Originally Posted by kcaldwel

We push these little motor LiPos pretty hard, 50C+. They sometimes die.

Kevin

As an example of that, two F5J batteries driven beyond their actual (not rated) capacity in the last month.

I see photos like that and my first thought is how they were cared for. Proper discharge to storage voltage is the single nicest thing you can do for your packs. Then being nice to them during charge, I rarely exceed 1C. Then pay attention to the IR when recharging. Packs usually start to leave some hints and rarely spontaneously fail without warning.

Lastly, packs are cheap. Replace them when you even think they may be an issue. For example, there is no such thing as a "little" puffed. Puffed is failed. Don't fly it. SPend the $15 and get a new one.

Originally Posted by wdwimbish

If this is true, why do so many fliers look to a backup system in case the bec fails?

Two reasons not to take 2S power directly from the battery. a) creating unnecessary imbalance in the cells, b) wire gauge and connector pins in balance lead not sufficient to supply required power for a six servo HV setup

Thanks,
Wayne

Have you actually tried this? I doubt that you have. I've used the balance tab for 7.4 volts for receiver power for 2 years. The only problem I've had is a brain fart when I got distracted and ran the battery down resulting in a crash. Carefully monitoring and it is not a problem. Regarding the imbalance of the cells, not a problem. 6 HV servos are not drawing enough to create any significant imbalance. The next charge balances them again. I haven't noticed more than a tenth of a volt difference.

Not sure about your comments on the wire gauge. How much gauge do you think you need???? Sources to back this up?? The wires in the balance leads are 22 gauge. The servo wires are 24 gauge. The balance connectors I make for the receiver are 22 gauge. More than adequate.

As a matter of perspective. The wiring harnesses in the Van's RV7 I just completed are mostly 20-22 gauge wire. Many of these are carrying up to 10 amps. The only 18 gauge wires in the plane are for the heated Pitot tube and it only draws about 10 amps.

Modelers seriously overthink the wiring in these planes. I usually don't use wire any greater than the gauge of the servo wires. No need to in our sailplanes.

Darwin N. Barrie
Chandler, AZ
Team Futaba

Hi

I've also noticed harsh overestimations of power needed in our toys. "I want to be sure that I have the authority available when needed, maybe 10 amps is needed". Where? I once tested the max load scenario with strong digital wing servos, jamming 4 of them completely in a vise. The max current was something like 1.5... 2.5 amps total in a completely bizarre scenario. A full 15 min flight (like fly off) takes about 200 mAh from battery after the motor run if your setup is even remotely ok, and even you use the flaps all the time for some strange reason.

About 2S-tapping. I use such setup in my light Q, battery being a 4S450. If I take a 30 s full throttle start, eg to dash 500 m upwind, the cell voltage (monitored via telemetry) drops to 3.5..3.6 volts during the motor run and returns to 3.8 ... 3.9 volts when motor is stopped. The rest of the flights only 2 cells are consumed, voltage of those being 0.1 - 0.15 volts lower than the 2 other cells after the flight. Nothing whatever decent charger is not able to balance immediately. The imbalance means nothing from the battery's point of view. The balance wires are thick enough, but as the balance connectors can be quite crappy I have soldered a paraller micro deans connector for the supply.

Due to my experience the reliability (the actual topic of this thread) is made available by following means:

1. Minimize the amount of connectors and use good quality connectors only. Powering the whole glider via AMRT is a way to look for trouble, so use the by pass connection and mitigate that one. Why should there be 2 extra servo connectors and something you move back and forth between every flight in the most critical supply line?
2. Learn and use std procedures. Always check the battery before flying, and do not fly more than a normal competitition task or amount of low power starts comparable to that. Keep a lipo checker in your pocket and change the battery before the cell voltage (without load) drops below 3.8 volts. Like empty guns, empty lipos are dangerous.
3. Look what the lipo-charger tells you about the batteries. It is very difficult to imagine that if a battery is not puffed, cell IR-values are ok and coherent, and all the cells get balanced there could be anything causing sudden problems during flight.
4. Inspect the wiring and the connectors in the beginning of the season, just go through the wires and replace if there is fatigue. Fuse mounted ballast tends to easily damage wires in some setups.
5. Batteries are cheap, buy fresh ones for each season. In our use after one season the "spark" is gone, although no puffing exists. Test the new battery's ability to give full power for 30 seconds in ground at least once before starting to use it.

Need for feeling of reliability is of course very personal thing, but like Tuomo wrote, there are lots of more relevant risks than some component failure in our (ab)use of our gliders. We get the "reliable enough" feeling by keeping everything very, very simple and clear and using components of decent quality.

Aaro

Originally Posted by dnbarrie11

Have you actually tried this? I doubt that you have. I've used the balance tab for 7.4 volts for receiver power for 2 years. The only problem I've had is a brain fart when I got distracted and ran the battery down resulting in a crash. Carefully monitoring and it is not a problem. Regarding the imbalance of the cells, not a problem. 6 HV servos are not drawing enough to create any significant imbalance. The next charge balances them again. I haven't noticed more than a tenth of a volt difference.

Not sure about your comments on the wire gauge. How much gauge do you think you need???? Sources to back this up?? The wires in the balance leads are 22 gauge. The servo wires are 24 gauge. The balance connectors I make for the receiver are 22 gauge. More than adequate.

As a matter of perspective. The wiring harnesses in the Van's RV7 I just completed are mostly 20-22 gauge wire. Many of these are carrying up to 10 amps. The only 18 gauge wires in the plane are for the heated Pitot tube and it only draws about 10 amps.

Modelers seriously overthink the wiring in these planes. I usually don't use wire any greater than the gauge of the servo wires. No need to in our sailplanes.

Darwin N. Barrie
Chandler, AZ
Team Futaba

I did not use the power backup system because of the weight.
I think it is very necessary to choose a highly reliable BEC brand. Then full ground testing and maintenance of the battery. Check every plug & connector when I come home.
Set telemetry and warning of the main battery, It can tell me the remaining power of the battery in real time to avoid a cell-level damage or over-discharge of the battery. Most time I only discharge to 3.8 each cell and replace it.

Originally Posted by wdwimbish

If this is true, why do so many fliers look to a backup system in case the bec fails?

Being very familiar with European F5J scene, I do not know having a back up battery in F5J. Not a single pilot. It is purely something that is discussed here in rcgroups. It would be interesting to know it is often done in US? In Europe everybody is flying with bec. Most recently there has been some experiments running rx & servos direct from 2 cells in drive lipo. That also seems to work perfectly despite some theoretical challenges (locally to me, take a look what Aaro did with his Explorer Q).

Originally Posted by Tuomo

Being very familiar with European F5J scene, I do not know having a back up battery in F5J. Not a single pilot. It is purely something that is discussed here in rcgroups. It would be interesting to know it is often done in US? In Europe everybody is flying with bec. Most recently there has been some experiments running rx & servos direct from 2 cells in drive lipo. That also seems to work perfectly despite some theoretical challenges (locally to me, take a look what Aaro did with his Explorer Q).

"direct from 2 cells in drive lipo" that's make your battery unbalance. That's very dangerous, it will make some cell of your battery discharge faster than others. for example, when you full power the 3or4 cell drop to difference voltage. when BEC check the voltage is too low, it may alarm to stop the power or even restart.

Not really. Read what I wrote a little bit earlier. And there is no BEC in such setup, which is actually the very point of such setup, so there is nothing to cut off.
Aaro

Originally Posted by dtx66

"direct from 2 cells in drive lipo" that's make your battery unbalance. That's very dangerous, it will make some cell of your battery discharge faster than others. for example, when you full power the 3or4 cell drop to difference voltage. when BEC check the voltage is too low, it may alarm to stop the power or even restart.

Has anyone experienced an ESC failure due to braking?
I have read that the rapid braking function can blow an ESC, but have not seen it happen in the flying outings that I have attended.

Originally Posted by Silent-AV8R

I see photos like that and my first thought is how they were cared for. Proper discharge to storage voltage is the single nicest thing you can do for your packs. Then being nice to them during charge, I rarely exceed 1C. Then pay attention to the IR when recharging. Packs usually start to leave some hints and rarely spontaneously fail without warning.

Lastly, packs are cheap. Replace them when you even think they may be an issue. For example, there is no such thing as a "little" puffed. Puffed is failed. Don't fly it. SPend the $15 and get a new one.

These packs were damaged as a result of being discharged at a rate beyond their capacity. Nothing more than that. This happened as a combination of a manufacturing error in the motor being used and the packs being advertised as having a C rating way beyond their real capacity.

Every pack I have ever owned has been charged at 1C and put on storage as soon as I get home from a session. Then into the fridge.

I'll explain how this happened. I have a F5J aircraft with a given motor and prop combo. I used two of the 4S Nanotech packs in it from the beginning of the year in multiple contests without any issue. Prior to selecting the battery/prop combo I ran extensive bench tests. The final arrangement got my aircraft to 215 metres in 30 seconds reliably.

I then modified a F3J model to F5J creating a pretty much identical aircraft to the first, same airframe and weight, so I decided to use the same motor and prop combo in that. Having done the tests I didn't repeat them with the new motor and prop (a mistake on my part as it turned out). I flew one test flight using one of the Nanotechs without issue except that I noticed on landing that the battery pack was warmer. Near the end of the motor run for a second flight (with another pack) the motor cut out and I had no control from that point. The aircraft performed multiple successive loops until hitting the ground. The pack had failed (see photo) and when I examined the second pack, that had failed in the same way.

The original aircraft current draw was a momentary 52a followed by a pretty constant 45a for the balance of the 30 sec.

When I tested the second motor on the bench it drew a constant 60a (limited by the ESC). I had been unable to get the Nanotech pack as HK was out of stock so I had bought several Dualsky 4S packs rated at 120C. I cycled one of these packs at low discharge 3 times then ran the motor for 30 sec. The pack got warm but not excessively so. After it had cooled and been recharged I ran it again whereupon after a few seconds lots of smoke appeared and after a few seconds it caught fire.

My point in mentioning this was not to encourage a discussion about LIPO care but as an example of what can happen in normal use. Weight, particularly forward of the CG is a problem in both these aircraft and despite my hatred of the idea both aircraft already have lead in the fin to get the CG where it needs to be. I didn't have a backup battery in either aircraft for that reason, but should have.

Originally Posted by mpjf01

............................... . Then into the fridge.............................

Good idea; I am going to adopt that practice over the summer, at least.
Thanks for mentioning it.

Jim.

Originally Posted by Aaro

The balance wires are thick enough, but as the balance connectors can be quite crappy I have soldered a paraller micro deans connector for the supply.

Aaro, are you taking the pack apart to solder the micro deans directly to the cells or are you stripping insulation off 2 of the balance wires and soldering the micro deans to that, or are you cutting 2 of the balance wires and then soldering in the micro deans and the wires you cut or?

Ryan

Originally Posted by mpjf01

These packs were damaged as a result of being discharged at a rate beyond their capacity. Nothing more than that. This happened as a combination of a manufacturing error in the motor being used and the packs being advertised as having a C rating way beyond their real capacity.

Fair enough. My comments still apply in general to the care and feeding of lipo cells. Although I have seen many references saying not to store packs in the fridge. But YMMV there.

I'll suggest that normal use does not include using a motor/prop combination beyond the capability of the pack to deliver voltage. Normal use to me is using the packs within the specified operation range for that combination. Blowing the pack up like demonstrates that normal operational parameters have been exceeded.

As far as C ratings go, there are several sources around that have tested numerous packs to determine their "true" C rating. Let's just say that the results indicate that for most packs the rating, especially high ratings, are overly optimistic. Bottom line of that testing was that for most packs the true rating is 30C to 40C at best. For my personal use I treat all packs like they are 30C.

To get back on subject, the closer to the edge you run your system the more important redundant power supplies become.

Originally Posted by Dave80H

Has anyone experienced an ESC failure due to braking?
I have read that the rapid braking function can blow an ESC, but have not seen it happen in the flying outings that I have attended.

Not seen or heard of it myself. I use a medium brake and avoid hard braking because it is simply not needed.

No need to take apart the battery. Cut - strip -solder - isolate (shrink tube), as it happened to ..happen, without any extra thinking or planning.
Aaro

Originally Posted by rdwoebke

Aaro, are you taking the pack apart to solder the micro deans directly to the cells or are you stripping insulation off 2 of the balance wires and soldering the micro deans to that, or are you cutting 2 of the balance wires and then soldering in the micro deans and the wires you cut or?

Ryan