logo
Thread Tools
Old 02-23-2009, 08:58 PM
Jason Niemier is offline
Find More Posts by Jason Niemier
Joined Feb 2009
6 Posts
Learning battery cycling FREE TOOL

When you consider the extremely important task the battery packs have in an RC aircraft one can begin to realize the monumental faith we place upon them. The battery pack is the onboard pilot in the model in a sense because there is really nothing to compare its significance in a full scale aircraft other than the pilot. Arguably I believe the RX & TX battery pack is the weakest link in our flight system, even the smallest anomaly can lead to certain disaster.

Unfortunately many beginners and even seasoned flyers consider the battery pack nothing less than a necessary enigma. But something this important should never be looked upon to just do the job blindly. Proper battery maintenance is paramount to the ultimate success of any well kept RC flight hanger.

Yes we have load engineered lighted switches and meters to check levels at the field which can be very helpful. However nothing can tell you more about your batteries condition over time than a well managed battery cycling regiment. Which is the charging and discharging of the pack down to a predetermined cutoff voltage to achieve an accurate Milliampere-hour or(mAh) reading with an instrument designed to do just that called an analyzer.

One ampere-hour is equal to 3600 coulombs (ampere-seconds), the electric charge transferred by a steady current of one ampere for one hour. So the mAh is one-thousandth of an ampere-hour or 3.6 coulombs. With a good analyzer (many are available in the hobby) we can measure the mAh drawn from the pack under a load similar to the flight system over time. Having this knowledge is like peeking into the pack through a window and seeing its fuel level, but even better.

The trick is to learn what this data is telling you so your battery is no longer an enigma just doing a job with blind faith. It is necessary to run at least three or more test per pack and then multiply that into an average. Then divide that average by the packs rated mAh capacity which gives you the percentage of the packs actual mAh capacity available for use.

The RC industry standard acceptable flight limits for most Nickel-Cadmium (NiCd) and Nickel-Metal Hydride (NiMH) battery packs is 70%. To determine the 70% capacity mark for your pack simply multiply its labeled capacity by 0.7. When packs mAh capacity is less then 70% of it's labeled mAh capacity a battery anomaly is present and it should not be used . Now under most conditions replacement is recommended with some exception which Iíll show you later on.

So we now have a better understanding of how to measure a packs mAh and using the above formulas to calculate its usable capacity. But doing this time and time again can really be a numbers game. So to manage those numbers we need some help and for my money there is no better help for that task than Microsoft Excel spread sheet software.

Hereís where things get real interesting and let me tell you NOTHING can give you this much information about your batteries condition at a glance. I took care of all the formulas for you in Excel. You DO NOT need to be a computer genius or a Einstein mathematician to use this spread sheet. Simply enter the tested data you get from your analyzer into the proper cell (light blue cells) and all the math is done for you. Even the line and percentage graph updates automatically so easy my 9yr old daughter can do it. How cool is that?

All the data in the graphs below are related and change as you input more data in the light blue cells. Now we can get a much better picture of how the pack is doing over time. Let me direct your attention to the line graph below. We can see pack 2 the 1800 mAh pack clearly has a problem but seems to level off at 70%. Not the best situation but with careful monitoring of this red line shows we now have a 800+ mAh pack we can use in a trainer or something. So the larger 1800 mAh pack near 70% is not always junk as long as it shows a consistent mAh capacity level (as the red line graph shows) itís still usable just not for the heavy duty tasks of a usual 1800 mah pack for the larger planes. Or simply bust it apart and use them in your glow igniter is what I do. Now look at Pack 10 the 600 mAh pack itís at 70%. I would junk this pack quick a 30% loss of a 600 mAh pack is not good for anything at all.

Now look at pack 3 the 1400 mAh pack see the drop off in the line chart but notice how it's coming back up. Old NiCd packs can in fact do this they will get better with continued cycling over time. Just keep testing it and make sure the mAh remains consistent over time when it does come back up. See pack 1 the first 1800 mAh pack this is ideal a true dream pack you can put this into any costly project and feel confident it will not fail you.

Lets talk about the all fearsome NiCd memory effect for a moment. Firstly, the term memory effect is not an accurate analogy of what is happening. People tend to attribute most anomalies of a NiCd to memory effect. Let us define memory as the phenomenon where the discharge voltage for a given load is lower than the rated pack. This can give the appearance of a lowered capacity, while in reality, it is more accurate to term it voltage depression. Memory is also hard to reproduce in a lab environment which makes it difficult to study accurately.

Let us look at various causes of "memory" or voltage depression. Memory can be attributed to changes in the negative or cadmium plate. Recall that charging involves converting Cd(0H) to Cd metal. 2 Ordinarily, and under moderate charging currents, the cadmium that is deposited is microcrystalline (i.e. very small crystals). Now, metallurgical thermodynamics states that grain boundaries (boundaries between the crystals) are high energy regions, and given time, the tendency of metals is for the grains to coalesce and form larger crystals. This is bad for the battery since it makes the cadmium harder to dissolve during high current discharge, and leads to high internal resistance and voltage depression.

The trick to avoiding memory is avoiding forming large crystal cadmium. Very slow charging is bad, as slow growth aids large crystal growth (recall growing rock candy). High temperatures are bad, since the nucleation and growth of crystals is exponentially driven by temperature. The problem is that given time, one will get growth of cadmium crystals, and thus, one needs to reform the material. Partial cycling of the cells means that the material deep with the plate never gets reformed. This leads to a growth of the crystals.

By a proper execution of a discharge/charge cycle, one destroys the large crystal cadmium and replace it with a microcrystalline form providing a better more accurate pack discharge for the plane. Which is way we see continued improvement in older packs during the cycling regiment.

If anyone wants this Excel spread sheet file just PM me Iíll send it to you e-mail. Anything I can do to keep the planes in the air and out of the trash can makes me happy.
Jason Niemier is offline Find More Posts by Jason Niemier
Reply With Quote Quick reply to this message
Sign up now
to remove ads between posts
Old 02-23-2009, 09:16 PM
alfycat is offline
Find More Posts by alfycat
TARECOS AeroClub
alfycat's Avatar
Costa Rica
Joined Apr 2007
1,020 Posts
Re: Learning battery cycling FREE TOOL

Wow, that was very scientific!
It's nice when you help others and that makes you happy!
Thanks.
alfycat is offline Find More Posts by alfycat
Reply With Quote Quick reply to this message


Quick Reply
Message:


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Learning battery cycling FREE TOOL Jason Niemier Motors, ESC's, BEC's, Batteries & Chargers 3 02-25-2009 11:24 PM
New NIMH mattman683 Onboard Electronics 35 01-05-2009 05:10 PM
Cycling LIPOS???? SmoothFlying Motors, ESC's, BEC's, Batteries & Chargers 1 10-12-2007 03:25 PM
Sweet!, just ordered "some" stuff! 3D_Kid General Discussion - Electrics 19 07-12-2007 09:30 PM
Battery voltage? Chris Cann Onboard Electronics 8 04-17-2006 04:01 PM