Thoughts from the workbench of Radical RC. The online retailer of electronics and kits for radio control aircraft. Dave Thacker shares his thoughts and knowledge of electronics, batteries, kit design and overall enjoyment of the hobby.
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.
Lipo DischargingWarren Behymer asks and interesting question:
I need to know how to dispose of LiPo battery that has swollen due to an overcurrent.
There are two issues.
1. We want to discharge the battery in such a way as there is minimal risk of fire during the discharge and later when the battery is in the recycle or trash bin.
2. We want to dispose of the depleted pack in a recycling container.
The first thing to do is remove all potential from the battery. We do this with an 1157 light bulb (Brake/Marker type bulb). It’s handy to use because it gives an indication of ongoing discharge by emmitting light and doesn’t tie up one of my ever working chargers. We have our bulb wired with alligator clips and a switch to choose between one or both filaments. I don’t remember what the draw is per filament, but we considered that a small cell would be more safely discharged at a lower rate than a larger cell. When discharging a large cell, we set the switch so both filaments burn. We use an ammo box as an oxygen poor fire safe to do this, since we are indoors and we’re working with a suspect pack in the first place.
Lipo Discharged
This is allowed to burn until the bulb is out, then allowed to set connected to the bulb until the following day. This way we are 100% certain the pack is completely exhausted.
Lipo Leads Soldered Together
Next we solder together the leads on the pack. Just in case any recovery or bounce back of capacity in the pack were possible, it is constantly discharged through the short. There should now be no chance of any kind of arc or spark starting a fire in the recycling container. Being that the pack is completely empty, there should not be any energy present of any kind.
Battery Recycling BoxIt’s now okay to discard /recycle the pack properly. We won’t have to worry about the pack accidentally being shorted and causing a fire in any container. It’s electronically inert.
Federal law (49 CFR 173.185) states lithium type batteries must be individually packaged in non-conductive material and transported to a “permitted” recycler. In our shop, we use Call2Recycle (also known as RBRC 1-877-723-1297). They provide free recycling materials and processing. A bag is provided for each pack, we wrap the back in the pack, seal it, drop it in the box. When the box is full, we contact UPS for a free pickup and delivery to the recycling station. Any local battery seller should have this capability on site. We accept lipos for recycling at Radical RC.
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.
RRC1001 Voltmeter Image
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.
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.
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.
We all have chargers. Most of these chargers will have a digital screen with a voltage read out. When you’re charging (or discharging) your favorite battery, where is the voltage being measured?
It’s an interesting question and I’ve found hardly anybody who knows or understands the answer. It’s really very simple, though not straightforward. What is your answer? I submit to you that your charger does not display the voltage of your battery. There really is a simple point of logic here. Certainly, nearly everyone reading this will look back on their experience charging batteries and think to themselves that I’ve gone nuts. But, I submit to you that I have not, and if you’ll participate in a little test you will soon be a “crazy” like me.
Let’s gather the parts of our test. We need a charger with a digital readout that accepts banana ended charge cords. To make this test simpler, let’s use a Li-poly or “A123” pack. I want you to use one of these kinds of battery packs because it has multiple connections to the cells inside and will make the test easier than if we use a NiCad or NiMH pack. We will also need a digital voltmeter with probes.
We can do this test under charge or discharge, you decide. I want you to apply a 2-amp charge or discharge load to the pack. If it’s full, you’ll be discharging, if it’s empty, you’ll be charging. Charging or discharging makes no difference. The results will be madness, I promise.
First, let’s take a resting no-loaded volt reading of the pack. Measure the voltage out of the output wires first(what you’d be connecting to your speed control) . Jot down this number. Now, measure it from the red and black (or outer most 2 wires on the balance plug). Jot this number down. These two numbers should be identical.
Next, plug a charge cord into your charger and then into the output plug of the pack. If your charger is on, you should be able to read out the voltage on its screen and it should agree very closely with your digital voltmeter measurement. Jot this number down. There may be a slight difference here and we can explain that because there may be some calibration difference between your meter and the charger. Also, most chargers do not read out to 1/1000th of a volt like your meter may read. Some chargers drop the digit, some round it. It’s hard to figure out what your charger does here but let me promise you this. For the purpose of this test it is of no consequence.
Next it’s time for another measurement. Slightly pull out the banana ends of your charge cord from the charger so you can easily probe them with your meter. Take a measurement at the partially retracted banana plugs. Now, you have made 4 measurements and jotted down each of them. All are essentially the same voltage. Now, apply your charge or discharge current of 2 amps to the pack. Connect your meter to the balance port of the battery. If you’re charging, you will see the voltage is lower in the battery (measured from the balance port) than on the chargers digital readout. If your discharging, you’ll see the voltage is lower on the chargers readout than at the balance port. How can this be? Are they are connected to the same thing?
Another test is to move the meter back and forth between the balance port and the partially retracted banana plugs. You’ll get a similar spread in your readings when you do this (while the pack is being charged/discharged). How can it be that essentially at both ends of the same connections you get two different readings?
If you repeat this test with a smaller weight charge cord, you’ll get an even bigger disparity between the charger and voltmeter. Poor quality connectors (like Kyosho/Tamiya/BEC/etc) will add to the voltage difference as well. We’ve performed this test in shop and have seen several volts difference before. What you are seeing is the voltage drop across the wires/connectors/solder joints, etc. between the charger and the battery pack. Now that you’ve run the tests, you’ve seen the voltage in the balance port is not the same as the voltage at the charger. What I’m pointing out here is the simple logic of understanding that the charger can only measure the voltage on its circuit board where the banana sockets are soldered on. It’s not measuring the pack voltage but the voltage as delivered by your connections to its internal circuit board.
Repeating the test at lower charge/discharge rates will show lower differences in voltage readings. Higher discharge rates will produce higher differences.
Higher resistance charge cords and connectors cause all kinds of problems. The higher the resistance, the more of your capacity is being wasted making heat (warming up the wires and connectors) rather than spinning your prop or being measured when you test capacity. The lighter the wire or more worn (or poor quality) the connections between the charger and the battery, the less accurately it can do its job and the less accurate the information it will provide you.
I’ve had instances of customers replacing battery packs with new ones which they tested to be just as bad as the ones they replaced because the charge cord was faulty or too cheaply made. The discarded packs were good when tested with a better quality cord and/or connectors. I’ve even had a customer with a 50cc gas model lose it in a dead-pack crash because he was using a “Quick” charge mode on an “A123” pack. In this mode the charger pumps the pack up to 80% where it’s safe to fly again. He flew and crashed because the resistance between the charger and the charged pack was so high that the charger was reading out a couple volts higher than the pack really was, under the condition of being charged. This condition was aggravated due to the high charge rate and small charge cord size. After the crash the pack was empty yet cycled good.
The point I’m trying to drive home here is don’t assume anything. There are many facets to doing accurate charging and battery testing which are overlooked by most. Certainly this short article will have confused some and enlightened others. One could go on to explain Ohm’s law and use these test measurements to calculate the resistance of your charging harness and thereby infer the error (believe me there is one) in your discharge readings when testing battery packs. But, I don’t think we need to take it that far in order to get the idea across. Are you crazy like a fox yet?
