Receiver – Radical RC Workbench

Balancing Severely Out of Balance A123 Pack

The following is a recent exchange discussing some charging logic that is impacted often by time-out settings in chargers. Not a bad thing all in all. The same logic can be applied to Lipo packs as well.

Hi, Dave:
Here’s my situation: All of my A123 batteries came from you and I’ve got several. All of them (except 1) work perfectly and I enjoy being able to take advantage of all that A123 batteries have to offer. My one that doesn’t work properly is a 2300mah, 2S receiver battery that I can’t get to balance. I charge it on a Turnigy Accucell-6 with the cutoff voltage set at 7.2 volts. My charger timed out at 120 minutes with one cell at 3.6 volts and the other at 3.25 volts. When I first started using these batteries, I was negligent about balance charging and would as often as not, just quick charge them and go fly. So, this is not a warranty question at all, but one as much for my knowledge as anything. First of all, is this battery safe to use (as a receiver battery), and second, is there anyway to get the second cell back up to voltage? I’ve cycled and balance charged it probably 3 times trying to get it to respond, but nothing I know to do has worked. I guess I could used it on an electronic ignition where sudden failure wouldn’t likely be as catastrophic as losing receiver power. What is your recommendation?
Thanks,
Steve

When balance charging, the first cell getting up to 3.6V causes the charger to start stepping down the charge rate. Ultimately, the charger cannot go over the maximum dissipation rate of your balancer. In other words, if it can only dissipate 100 mah, then the charger will drop back to 100 mah. It’s charging the pack at 100 mah but at the same time discharging the full cell at 100 mah to keep it from going over 3.6v. If the low cell is 1000 mah behind, in the two hours of the time out, it will only be able to bring the lagging cell up about 200 mah. It will still be lagging by 800 mah and some measurable voltage difference will be the evidence. Because the charger times out and stops working, your still out of balance.

Procedure options:

A. You could just keep repeating a 100 mah charge rate and let it time out 4 or 5 times.

B. You could also go into the setup and disable the time out.

C. There are some safety concerns with both “A” and “B” above. The best and quickest method that we use at our shop is to connect the charger through the balance port to only the low cell. You can do this through the standard XH balance connector by taking a JR or Futaba RX charge cord, crack off the outer shroud exposing the two pins. These two pins will be .100″ apart, just like those in the balance harness. Plug the bullet end of the cord into a volt meter, plug the business end into the balance harness, probing the different combinations. In the case of a 2 cell RX pack, you’ll only find 2 combinations. Offset to the black wire and offset to the red wire. One of these will read about 3.6v (the full cell) the other will read 3.25v (in your example, it’s the low cell). When you find the low voltage position, carefully pull the banana plugs out of your volt meter and plug into your charger. Set the charger to charge 1 LIFE cell. Set the rate (for a 2300 A123) to something between 1 and 2 amps (we don’t want to overheat the delicate balance connector) and let it charge that individual cell through the balance harness until it’s full.

When it’s done, both cells should be at similar voltage.

If you want to get really fine, there could be a slight calibration difference between your charger charging a single and a two cell pack. To really refine it, reconnect the pack to the charger as a 2 cell pack in the conventional way. Put the charger in discharge mode set at 2 amps. Let it take our 100 mah or so out of the pack. Then, switch back to Balance Charge mode and charge at 2 amps. Now the charger will put the 100 mah or so back in and at the same time balance both cells to each other. Since the pack is almost full, it won’t actually charge at 2 amps, it will read something lower. When complete, if the cells are good and the charger is working properly both cells should be very close.

It is possible the cell is bad. If this is the case, the above procedures and logic won’t result in a balanced pack. (presuming the charger is working correctly) It’s OK to repeat the procedure if you want to try again however, it’s likely your results will be the same.

If you are able to balance it successfully, do a discharge on the pack at capacity/2 or near. This is the standard for testing lithium type cells. So, a discharge rate of about 1.1 amps would be correct. Realistically the A123 2300’s should test within 50 mah of 2200 if they are in perfect condition. If the pack tests below 80% of 2200 (below 1760 mah) it should be replaced.

As to safety, I hesitate to ever say any battery is “safe”. I would say that if I could not get the pack behaving properly, I’d replace it. The cost of any pack is always a tiny fraction of the value of a model. It never makes you feel like a winner to put one in the dirt over saving a few bucks on a simple part, especially if you were suspect of it before you flew. Get it right, get confident or replace it.

Another safety warning here is you should be extra diligent when working with any battery where it’s condition is suspect. Do it outside and/or supervise closely. Never charge unattended inside a structure or vehicle. Always use a fireproof container for charging, especially when dealing with anything suspect.

If you follow through those procedures and that logic, you should be able to rule the pack in or out and have good confidence in your decision. Hope this helps you sleuth out the pack. Dave

Is Daily Low Self Discharge Rate and Important Factor in RC?

A customer asking about an unusual cell size of ours asks: “Are they are low self discharge?”

They are not Eneloops which would be the only NiMH I would classify as Low self discharge. This is almost a silly thing to consider because we come off the charger and go to the field in this hobby. We do not charge a battery, then start a 1 week hike at the end of which we fly the model. That specification makes sense for a flashlight or an emergency radio, not an RC aircraft. Others may have a different view. I don’t fly unless I’m coming off a fresh charge at the beginning of the day. Other systems are to the choice of the users but reckless in my view. Yet I offer the Eneloops for those seeking this value. There are no Eneloops this small. Also, I don’t like the Eneloop under fast charge “ever” conditions.

For RC, lets go over how silly this is;

Standard self discharge x 2 would be only 2% per day.

After charging, if we let a 500mah pack set for 2 days before flying, it would lose (500x.01) 10mah the first day, (490 x .01) 9.8mah the second day. Value at end of 2 days 500-10-9.8=480.2mah. 19.8ma lost over 48 hours or about .416mah per hour dissipated.

If we come off the charger and go to the field, a trip that takes 2 hours, the same rate of loss would mean our pack would be about 499mah since we loose about .416mah per hour.

So, perhaps, a low discharge pack is good for about a .2% advantage when you get to the field. And, to come up with that .2% I had to exaggerate the loss by double and suggest a very long trip to the flying field.

This is why this specification is essentially moot when it comes to normal day in and day out use of receiver packs in RC aircraft. Is it better? We’ll yes in some microscopic way, but to get it, what are you going to have to do? Accept an Eneloop you can never fast charge? Use a non-Sanyo cell? I see a lot of effort hunting something that has no real measurable benefit in our application.

It’s more arguable in a TX battery because we often use that battery over a number of weeks between charges.

Dave

Checking A123 RX Packs For Recharge Point

A123 RX Packs can be tricky to deturming how much is left in the pack by checking voltage alone. Variations in connectors and length of wire can have a big impact on actual volt readings when loaded. Using an RRC1000 digital voltmeter with load capability of 0.0A, .5A, 1A and 1.5A we get the following results measuring a 2300 2S RX pack with 6″ 20 g silicone JR pigtail and the included 22 guage battery checker pigail with the meter. Note: the meter (which ever you are using) is reading the voltage on it’s board, not at the pack. The voltage at the pack will actually be higher by the voltage drop across your checkers connector, pigtail, checker/pack connector and the pigtail on the pack. Here are the results we measured at varous loads. Room temperature was 74 degrees F, each load held aproximately 5 seconds before reading taken.

State Of Charge	No Load Resting Voltage	.5 Amp Load	1.0 Amp Load	1.5 Amp Load
40%	6.58v	6.37v	6.18v	6.09v
30%	6.52v	6.38v	6.17v	6.06v
20%	6.45v	6.32v	6.19v	6.08v
10%	6.38v	6.25v	6.14v	6.04v
0%	5.43v	5.19v	5.08v	4.98v

As can be seen from the data above, at some loads, the pack actually increased slightly in voltage as we went down even though the overall trend was lower in voltage. Note this test was not over a multitude of packs which would be more accurate and likely nuetralize the unexpected results mentioned.

Notice how little the pack is falling off in voltage and that the biggest consistant drop is in the resting voltage column, not a result I expected.

Notice the results at 0% capacity remaining as measured by my charger/discharger. As it is important to understand the context of the data and how I was checking the voltages, it is also important to understand the context of the data and how I was discharging the pack in 10% steps until empty (more explanined below) All discharges to make this chart after the initial 60% discharge were at 1.1A and in 230mah steps. The discharge harness was made from 22guage wire, 24″ long and plugged only into the JR output lead on the pack. Even though after 5 seconds of holding the load, I got the voltages above on the 0% line, putting the pack back on the discharger and trying to discahrge it some more resulted in the pack falling off to the 4V cut off (the empty point) in only about 10 to 15 seconds. Yet, I was still able to measure almost 1.5 higher than that when the pack had come off the first discharge to empty and been allowed to set for only 10 minutes before I measured anything. We can see that a wide range of voltages over 5 to 15 seconds with differing loads were all the same thing – EMPTY! Pay attention to the context of everything or you’ll get fooled! Because the context of how you are checking the voltage has such an impact on the reading, you should check your packs the same way every time with religious zeal.

A123 Systems cells ability to hold a strong voltage under load all the way until they are empty is one of the primary reasons they are so popular as RX packs, yet it is the very reason they are somewhat more challenging to voltmeter check from flight to flight.

To devise your own chart, cycle the pack to deturmine it’s actual value (ours was 2250), recharge, then set your chargers limiter to 60% of the actual value (ours was set to 1350) and discharge at capacity/2 (we used 1.1amp for our pack). After you’ve discharged it to this point, take the reading with the equipment and through the switches or whatever you have installed in your ship. Now you will know the readings at the 60% discharged (40% remaining) point. This is where you should be recharging any mission critical pack such as a TX or RX pack. To arrive at another row of data aproximately 10% further down in the pack, we simply set the limiter to 230mah and repeated the discharge. Repeat for each line of data you’d like to collect. You could start from full and discharge in 230mah steps generating data for 100%,90%,80% & etc….. Science, don’t you love it!

It would be my advice to think about making your own chart so you can learn something and become firmiliar with the voltage drop across all the gear in your model. You’ll be measuring the pack across a switch harness in most cases which will give you lower voltage readings than these.

General practice should be to taxi the model back to the pits, and before you’ve turned it off, plug your loaded volt meter in, turn off the model and take your reading immeadiately. Note your own chart for the correct cut off voltage and always recharge at the 40% remaining point. Flying below 40% is dipping into your reserves and should be avoided for any mission critical pack.

Do A123 LIFE Packs Free Us From Cycling?

Dave
One more quick question. I ordered some 1100 2s A123 packs from you today.
Do these need to be cycled? I have a FMA 4S CellPro charger that is A123 compatible. It will charge and balance, but not cycle.
Thanks again.
IRL

Irl,

If you want to check them before flying, Yes.
If you want to find out when they go bad on the workbench rather than at the field, yes.

There is no skipping regular battery testing and maintenance regardless of battery chemistry. All battery types will fail eventually and discharge testing is the only chance to discover packs needing replacement before having an accident.

My answer might seem a bit strange, however, every time there is a new battery chemistry many modelers think the new “miracle chemistry” means the end of regular battery maintenance and testing. I got the question many times at the beginning of the NiMH revolution, the Lipo revolution and at the introduction of A123 Systems LIFE cells. There could be nothing further from the truth. There is never a time when battery maintenance and testing is not prudent.

No jab against the CellPro chargers is intended here. They are very good quality and I recommend them. I don’t know the specifications of all the models they sell but am aware some of them will discharge test packs. It is possible to discharge these in NiCad or NiMH mode on modern digital chargers as long as the mode has NO CHARGE at the end of discharge. In other words, as long as it’s not a “cycler”. A cycle is a full discharge then charge or full charge then discharge. To do this, we want to us a charger that simply does 1/2 the cycle, in other words we want it to discharge and that is all. Just set the (NiMH or NiCad) cell count to 4 for a 2 cell A123. Some let you set the cut off voltage directly and in that case, set it to 2v per cell or 4V for a 2cell A123 pack. The correct discharge rate for any kind of lithium is Capacity/2. They are rated over 2 hours. Since many chargers/dischargers only allow discharge rates at even .1 amp (100mah) increments, set discharge to 500 or 600mah (.5 or .6 amps) to do a reasonably accurate job on an 1100mah rated cell.

I’ve noticed over the years the 2300mah cell (26650 can size) generally cycles to 2100-2200 range. They seem slightly over rated. Don’t be alarmed if your 1100mah (18650 can size) pack tests to 1000 or 1050mah. It’s probably just about right.

Happy Flying
Dave,

The Texas Sharpshooter Fallacy

I wrote previously on the subject of “Confirmation Bias”. Sometimes it is difficult to discover the answer to a technical problem because the person bringing you the problem has a hitch, assumption, or faulty logic step in their diagnostic process. Often people draw conclusions from spotty evidence. For instance a customer shows me a receiver and say’s “This receiver is bad.” I ask: “Why do you think it is bad? The answer almost always comes back something like “I plugged it in and it does not work.” The person is saying from that one test or measurement they have drawn a conclusion. It seems reasonable doesn’t it? But, really it’s pretty silly when you think about all the things that can cause an RX not to respond to a TX. You see, the real and only conclusion you can draw from the customers test is this. “In one trial, the RX produced no apparent response.” That is quite a bit different from “This receiver is bad.” Understanding the difference in those two conclusions is why some people are good at diagnostics and others are not. To be good at figuring out a problem, you are greatly advantaged by not making any assumptions or broad conclusions.

In the case of a receiver, lets go over many measurements and tests that you might perform to decide if it is in fact “Bad”.

1. Has it ever functioned successfully?
2. Does it really match the Transmitter? (is it talking the right language DSMII vs DSMX or PCM vs FM vs AM and etc…)
3. Is it on the same channel? (in the case of non-2.4ghz gear)
4. Besides looking at the stickers, did you actually look at the tags on the TX and RX xtal?
5. Is the shift the same? For example, a positive shift JR TX is never going to drive a negative shift Hitec or Futaba RX.
6. Have you driven the servo you used on the tested RX with a servo tester to make sure it actually wiggles?
7. Have you load tested the battery your driving the RX with to see that it is high enough to actually turn on an RX?
8. Are you using a switch between the battery and RX? Plug the battery in directly so your not actually testing if the switch is good.
9. Can you demonstrate the TX driving another RX to establish that your testing with a working TX? One might complain, “I flew it a week ago!” However, we’re not testing the troubled RX a week ago, we’re testing it now. 😉
10. Have you plugged a voltmeter into an empty servo port to see if there really is voltage finding it’s way to the RX?
11. Is the crystal really fitting tightly in the socket or is it loose and wobbly?
12. Does the TX have the capability of being on for programming without broadcasting?
13. Is the meter on the TX a voltmeter or RF Output indicator? What does it say?

I’m sure a sharp thinker can come up with some more things to consider. Many of the things above we’ve found at one time or another to be the cause of a non-responsive RX. Assume nothing.

Recently we had an A123 RX pack returned by a customer. He said it tested poorly, only a few hundred mah. The customer appeared to be correct, it was testing bad after several charge/discharge cycles on our bench. And, the charger would increase in voltage rapidly when we applied charge current. Strange. However, even after several trials, a good mechanic still hasn’t drawn any conclusion. He may be moving towards condemning the battery but all tests were not complete. He cut the shrink off the pack. The tabs all looked fine. He re soldered the tabs anyway just in case there was an unseen cold joint. Note: He had originally built the pack, but without emotion, he redid his original work anyway. Many people fail at this step because “they couldn’t possibly have done anything wrong.” (yea right!). The pack was cycled again with the same poor result. Now, finding a bad battery pack is rare, exceedingly rare. We know this to be true from many years of experience. So, we keep looking. I examined the pack under magnification (even though it had been re soldered by a respected pro) and all looked good. I looked at the plug under magnification and found a thin transparent film on the plastic shell. The more I looked, the more I saw this film all over the shell. Is this paint? We decided to solder a second lead onto the pack and test again. The pack tested good. What was the problem you wonder? We can only conclude the film on the plug was thin CA the customer had somehow accidentally allowed to come into contact with the plug. It had a high resistance because one or more connector pins was evidently coated in glue. After replacing this plug, the apparently bad battery pack was proven that it was always as good as new.

So, if a battery pack fails a discharge test or an RX fails to respond, is it bad?

To read about The Texas Sharp Shooter Fallacy, check out this wiki link. Reading it is what inspired me to write today’s article. It describes in somewhat technical language a common way to foul up a test.
“Texas Sharpshooter Fallicy” Wiki Link.

RadicalCast #003

RadicalCast #003 [ 25 min 06 s | 28.65 MB ] Play Now | Play in Popup | Download

Topics discussed include: Using Lipo’s & A123’s as Receiver Packs, Choosing a Soldering Iron & Lost Watts – Why Efficientcy Matters.