capacity after flying

[dan c 1]

I have been using 2200mah 3s lipos in the bixler2. After flying for about 35 minutes I put a battery back on to charge. It says it was at 9.9v and the cells read - 3.35 3.30 & 3.40.

Does that sound about right ? Too high/low

[Masher]

No - that's too low, I fly mine down until about 20% capacity remains and expect voltage to be 11.2V min.

Trouble is with a Bixler you don't have power on all the time so it's difficult to judge capacity left - only you can judge how much you've used the motor. Sounds like you enjoyed the flight too much!

[Chris Bott - Moderator]

Dan does your charger tell you how many mAh it puts back in when you charge the batteries back up again?

This is a far better indicator than a voltage measurement.

[dan c 1]

Yes there is the counter going up when it starts charging. Ill look at it next time and see how much it shovels back in and report back.

[Keith Bell]

I agree with Masher that is definitely too low. At that rate you could have a puffed/potentially dangerous lipo on your hands after a few more cycles.

You should be aiming for around 3.75 per cell as the lowest to take it during a discharge cycle. Also the cell voltages you listed are way to far out of balance and really could do with being balanced as soon as you can, so you should also be aiming to keep individual cells within 0.2 of each other so cell readings of 3.75 - 3.75 - 3.77 would be more than adequate.

Do you have a charger that has a balancing function?

[dan c 1]

So perhaps I should fly for about 25 minutes? See what that shows ? I thought Id get longer, maybe not then.

Yes, I have an accucel 6 - I balance charge my batteries every single time. No reason not to.

[Keith Bell]

Yes I'd certainly reduce your flight times and check what you are getting left on your pack. You can then adjust your flight times accordingly to ensure your batt is not over discharged.

With you saying you always balance and yet your cell V's are so far out I'd wager a guess that either you lipo is puffed and/or is on its way out and I would keep a careful eye on it. Good call on balancing every time, not crucial but a really good habit to get into.

[Masher]

Don't forget that flight time is dependent on how much you have the motor on - in the wind we had today, you should just be able to sit there for ages with no power

[J D 8]

My Futaba T8j has a handy function in that you can set the timer to run only when the motor is on,useful with motor gliders.

[Mike Smith]

I also used to use the TX timer function, but then discovered I could monitor the LiPo voltage by telemetry and can even set minimum voltage alarms on the TX (Hitec Aurora 9). I'd definitely recommend using this feature if you have the facility.

[PatMc]

Posted by Masher on 22/02/2014 14:21:49:

Don't forget that flight time is dependent on how much you have the motor on - in the wind we had today, you should just be able to sit there for ages with no power

Ehh

Except for slope soaring, how can the strength of wind increase flight duration ?

[Masher]

W

Edited By Masher on 22/02/2014 17:44:29

[Chris Bott - Moderator]

I generally find that if it's windy, far more power is used from the battery. So flight times should be shortened to look after the battery.

[John Privett]

But as Pat says, unless you're flying on a slope how will the wind increase the amount of time you fly with the power off?

[Masher]

Ok - my mistake, didn't mean to bring the wind into it - please don't run up pages due to my mistake.

All I meant to be reminding Dan was that he needs to keep a mental note of the time he spends gliding (motor low or off) compared the amount of time with powered flight (motor on)

Won't do that again!

[dan c 1]

Posted by Keith Bell on 22/02/2014 13:57:19:

Yes I'd certainly reduce your flight times and check what you are getting left on your pack. You can then adjust your flight times accordingly to ensure your batt is not over discharged.

With you saying you always balance and yet your cell V's are so far out I'd wager a guess that either you lipo is puffed and/or is on its way out and I would keep a careful eye on it. Good call on balancing every time, not crucial but a really good habit to get into.

How far out can the cells be or be allowed to be? They are usually about the same and when I charge them they end up exactly the same.

[John Privett]

No problem, Masher. I set the timer on all my models, electric and ic, to start only when the throttle is opened above a small amount. As well as not recording 'gliding' time, it means the timer also isn't running whilst I'm getting the model ready, plus I can't forget to switch the timer on at the start of the flight - which I often used to do when I had it operated by a switch!

[gliggsy]

price of batts these days best to have a few and swap often, overflying lipos is the second best way to give them a premature death.....g

[Chris Barlow]

I always try to run my batteries to 3.8v/cell as this is the storage voltage and the voltage where the cells are least stressed. I usually measure the current draw at full throttle and do the maths to work out the flight time. I then set the timer on the radio to about 80% of the calculated flight time. This is further adjusted at the field by plugging in a lipo tester after each flight and changing the timer accordingly for an "average" flight.

The Turnigy 9XR has a timer feature based on the % throttle used. At 0% throttle the timer is stopped, at 50% throttle the timer runs at half speed (i.e 1 second on the timer takes 2 seconds) at at 100% the timer runs as normal. This is perfect for something like the Bixler or any other electric powered glider.

[Keith Bell]

Posted by dan c 1 on 22/02/2014 18:15:29:

Posted by Keith Bell on 22/02/2014 13:57:19:

Yes I'd certainly reduce your flight times and check what you are getting left on your pack. You can then adjust your flight times accordingly to ensure your batt is not over discharged.

With you saying you always balance and yet your cell V's are so far out I'd wager a guess that either you lipo is puffed and/or is on its way out and I would keep a careful eye on it. Good call on balancing every time, not crucial but a really good habit to get into.

How far out can the cells be or be allowed to be? They are usually about the same and when I charge them they end up exactly the same.

Technically speaking they should be within 0.2 of each other to be classed as balanced but I wouldn't worry if you're about 0.3. Any more than that then I'd start being concerned about the lipo's health. Initially that's what caught my attention when you posted your cell voltages as 3.30 - 3.35 & 3.40. That to me is alarm bells as they are way out of balance.

Do you have an independent cell checker by any chance? The reason I ask is there is a little slop in the resistors that measure voltages in the accucel 6 and are known to give false readings. I suspect this as I'm currently having issues of the same description with my A6 which is resulting in the charger reading fully balanced after a charge but when I check them with my fluke or cell checker there is a difference between cells. 1 & 3 are within 0.2 which is fine but cell 3 is a whopping 0.6 out so I end up 4.20 - 4.28 - 4.22 but the charger reads them perfectly balanced at 4.2 per cell.

[Chris Bott - Moderator]

Dan I wouldn't worry too much about the 0.1V difference between your top and bottom voltages.

What I would do in future if I were you would be to fly a normal flying style for a fixed length of time, say 10 minutes. Then charge the battery and see how much goes back in. This way, you know how much capacity a 10 minute flight takes. From this you can work out how long it would take to use say 75% of the capacity of the battery, and stick to that as a max flight time.

[John Privett]

Posted by Keith Bell on 22/02/2014 20:23:13:

Technically speaking they should be within 0.2 of each other to be classed as balanced but I wouldn't worry if you're about 0.3. Any more than that then I'd start being concerned about the lipo's health. Initially that's what caught my attention when you posted your cell voltages as 3.30 - 3.35 & 3.40. That to me is alarm bells as they are way out of balance.

Keith, you have a decimal place adrift in your posting. You keep referring to a difference of 0.2v being ok when clearly you mean 0.02v. Otherwise the voltages 3.30, 3.35 and 3.40 lie within a range of 0.1v which falls well within your stated 0.2v yet you say they cause alarm bells to ring!

[Keith Bell]

Apologies on the typo. You are correct I meant 0.02V.

Thanks for pointing it out.

[Rob43]

Hi Dan
I use one of these
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=40907

The alarm sounds at 3.3 v per cell so will let you know if ANY of the cells go low, better than an overall voltage measurment. Remember
that the 3.3 volts will be measured under load and when it sounds is loud enough to be heard from the air, so if I hear it go off I just land and normally the off load voltage reads at a safe level once the model is back on the ground. To me it takes out the guesswork of timing etc. Although the capacity used is the required information the point is you need to know if the cells are nearing critical voltage level, regardless of how much capacity you may have used.
another advantage of fitting one of these is if you loose a model if all else fails as the battery drains the alarm will go off helping you to locate it from an unseen location.

[dan c 1]

I have a timer on the futaba t7cp that I use. In amongst learning to fly and what have you, It seams to be the last thing I remember to do !

So next time I will set it!

I use a phoenix 45 esc - Can I set the lvc to 3.75v on it ? Wouldnt that be good enough? It is difficult to guage like you say when flying this plane how much you are using - Motor on / motor off / gliding etc.