Testing – 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

Being A Good Test Pilot: Part 2, Setup

Part 2 in an ongoing series about being a good test pilot. These articals are less about step by step hand holding instructions and more about how to think. We continue….

I always conduct an interview of sorts to fill in the blanks of things I may not be aware of such as level of experience, how long the project took, what the MFG says about CG location, suggested weight, what does it actually weigh and etc…. What your fishing for anything that could be a surprise. After the crash, you don’t want to hear the builder say “I thought 1/2″ behind reccomended CG was close enough?!?!”. Your looking for any shortcuts or oversights in the model. Did they use the reccommended servo’s or at least close? Is the battery large enough? How old is it? If the RX is new, has it been run a while? If the RX has come from another model, was it flying fine or did it’s last flight end in a crash? Has the TX been performing well? Are there any used or harvested components in the systems?

Always check the CG against common sense and what the builder says it should be. NEVER trust that it is right. Hold the model up in the air and check it yourself. Measure if you must. There is NO excuse for crashing over a missed CG check.

Control surface throws: I’ve run into many first time builders that started with a few foamy’s, have built their first sport or scale model and they put throws in the model that look like the 3d foamy they’ve most recently been flying. Not realizing the throws apropriate for a 25-30mph ship will be grossly excessive in a 70mph aircraft. Besides asking about suggested throws, apply your own stink test. If it doesn’t look right, it probably isn’t right. I like throws for a test flight just a tad on the high side of suggested first setups. However, you can take this way too far and end up with an over sensitive model. A model that is way out of trim and also way too sensitive is a real handful. Don’t walk into any such traps. Also, if a throw such as the elevator is too low and the model turns out to have an incidence or nose heavy problem, you might not be able to land at a reasonable speed and hold the nose up. Too little therefor can be just as bad a mistake. Try to be reasonable about what’s correct for “this” model. Generally, the faster it flys, the lower the throws should be. Something else to aid here is look for similar level of throws. If the ailerons are deflecting 30 degrees and the elevator only 15 degree’s perhaps the throws will be “out of balance”. Out of balance controls would be one control being sensitive and antoher being soft. This is more difficult to process in your brain when your under pressure trimming out a difficult model. Consider it before proceding.

To Be Continued……..

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,

Battery Storage In Reverse

For many of us there is a winter storage season. How do we bring our fuel powered models out of storage confident our RX battery packs are up to snuff? Were they nearing the end of life at the end of last seasons flying? Did they survive being in the trailer or garage ceiling for a number of months? Here are important steps to greatly reduce your risk of shouting “I Ain’t Got It!” when you hit the field this spring. These recommendations are intended for NiMH and NiCad packs although the similar principals apply to any mission critical TX or RX pack regardless of chemistry.

1. You should have cycled your packs and noted the value on them when you put the model in storage. Did you do this? A simple round of cycling in the fall will help weed the weakest packs from the herd.

2. Check the purchase date on your pack prior to model reactivation. Did you date your packs? Noting the purchase date in permanent marker should be a routine with new packs. Has this pack made it 3 seasons already? If it has made it 3 seasons, it’s time to replace it with a fresh one even if it’s still cycling well. It never seems like a good deal to “squeeze one more season” out of a pack if a model is lost doing so. There are no battery experts in the industry, nor any magazine writers that are willing to dare recommending using packs beyond 3 years. Most recommend only 2 years. The incident of surprise failures increases with each season. It’s much cheaper “not” to find out how long it will take to have a failure. Think about it.

3. Similar to a new pack, a pack having been in storage for some time is in need of a slow “forming charge.” A forming charge is a simple full-to-overflowing charge on a non-peak detecting charger like your factory wall wart. While in storage the cells slowly discharge. Not every cell will discharge at the same speed. After a few months, you could have one cell at 80%, one at 60% and two at 50%. When form charging, It’s important the charge rate does not exceed 10% of the packs mili-amp-hour (mah) value when doing this procedure. This type of charge allows all the cells to fill fully and the first cells to fill won’t be overheated by the ongoing charge. The danger of peak charging a pack that has been in storage is the best cell (the 80% full one) can be ruined as it’s overcharged while the other 3 are still filling up. Also, your pack may false peak meaning that although the charger reports it is full, it really might not be. Re-equalize the cells with a good long slow wall charger charge prior to any peak charging to avoid most problems.

4. Test for Capacity. Discharge the pack on your favorite charger (with discharge function). For the purposes of this kind of test, the correct rate to test against factory rating is 20% or 1/5 of the rated capacity. It’s ok if you can’t get that setting exactly, just get it close. Example: A 1000mah pack would be tested at 200mah discharge. Most chargers will display this as .2A. Your pack should test at least 80% of it’s rated capacity. If it does not, then a few more charge / discharge cycles are in order. If you can’t get the pack to test above 80%, it’s time to replace it. Although it might seem like a money saver to succumb to temptation and overlook marginal packs, one crashed model will pay for a great many replacement battery packs. And that’s to say nothing of the risk to others when a model goes out of control. Good pack or no go!

5. When you recharge the pack after your final discharge test, check the charger input mah. Did it put in about the right amount? A pack that’s been in storage, particularly if you’ve skipped the step of re-forming it is very prone to a false peak. A great pack that tests perfect but only takes 50% of the expected recharge amount could cause some unwelcome excitement.

6. Test your Switch. First, use a loaded tester to check your fully charged pack directly. Note the value then test it through the switch harness. If it tests good directly but marginal through the switch, it might be a sign the switch is getting dirty internally, worn or perhaps some connectors are going south. Like battery packs, finding out how long a switch will last is costly knowledge to acquire. It’s a good idea to replace the switch with every other new battery just to avoid trouble. Load testing your pack with and without the switch harness looking for any substantial difference is a good way to detect a problem before starting the season. Did you notice what I omitted? After checking the battery through your switches charge lead or charge jack, unplug it from the RX, turn the switch to the “ON” position and check it again. Is it load testing similar to the charge jack/charge pigtail? The most important place for your pack to deliver it’s energy is to the RX. Make sure it’s solid to this point, not just the charge harness.

Integrate these practices into your seasonal routines and many common pitfalls are avoided. Don’t forget to scrutinize your TX battery in similar fashion. Ongoing TX function is every bit as important as RX functionality.

Dave Thacker, Owner: RadicalRC.com
Blogsite: Radical RC Workbench