Basic Understanding of a Li Po battery

[eddiemoth]

After reading about Li Po battery for awhile and with my limited experience with a Li Po battery, I wrote this small article here just to help beginner RC helicopter pilots who want to know the basic information about a Li Po.

All the experts, please correct me if I am on the wrong track and please add if you have more information.

Regardless of RC helicopters you are into - either gas/nitro or electric, you are not going to avoid a Li Po (lithium Polymer) battery. This is the breakthrough battery technology which makes the battery smaller, lighter yet more powerful. Because of its unique chemical set up and reaction, Li Po battery can be dangerous if you are not careful or do not use it properly.

Most of Li Po battery manufacturers do not provide product warranty for their Li Po battery packs. I rarely hear that you can return a battery for a replacement.

This page is to provide newbie pilots some basic information you need to know about Li Po battery. HeliTown.COM is not responsible for any damage or problem you get into by using your Li Po battery. The information and discussions here are based on the experience of our individual members.

Though it sounds scary, a Li Po battery is safe if you are careful and follow the safety guidelines provided by the manufacturer, and it will provide you with more power for longer flight time.

Before you start using a Li Po battery, you need to read the Important Safety Instruction and Warning provided with battery when you buy the pack.

There are a few things you need to know when you use a Li Po battery pack: number of cell, battery voltage, battery mAh and battery discharge rate (C) and charge rate (C).

Number of Cells: A cell is a unit of each Li Po battery pack. Normally, it is abbreviated "S". For example, my APEX 3 Cell battery is marked "3S" on the label. This information is used to determine the voltage of each pack and it is critical to know when you charge your Li Po battery.

Battery Voltage: Electric potential of a battery pack. For a Li Po battery pack, you can tell voltage if you know how many cells your Li Po battery has. For example, I know that my APEX 3 cell Li Po battery has 11.1 Voltage. This is because each cell contains approximately 4.2 Volt. Always, check the label for the exact voltage.

MAh: Milliamp Hours. A unit for measuring electric power over time. mAh is commonly used to describe the total amount of energy a battery can store. A higher mAh rating means the battery can power a device that consumes more power and/or for a longer amount of time. For example, my Li Po battery pack rated at 2200 mAh can power a device drawing 100 milliamps for 22 hours.

Discharge Rate (C): C-rate - Capacity Rate - of a Lithium polymer (Li Po) battery. This is a critical information of a Li Po battery pack each RC pilot should know. The C- rate is used to determine how many amps a battery can deliver the power. For example, my APEX 2200 mah pack rated 12C can deliver 2.2 * 12 or 26.4 amps of power.

Charge Rate (C): Normally you can charge your Li Po battery at the rate of one time of its capacity - 1C to avoid damaging your pack or cause fire. For example, a 1C rate of a 2200 mah is 2.2 Amps. It is advisable to always charge less than one C rate to increase the Li Po battery efficiency. Check your Important Safety Instruction and Warning for more information.

Connectors: There are normally two wires connectors for a Li Po battery pack. In the picture below, the longer and bigger wire is for connecting to the ESC. The smaller and shorter wire is for connecting to the Li Po battery balancer.

[thanhTran]

Great write up Eddie! I think you should make it sticky for this battery forum. Also I notice that charging at 1C or lower usually helps a lot in balancing out the different voltage between the cells in the pack (providing that the pack is not too deeply discharged and still good)

Thanh

[medtech]

So, I'm still missing the great benefit to lipo's? A 4300 NiMH pack runs around $40 these days. For $40 you can get a 3150 Mah 3cell 8C lipo.

The lipo's are lighter, but you can't use all their capacity and there is more risk of damage and danger if used incorrectly.

The NiMH are heavy (almost twice as heavy) but can be charged much faster.

It seems even though NiMH are heavier they give just as much flight time if not more as a lipo would in my corona.

Could someone enlighten me!!

Thanks!

[swatson144]

On a Corona it's a matter of preference since they were designed to use NiCads. On other helis designed for Lipo you don't have a choice.

The 75% limit isn't written in granite and is a fairly new premise. It's for longer battery life. The thing with LiPo is not to discharge below the manufacturers minimum, as that is instant irreversible damage. It was 3v/cell now some are a tad higher. I use the 75% rule myself because it gives me a suitable safety margin and if all goes well and I don't dip into it, The pack charges in 3/4 the time anyway. Last I saw from a reputable source the difference wasn't that great between using the pack to 3v/cell or 75%. The idea makes sense to me as the pack heats up more as the voltage decreases and heat is the enemy. Lipos do drop off very quick so you'd probably see most of the benifit at 90% but not much margin for error.

Some people like the suitable helis heavy, some like them lighter. It's an option. For hovering and learning the NiXXhave an advantage. If you were closer I'd sure let you use mine so you'd know which you liked.

Steve

[eddiemoth]

Does anybody has tried Bphobbies's Batery Flight Time Calculator? http://www.bphobbies.com/view.asp?id=A0320048&Nav=7

[swatson144]

Simple arithmetic and works every time. If you know the data.

Steve

[lenny]

That answers a question I was going to ask about discharging LiPo cells :D I will up my ESC percentage a bit .Do they have the same memory set problem as nicad packs(the need to almost completely discharge on a regular basis)

[eddiemoth]

lenny,
I read somewhere and forgot the source of the info that a Li Po pack has no memory problems like nicad packs. It might be the BatteryUniversity. (http://www.batteryuniversity.com/index.htm) I never discharge my li po before charging like I used with a nicad pack. Correct me if I am wrong, folks.

[G-MRM]

Hi, Li-Po's dont have the memory problem and you should not discharge them other than in use.

These battery have a cycle life. If you only discharge at say .5 amp for 4 hours and always charge at 1C, never over discharge etc you may get over 600 or more cycles, however we take out the power in say 10 mins at say 10C life will go down, FlightPower say 100 to 200 cycles, my Align packs in the T Rex 450 just about made 100.

So, don't discharge unless your flying as it will use up another cycle life plus there is just no need ! :D

[swatson144]

If you have a safe way to accurately measure the pack voltage here is a chart to estimate the percentage of charge from the resting voltage per cell.

4.20v = 100%
4.03v = 76%
3.86v = 52%
3.83v = 42%
3.79v = 30%
3.70v = 11%
3.6?v = 0%

So for your 3S you should be able use the pack until it has no less than 11.3V after sitting for several mins, and enjoy a long pack life, with some margin for error. Please remember that as your skills change and put more demand on the pack your flight times will decrease. IE just scooting around on the floor your flight time may be > 30 mins, once you start hovering consistently it'll be more like 15 mins. etc. Much better to err on the conservative side than buy new packs.

A good LiPo alarm is highly recommend along with a timer. You shouldn't rely on the alarm but use it incase the pack isn't fully charged or some other error.

Steve (who just said hmmm some of this should be in the LiPo basics and pasted it there)

[G-MRM]

That's a handy table Steve !

I use the timer function on the Tx and always look at the amps that I have put back in after to make sure I did not remove more that the 80%.

[swatson144]

I've been running home made lipo alarms, and the very nice HrPolyX (made in the UK) as a back up for the timer. The HrPolyX is the bomb! Great gadget that protects against damage caused by replacing the pack you just removed with itself :lol: etc . Big danger for me as I buy packs in pairs and am easily distracted. An added advantage to the alarm is as the packs start reaching the end of their life (internal resistance goes up) you'll start hearing the alarm during hard manuevers early in the flight. Back off and the alarm quits. Time to budget new packs as the end of those is near.

Steve

[swatson144]

If you have no other way of checking the condition of your packs to properly manage flight times etc. Just start on the conservative side and fly longer until your charge takes about 1 hr to complete at 1c (less than an hour if you want to be more cautious). This isn't saying to only charge for one hour! I'm saying if you fly for 5 mins and it only takes 40 mins to charge you can add another minute to the timer and fly again. When you get close to 60 mins charging at 1c you are only using ~75% of the charge. Not the best method of determining flight time But it's reasonably accurate. The way it works is that most all chargers use the same method of constant current then constant voltage to finish. The constant voltage phase takes about 25% of the time of charging for a total of 1 hr and 15-20 mins to charge an empty pack at 1c.

If your charger doesn't have a setting for 1c the math is pretty simple 1300 mAh pack charged at 1000 mA. 1300/1000= 1.3 * 60 =78 mins so you'd fly until it took roughly 75 mins to finish charging.

I tested this with 2 good packs flown to what I knew was about 75% discharged (9mins on the timer). 2100 mAH charged at 2.1A (1C) charged in 62mins with an input of 1590mAh or roughly 75%.

YMMV but it's a bit better than just flying for 5 mins and charging for 30 mins. You will need to bear in mind that your lipo safe flight times will decrease as your skills improve. If you are trying to learn to hover you may be safe at 30mins sliding around on the floor but once you start spending time in the air your safe flight time may be only 12 mins.

Steve

[eddiemoth]

Just want to drop a link to Easics of electrics (without Math) by Albert Tejera.

http://www.tmenet.com/pdf/Basics%20of%20Electrics.pdf

[Mysterious]

whats the lifespan of a lipo battery? Does after XX months is basically need replacing beause its charge hold diminish?

[ranger306ci]

Not in my experiance. So far, oldest have lasted over a couple of years without any problems. That includes many flights, and many sudden impacts with mother earth. The worst looking cells are holding up with the best looking ones.
Just heed peoples warnings about certain brands. If there are alot of reports of x brand puffing in a short amount of time, I would not recommend buying that brand for a while. Sometimes companys get bad patches that puff quickly. From what I have seen, there are a lot of "rebadged" cells, ie one source to many companys that slap their own lables on the cells. That means that it is harder to weed out bad from good.

[basic]

in reference to the rebadging of cells mentioned by ranger306ci...rebadging can make it hard to weed out good from bad, but it can also lead to getting packs identical to expensive ones for less. just a thought

[ranger306ci]

I totally agree, basic.

[Pearl]

Hey this article is of great help for me , as I am using Li-Po battery for my laptop and found it realy helpful all that information you have given in this article. Even I bought a new Li-Po battery for my laptop from http://www.newpower99.comand through your article , I got to know some very useful information for my Li-Po laptop battery .Newpower99 is an Online store for purchase of laptop batteries, laptop power adaptors, batteries for brands like sony clie ,palm ,HP ,iPod, sony PSP, creative labs , iRiver , casio , asus ,RIM blackberry etc. Buy Batteries, accessories, chargers, adapetrs etc online at newpower99.com Quick answers to customers questions regarding purchase of batteries, chargers and replacement batteries , repair services , i pod repair accessories, asus , RIM, HP, iriver, blackberrry, palm, sony PSP, creative labs, casio, etc http://www.newpower99.com/aboutus.asp

[cjcj]

Number of Cells: A cell is a unit of each Li Po battery pack. Normally, it is abbreviated "S". For example, my APEX 3 Cell battery is marked "3S" on the label. This information is used to determine the voltage of each pack and it is critical to know when you charge your Li Po battery.
The S stands for series connection. P would be parallel