Posts: 4445
Joined: 5/24/2003 From: Rochester,
NY, USA Status: offline
Hi,
I have seen many folks here on RC Universe asking about Lithium application basics. This same theme is predominant at the local fields where I fly.
The benifits of using Lithium power for e-flight are fantastic, but to many folks, they are not obvious. The power density to weight ratio surpasses both NiCd and NiMH technologies.
Currently, some of the barriers for using Lithium cells include price, safety, and, complexity. It was not common to parallel cells until Lithium Ion and Lithium Polymer technology arrived.
Let's start the thread with an application example. Feel free to help out by adding your own knowledge and applications.
Although I use Kokam cells, this is not a Kokam thread. Feel free to add E-tec, Thunderpower, or any other Lithium Polymer cell application.
Here is the new 3 cell Kokam 1200HC pack. By itself, it has an equivalent voltage to a 10-cell NiCd pack. It can deliver an average current of 6amps continuous. This is about 5C or 5 times the capacity "C" of the cell. (i.e. 5x1200) It can also deliver up to 10-12amps for short term peak current using throttle management. This is between 8C and 10C.
Posts: 4445
Joined: 5/24/2003 From: Rochester,
NY, USA Status: offline
Another view of the packs reveals a total cell count of 12 cells. The cells are arranged in a 3s4p configuration which provides an average current delivery of 24amps. (1200x5x4). The peak current delivery is about 38-48amps (8C - 10C).
This current delivery capability is a good fit for my AXI 2820/10 powered Wipa Firecat application.
The good news is that this pack increases my CP1700SCR duration to 4800mAh and drops the weight from 16.8oz to 11.7oz. This is 2.8 times the flight time with a 5oz drop in weight! A fantastic gain indeed!
The bad news is that the price is about 4 times the pre-assembled CP1700 pack. (about $200 vs. $50)
Posts: 4445
Joined: 5/24/2003 From: Rochester,
NY, USA Status: offline
The Kokam USA Connector Modules allow you to easily reconfigure your pack for various applications. This helps you save money by using the same cells in different setups instead of hardwiring the packs together.
Posts: 4445
Joined: 5/24/2003 From: Rochester,
NY, USA Status: offline
As with NiCd or NiMH cells, Lithium batteries can be wired in series to increase voltage. Unlike NiCd or NiMH cells, the Lithium cells can also be wired in parallel to increase capacity and current delivery.
A combination of series and parallel Lithium cells can be used to create a more capable flight pack for powering larger motors or increasing flight duration. A common designation seen to describe the configuration of multiple cell Lithium packs is the XsYp label where X is the number of cells in (s)eries and Y is the number of cells in (p)arallel.
The series/parallel diagram below shows one pack of four Lithium cells in series and one pack of four Lithium cells in parallel. When the cells are placed in series, the voltage per cell is simply added. The four cell Lithium pack would measure 4*4.2v or 16.8 volts unloaded. Its pack designation would be 4s1p or simply 4s. When the cells are placed in parallel, the voltage remains the same but the capacity and current delivery are added. If the four cells are the Kokam 145mAh Lithium cells then the resulting pack would measure 4.2v unloaded and have a capacity of 145*4 = 580mAh (or 4C). The current delivery capability would also jump from about 700mA continuous current per cell to about 2800mA continuous current for the 4-cell combination. The short term peak current delivery for this particular cell is about double that so our 4-cell combination would deliver about 5600mA for 10-20 second bursts. Its pack designation would be 1s4p or just 4p.
Posts: 4445
Joined: 5/24/2003 From: Rochester,
NY, USA Status: offline
Use the "Click here for a larger image" link not the built-in "click for full size" feature and you'll get your big image. I host the larger images on my site instead of RCU.
lets say a 1200 mah battery yeilds 20 min flight time in a gws zero. One etec 2cell 1200 will take 1 hour to charge so how is this a good ratio? every 20 mins you have to wait an hour ? wouldnt a 600mah nimh yeilding 10 mins flight time and only 20 min charge time make more sence?
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Posts: 4445
Joined: 5/24/2003 From: Rochester,
NY, USA Status: offline
Interesting scenario. Let's look at it in a 2 hour time frame assuming that we are using rough guestimates.
For the 600mAh NiMH pack, you fly 10 minutes and charge for 20 minutes. Neglecting any cooling time, that gives you 10 minutes of flying per 1/2 hour or 40 minutes of flying for every 2 hours.
For the 2-cell E-tec pack, you get 20 minutes of flight time and charge for 60 minutes. You can then fly another 20 minutes for a total of 40 minutes flying and have 20 minutes "free time" to help out other hobbyists in your 2 hour period.
Basically, you can stay in the air longer without re-charging. Often, we bring multiple packs and simply leave the chargers at home.
Posts: 4445
Joined: 5/24/2003 From: Rochester,
NY, USA Status: offline
If you are converting an existing model from NiMH or NiCd batteries, for example, you should plan on supplying at least the same voltage with Lithium cells. This can often be accomplished by dividing the number of NiCd cells by a factor of 3. A 2-cell in series (2s) Lithium pack is often used to replace a 6 or 7 cell NiCd pack. A 3-cell in series (3s) pack is used to replace a 9 or 10 cell NiCd pack.
Lithium cells have the advantage of very slow storage discharge. You can charge a Lithium pack weeks before using it to fly your plane. This has been a much-noticed benefit among R/Cers that use Lithium packs since preparation for flying can now be done days, or even weeks, ahead of time.
Throttle management can be used to bring the average rate down, thereby reducing the load on the battery and increasing flight times. It is also best to gain power through increased voltage, by adding cells in series, rather than increasing the peak current.draw. This is most often seen in brushless motor applications since they typically handle higher voltages than their brushed counterparts.
lithium polymer technology for RC is improved day by day.when kokam,145/560/830/ 1020mah cell was introduced last summer,it was a big surprise against conventional other batteries. kokam 1020 has steady power than any other batteries but continuous power drain was not strong enough (conti 4A) than e-tec1200(6A) but new kokam 1500HD can draw continuously 9A/CELL and peak 15A.EVEN kokam 2000HD can draw 14A conti,20A SHORT BURST without problem.If you connect pararell and series connection, you can draw 100A draw according to the pack assembly.kokam reduced the price of high capacity battery so that higher capacity motors will be popular in the market.further many other charger makers develops lipo chargers for high capacity and high voltage range.In GERMANY F3A competetion one of kokam user used 10s2p ,37v,4000mah pack.He said he could do same as other engines'powerful flight.
For enviromental control reason lipo flight will be the future technology for RCers as well as other batteries.
JJ HONG as you said the candy store is much bigger don't know what pice is X. I got my 1020's last summer also now got about 60 hrs on 2 of them. We talk about heat before I still am going at 6-7 c and burst of 10-12 C. I have followed you around some. You take care Best wishes. CU
Dear E-bird, kokam 145HD cell has no heat problem at 7c continuous and 10c burst.kokam 340 mah HD cell has no heat problem at 20c continuous and 34 c burst. kokam 700HD has no heat problem 7c/10c kokam 1500HD has higher heat at 6 continuous c-rate,10c burst but no special quality issues. kokam 2000HD has no heat problem like 700HD. kokam 3300 mah cell has same formula as below 1020mah. kokam 30AH/70AH cell has no problem like 700HD
last 1020 mah cell can be applicable for 3-4 c continuous,burst 6c rate.If you use them higher rate,you may have heat problem. The heat means below 60 degree C.If you store battery at 60 degree C longer than a week,you will lose 12% capacity loss at normal discharge r
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