Solo!

Let me add that most battery manufactures, and most long time heli pilots agree that for longest life of your LiPo they should not be discharged below 3.7V per cell. Of course, it's up to the individual to decide whether they want more fly time at the expense of less battery life, or less fly time and longer battery life???

It's also good advice to buy batteries with the highest "C" rating a heli pilot can afford. To low of "C" (10C, 15C, 20C, etc.) rating for a given setup can cause premature battery death and, or, "puffing" of the battery itself.

One other piece of advice is to store LiPos that will not be used for a week or more at a voltage of 3.85V per cell. Batteries stored for long periods at full charge (4.2V) will build-up internal resistence and quickly lose their ability to discharge like when they were new. Many chargers have a charge mode named "storage" which charges batteries to 3.85V per cell and shuts off so the battery can be properly stored.

Don't ask me to explain any of this because I don't have the knowledge. Just reporting from much reading on the subject and my own experience with LiPo use (and abuse).

soloboss

Good post. There are always new guys coming onboard and a good battery knowledge base and a basic "How it works" thread could be good. I know there is a forum just for electric power, but I put this here for those who don't get far from home.
Thanks for the additions.
Solo

J-MICHAEL

Good stuff Solo ! That helped make things a bit more clearer for me now...


Cheers,

~ Jeff

dknovick

To add to this...

The mAh rating (milliAmp Hour) of the battery is the capacity (C) of the battery. The battery only supplies current it does not "push it" out. A battery that is rated for 1000mAh can supply 1000mA for 1 hour, or ~2000mA for 0.5 hour, or ~500mA for 2 hours. If this battery was a 1C battery, it could, at most, supply 1*C = 1*1000mA = 1000mA = 1A of current. A 10C battery could supply 10*1000mA = 10,000mA = 10A of current at one time.

In order to fly, you heli requires a certain amount of current (for the main motor, for the tail motor, for the servos, for the receiver, etc.). Let's say it's 8A. You're 1000mAh battery would have to be at least an 8C battery (8*1000mA = 8A) so that you don't starve your heli (don't supply it with enough current for lift off). However, if it was just an 8C battery, you would be right at it's limit, and it would probably get hot. A 10C or 15C battery would be a better choice.

marcolima

Hi Soloboss,

Sorry, but I didn't understand the count: 2800w/kg = 2,8w/1000 gr. Wouln't 2,8w/1 gr?
If the correct is 2800w/kg...wouldn't 280w/100 gr pack?

soloboss

Hmmm. Maybe I slipped a decimal point. Let's do it again by the book.
2800 Watts/kg
2800W/1000g
280W/100g

Yep - you're right. From a power to weight ratio, I liked it better my way, but the numbers don't work. I fixed my post. If you would double check me I'd be happy! Good information is always good. This little post is turning out to be informative for a few. I see it's been hit a few times. I was just writing it down to get it straight in my head, and decided to post it 'cause I'm not the only one who didn't have all the information straight.
Solo

BEAR-AvHistory

Basic question.....if a model comes with a 3S 1800x12C battery will a 3S 2200X20C battery cause it a problem?

evdreamer

The higher capacity and higher C rating won't cause a problem. It might make the heli a little nose heavy, but nothing bad electrical will happen.

Nick

rotarydoc

BEAR,

No problems, just more flight time with the 2200 mAh pack...

The only drawbacks;

The charging time will be longer on the 2200 pack.

The 2200 pack will be a bit heavier, but not a huge amount.

The voltage will be the same, so the performance should be similar.

Hope this helps!

rotarydoc

Nick,

Now this is getting strange...LOL[X(]

A minute apart this time...

I wish it still had seconds..

Glenn

evdreamer

Glenn,

What is the chance this would happen three times in two days?

Nick

BEAR-AvHistory

Thanks guys, it helps a lot.

Did not think about the weight all hanging off the front of the helicopter , good point. Have some fixed wings & a Blade 400; all using batteries in the 1800/2200 range & I was hoping to standardize on the 2200's.

I will take a close look at the weights of the E-Flite factory 1800 battery & the 3rd party 2200's

rotarydoc

Nick,

Pretty slim, I was laughing the second time...this time I was gettin' kinda scared...LOL

Maybe something to to with St. Patrick's day...LOL

Glenn

rotarydoc

BEAR,

Never mind Nick and I...seems we are psychic or something, we have posted replies within 2 minutes of each other 3 times in two days, all with the same basic answers/replies!

I guess we are on the same track lately, so to speak...

Glad we could help on the battery question, hope it helps you decide!

Good luck with it...

Glenn

BEAR-AvHistory

Thanks again.