Checking A123 RX Packs For Recharge Point

Radical RC A123 2300 2S RX Pack Example
Radical RC A123 2300 2S RX Pack Example

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
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.

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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

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