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Cut-off voltage

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PB31/08/2012 07:57:51
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I'm not sure if this should be in the battery forum or the ESC forum, but here goes.....

I've just completed a model which uses two Multiplex BL-20 SD brushless speed controllers. I was surprised to see that in the programming guide the battery cut-off voltage had three programmable settings which for Lipo batteries were: LOW = 2.6v, MEDIUM = 2.85v(default) and HIGH - 3.1v.

Now I'm pretty new to this, but I thought that Lipo batteries weren't supposed to be discharged below 3v per cell, but I've left the programme on the default setting nonetheless.

I should be grateful if someone could explain these three settings - am I missing something?

Thanks

Edited By Morvargh on 31/08/2012 08:21:15

Greybeard31/08/2012 08:11:23
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Could it be something to do with on-load and off-load voltages?

Mark Powell 231/08/2012 08:49:02
430 forum posts

That's just Multiplex being helpful to justify its 'higher than most' prices. Use the default, that's the real one. They do know what's best, they make batteries too.The cut off means it is not on load anymore. That's the point of it.

Pete B - Moderator31/08/2012 09:56:04
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Not having ever acquired letters behind my name, I can only answer in layman's terms, Morvagh.smile

As the charge in the battery pack depletes during flight, its ability to sustain a constant voltage reduces and it 'dips' as you open the throttle. Once you reach, say, half charge, at 3.8V/cell, any prolonged wide open throttle may bring the voltage down below the cut-off point that has been set.

At that point, the ESC will detect the low voltage and activate the cut-off, to protect the battery (not the model!). If you then close the throttle, the off-load voltage should be restored and the ESC should re-activate, which will hopefully give you enough power to recover the model for a landing. Don't rely on it, though.........

Personally, I always set my cut-off default to 'High' - 3.0 or 3.1V, depending on ESC, and limit the flight time to ensure that I don't reach LVC cut-off. This avoids caning the battery too much.........thumbs up

Pete

Mark Powell 231/08/2012 10:44:36
430 forum posts

A variable cut off has its uses, as Pete B says. Good on powered gliders. (Multiplex are keen on them). You can stay up for ages, even give the occasional spurt of power, after the cutoff has happened once, if you have a 'high' setting. Done it often myself.

John Cole31/08/2012 11:00:39
615 forum posts
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Discharge of LiPos below about 3.0 V / cell is not that good for them, but the real damage comes if you take them down to really low voltages. At 1.0 V / cell and below they suffer internal corrosion that can lead to an internal short circuit, destroying the cell completely. Laptop-type power packs contain protective circuitry to prevent overdischarge of thei type.

PB31/08/2012 12:28:39
155 forum posts
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OK thanks for the useful and informative comments. I wasn't going to mention this, because I had the feeling that it was Multiplex covering their 6 o-clock, but the very last paragraph on the 5 page documentation for these ESCs says:

Quote: " Note: If you are using the latest generation of LiPo batteries, the low voltage cut off function will not work due to the cells' constantly high voltage curve, in this case the speed controller is unable to cut the motor in good time. In these circumstances you should fly using a current sensor or timer to avoid discharging the drive battery to a dangerous level" unquote

What is "...the latest generation of LiPo batteries..."? I'm using RC King 3S/3000/35C batteries so does this statement apply do you think?

Myron Beaumont31/08/2012 12:51:46
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As a dyed in the wool ic man for years ,I decided a while ago to really get into electrics .This Lipo business is all very confusing having read so many do'& don'ts,scare stories,conflicting opinions etc.& the more I read then the more unanswered definitive questions come to mind .I really am trying to get to grips with the technology & fully understand Ohm's Law etc.(TV repairer sometime ago) However it is conflicting advice that gets to me from the various forae(ums?) I have spent the last few days perusing .For example,having bought a new balance charger (GT 606D) I find conflicting advice on storage voltages .One source of info says store at around 30% charge & another (the battery instructions) says 80%.Others have said they charge fully & leave them ready for the next time they fly ,whenever with no noticeable detriment to the battery .Another source of confusion is over the best charging rate -1C normally or better still 1/10C overnight so as not to overheat anything & reduce battery & charger lifespan.In short,just what are the proven facts .If we all knew them then there would be a lot less controversy on our and other forae(ums?)

Pete B - Moderator31/08/2012 12:52:25
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Hmm, not sure.....what they are suggesting is that the voltage remains high until it drops off the cliff, rather like A123 cells. However, even if this happens, there will still be plenty of volts for the Rx, so, apart perhaps from the problem of there being nothing left to limp back with, I'm not convinced that this is any different from before - perhaps they are just covering their back.

The only bit of that comment I'd go along with is the use of a timer. When you first fly a pack, set the timer conservatively and note how many mA you put back in on re-charging. That will give you an idea of the safe duration - I make mine about 80% of the pack capacity and do not take liberties after the time has said "That's enough!"......smile

Pete

Pete B - Moderator31/08/2012 13:10:46
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Posted by Myron Beaumont on 31/08/2012 12:51:46:

.For example,having bought a new balance charger (GT 606D) I find conflicting advice on storage voltages .One source of info says store at around 30% charge & another (the battery instructions) says 80%.Others have said they charge fully & leave them ready for the next time they fly ,whenever with no noticeable detriment to the battery .Another source of confusion is over the best charging rate -1C normally or better still 1/10C overnight so as not to overheat anything & reduce battery & charger lifespan.

I work to the KISS principle, Myron.

Storage charge is 3-7 to 3.8V/cell. I wouldn't know what 30% or 80% is anyway but it doesn't sound right........

Storing them fully charged might be alright for a day or two but any longer and I'd drop them back until I need them.

I've never, ever, heard of charging Li-Po's at 1/10C - don't know where that's come from and my simple advice is - DON'T DO IT!!! Are you confusing this with NiMh or NiCd batteries?

If you stick to 1C, even with packs that are rated at 5C charge rate, you won't go far wrong......

Pete

PB31/08/2012 14:15:24
155 forum posts
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Setting a timer for the flight and then setting the post flight charge current and monitoring the time to full charge is a great idea Pete, thanks I'll definitley do that. I think what you're suggesting is to fly for, say, 3 minutes on my fully charged 3000mAh battery then set the charge current to 3 amps and if it takes, say, 20 minutes to get back to full charge then the 3 minute flight used 33% of its capacity. Definitely like that - makes perfect sense - thanks again.

Edited By Morvargh on 31/08/2012 14:18:44

Pete B - Moderator31/08/2012 14:30:38
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Sounds as if you have a Twinstar if I'm not wrong, Morvagh. I have no experience of them but I would suspect a 3000mAh pack will give you a fair bit more than 3 minutes. I'm sure there must be a user out there familiar with the average endurance of the model.........

You'd probably be safe for about 6 or 7 mins initially but that depends on the wattage your motors are producing and how much full throttle you use!

Your maths sound right to me. If your charger shows the number of mA put back in, as mine and many others do, that will give you the best information. If you do not have that facility but have a Wattmeter, you can connect that inline with the charger and that will record the mA.

Pete

PB31/08/2012 14:41:44
155 forum posts
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Yes it's a Twinstar II - the three pics. Maiden flight tomorrow under instruction so no heroics, but it will be good to time it as you suggest and do the arithmetic afterwards to see what I can get out of the 3000 packs with two motors in parallel. Your 80% rule sounds pretty good to me too.

Wish me luck !!!!

Pete B - Moderator31/08/2012 14:59:07
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Good luck indeed, Morvagh. Should be fun!thumbs upteeth 2

Pete

PB31/08/2012 16:35:13
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You know you really shouldn't give an engineer these little tricks - it drives them mad until they've worked out the general rule !!! In this case it's..........

M x (CC/BC) x 1.7 = % battery used

where M = minutes to reach full charge, CC = charge current in amps, BC = battery capacity in amps

(Ok I need to get a life........but I'll settle for bringing my Twinstar home in one piece tomorrow ..frown)

John Cole31/08/2012 16:43:46
615 forum posts
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Myron: if you want hard, detailed info on LiPos then read this manual from Sony, who make them. The manual covers LiPos and 2 other types of Lithium Ion cell. Note: this is not the easy-reading version!

Manual

Storage temperature is rather more important than state-of-charge. Store them COLD.

PB01/09/2012 14:45:35
155 forum posts
17 photos

OK, just to finish this thread off, the Twinstar had a successful maiden flight this morning in the capable hands of a very experienced flyer, who put it through some great manoeuvres, loops, spins, stalls and was very impressed with it ..."for an electric foamy..." He handed me the tx and I flew it for about 30 seconds. The windsock was horizontal and it was very blustery so I lost confidence on the downwind leg and very quickly handed the tx back!!

Bottom line is a 4 minute 15 second flight used fractionally under 50% of a freshly charged 3000mAh battery, so you were just about spot on Pete, 80% of the battery charge should give me around 7 minutes. I'm really looking forward to a calmer day now.

Thanks all for your help and advice, and thanks for the link to the LiPo manual John. As you say, not an easy read but very very informative, especially the graphs.

Pete B - Moderator01/09/2012 17:43:04
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That's great news, Morvagh - watching your new model take to the air is very satisfying whether you are flying it or not.

It doesn't matter how many times you've done it - it always gives you a buzz! thumbs upteeth 2

Pete

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