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Red's Corner, July 2010
Monday, 14 June 2010

All about Lithium Polymer Batteries

Hi Red, I have a question about LiPo batteries. I don't think I'm the only one either because nobody I've asked seems to know. Perhaps you could talk about how they are rated. For example I know what 3S means (3 in series = 11.1 volts) but I don't know what 3S1P means, could you explain the basics?

Thanks, Marty Thank you very much for the great question Marty! I am sure you are not the only one either, as the electric industry as a whole is really not that well defined when it comes to explaining its various ratings and acronyms. It can be very difficult, for instance when converting an IC (internal combustion) powered model to E power, to spec an equal performing power system. More about this later, but that should be really how you think of this type of thing, as system. (motor, ESC, battery and prop all have a great deal with the “power” your system will produce) Thankfully the battery nomenclature is pretty well defined and easy to follow once you understand the system.

On to your question! By definition all batteries are made up of individual cells, whether they are NiCad, Nickel Metal, Lipo or any other chemistry. A battery is a cluster of cells wired together either in Series and/or in Parallel. Let’s take a simple 2100 maH 11.1v pack as an example and look how it is defined. The 2100 is the total capacity of energy the pack is able to deliver effectively rated in milliamp/hours. If you have a system that draws 30 amps you can expect to fly for about 7 minutes. (Provided the 30 amp draw falls under the packs “C” rating, more later!) The 11.1v rating as you noted comes from the fact that this pack is wired in Series as the 3 individual cells are wired in such a way (pos & neg soldered together) that their individual voltages (3.7v) are added together make to the 11.1v rating.

Parallel (pos & pos/neg & neg soldered together) wiring is used when a larger capacity is required. The largest typical single cell size is around 2700 maH (there are other larger single cells, but they are rare and don’t really apply in this example). Parallel wiring sums the capacities together. So if you buy a 4200 maH pack it likely is two 11.1v 2100 packs wired in Series/Parallel or 3S2P configuration. That means there are 6 individual cells wired into two 3S packs to get 11.1v, and those two 11.1v packs wired together to get in parallel to get a capacity of 4200 maH. So for any battery the first number indicates how many cells are needed to get the desired voltage and the second number is how many packs of cells at the desired voltage are added together to get the desired capacity. You can stack any number of cells together to get the desired voltage and capacity. The only rule is all cells must be of the same starting capacity. (i.e all 2100 cells) It is not uncommon to see up to 10S4P setups in larger aircraft.

I suppose some of the confusion is that a 3S1P battery really only is referred to as a 3S battery leaving out the 1P part of the designation. By default it is a 1P battery. Which is fine as 1P batteries are all most people fly with anyway. The 2P (or 3,4,5 or more) is not really seen very much at all. Earlier I mentioned C rating. The C refers to capacity, and the rating is used to tell how quickly the pack can safely discharge current. Using the same 2100 maH pack as an example, one might have a 15C rating while another may have a 35C rating or more. The reason it is important to pay attention to this is because it is possible to permanently damage your packs if your system exceeds the C rating of the pack. How it works is simple. You take the C rating and multiply it by the capacity to get the maximum available current the pack can deliver safely. Let’s say our 2100 pack is rated at 15C, you multiply 2100 x 15 = 31.5 maximum amps continuous. So for our example 30 amp setup mentioned above, this battery would suffice but I would consider it a bit too close for comfort. You can see that the 35C battery with its 73+ amps continuous available is a much better solution. Most batteries also have a burst C rating, most of the time it doubles the continuous rating. The burst rating lets you know that if you have a hotter system and your battery is on the edge of being adequate, that you can exceed the normal C rating for short periods of time (i.e. take-off, hovering etc.) but you should not exceed more than 10 seconds or so at this higher current draw.

In normal operation, LiPo batteries should never be brought down below 3v per cell (9v for a 3S pack). Doing so can permanently damage the cell and can lead to poor performance and shorter run times. In extreme cases they can even swell and catch fire. Most ESC’s have this 3v limit set at the factory, but it is possible to disable it on some controllers like the Castle series for instance. At any rate, flying until you hit the low voltage cut off each time is not the best way to preserve your packs. It’s best to land when you notice the power getting soft.

Balancing is always a good idea, you cannot over balance. I balance every time I charge. This will maintain your packs to the highest possible standard. I have packs that are going on 5 years old and still perform as new. Balancing becomes even more appealing when you are running multiple parallel packs together as the different performance characteristics of each individual cell will unbalance them rather quickly.

Well Marty I hope this answers your question, and thanks for sending it in.

Email Red at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it with your comments and suggestions.
 
Float Fly, September 6th
Tuesday, 24 August 2010
Please join us on September 6th from 10am for "Day on the pond II" at the Sal Lake float fly site. Take the Lytton Springs exit just north of Healdsburg, enter through the CDF parking lot.
  • cold drinks and restrooms available
  • 2.4 GHZ preferred, call 584-4428 for FM channel arrangements
  • 2010 AMA memebrship is required to fly
Email Sid "the Airman" Maxwell at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it or just give him a call for all details at 707-584-4428.
 
Red's Corner, June 2010
Monday, 14 June 2010

I’d like to write a monthly column that addresses the more technical side of our hobby. I am probably more addicted than most, but I really enjoy the how’s and why’s of our toys and would like to share as much as I can with you. I take great pleasure in tweaking whatever project I am working on to get the most performance out of it whether it be radio setup, or getting an engine to run perfect or dialing in C.G and control throws. What I would really like is for people to submit a question or topic that they may be interested in, and I could write about it here. Some columns may go hand in hand with a live demonstration at our monthly meeting, perhaps something like an introduction to composite part making or other relevant topics.

To get the ball rolling I thought I would touch on some radio basics that there still seems to be some confusion about, and some reluctance to use. There really is no excuse for not using whatever features your radio has to offer. Even the most basic aircraft can benefit from some of the advanced features that even the mid level radios seem to be equipped with nowadays. My goal here is to hopefully explain some of those features a bit better and inspire some to give it a shot. Most people don’t realize that the ergonomics of your transmitter can be changed quite a bit to suit your flying style fairly easily. For instance, the control stick length can be adjusted to a more comfortable position. As a “pinch” type pilot I prefer my sticks very short. If you are a “thumber, you might like them a bit longer. If you tend to be a jumpier flyer, longer sticks can smooth you out and conversely, 3D pilots generally like them shorter. The point is you might not realize what you are missing, so tweak them and find out. Another stick related item is tension. Many radios allow you to tighten or loosen stick tension to suit your style as well. Jumpy/precision guys like tighter, 3D looser. I prefer them on the tighter side. Your switches can also be rotated slightly in the case to be less cumbersome to flip depending on whether you pinch or thumb. The best switch flipping path may not always be straight up and down.

Another area to look at is exponential, or expo for short. I know, I know seems mundane but in reality it seems to be a bit misunderstood underutilized. Just recently I helped a fellow member adjust expo values with favorable results and that got me to thinking it’s not as widely used as I had assumed. What expo does is essentially “soften up” the movement of the control surface in relation to the movement of the stick around the neutral point of the controls. This is needed because of the way our servos act our flight surfaces. The servo takes rotary motion (the twisting of the servo arm) and transfers it to linear motion (back and forth of a pushrod). This inherently causes some loss of throw as you reach the ends of the travel as the pushrod moves less back and forth and more in towards the center of the servo. In fact if you do not use any expo, a standard set up will actually have negative expo and be more jumpy around neutral! A good rule of thumb is that it takes 20-25% expo to get back to a linear motion. I never use an expo value of less than 35% on low rate and as high as 60-70% on some models. My racer runs about 45% on all surfaces. Give it a try; you might be in for a treat. (Ed. Futaba uses the opposite numbering for this feature, negative values result in a softening of the controls. If in doubt, ask someone for help)

I’d love to hear from you. If you guys find any of this stuff helpful drop me a line. I take requests.

Email Red at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it with your comments and suggestions.
 
Wings over Wine Country 2010
Tuesday, 08 June 2010
The Wine Country Flyers will be performing in the Wings over Wine Country Air Show once again this year. The Air Show will take place August 21st -22nd. If you are interested in participating, members can sign up at the June general meeting. We need volunteers to help out with our booth, and proficient pilots for the flying demonstration. The deadline for pilot sign-up is July 1st. Pilots will be required to demonstrate exceptional flying skills during the August 8th Practice Day to participate in the Air Show. A rehearsal will be held at the WCF site on August 15th. Attendance on August 15th is mandatory for all pilots participating. We look forward to putting on a safe and exciting show! Email Jon at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it for more info.
 
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