Battery Pack capacity and C ratings and so on

[Malcolm Fisher]

Having dipped my toes into the elctric power flying pool, I decided that it would be likely that eventually I will have to adopt LiXX batteries. I also wanted to be able to monitor my Tx/Rx batteries and have bought a multi pirpose charger.

Having read through many threads trying to find out how best to do this I think I finally understand about charge rates.I am now seeking confirmation as I have several Nickel based batteries which, so far I have only used the supplied chargers which have fixed outputs.

Original Tx/Rx chargers charged at 50 mAh, 1 tenth of their capacity. After much head scratching I now think that it would be OK to charge these at up to 0.5 amps. and one battery of 1100 mAh capacity ought to be OK at up to 1.1 amps. Similarly a 3800 mAh capacity Ni MH I have could be charged at up to 3.8 amps.

I believe that Tim suggests charging at about 0.5C and discharging at about 0.25C i.e. half or a quarter of the theoretical maximum.

I am pretty sure that you have probably answered this many times, but please Tim, have I properly understood?

Malcolm

[Tim Mackey]

Yup, about right OK Malcolm.

Lithium generally @ 1c ( 1 x capacity ) although these days many packs can be fast charged. NiMh also like "fast" charging at around .5 - 1C as slower rates tend to cause build up of nasties on the plates. A good peak detect charger will stop the charge as the battery "peaks" and then falls slightly.

[Malcolm Fisher]

Thanks Tim.

Now I only need to understand how to use my new all singing all dancing charger

Malcolm

[Ross Clarkson]

Hi Tim,

 

As you know, i am soon going to be getting an electric model and 2 x Overlander Sport 2200 3S 11.1v 25C batteries used with an EFL 30A Switch-Mode BEC BL and 15-size 950Kv Brushless motor.

 

I already have an Imax 6 charger.

 

So, as i am new to Lipos.

 

I should charge at no more than 2.2A yeah?

 

I will obviously take a look at my charger and read up on it, because i have only used Nimh's so far, but when do i need to balance them, or is that automatically done with my charger/balancer?

 

Also, can i use my Imax 6 charger at the field along with my 12v battery to charge them?

 

Thanks,

Edited By Ross Clarkson on 15/08/2011 17:32:16

[FunnyFlyer]

Many LiPo packs can be charged at 2C or even a claimed 5C. Personally - I always charge at 1C - just in case. This is a bit tiresome when my 4000 packs have to use 3A, because my ancient PSU trips out at anything over 3A.

 

Ross - in my experience, get as higher value "discharge C" as you can - even if you don't plan on slaughtering it with the claimed current capacity. I find that at 10-20C discharges, a 30 or 40C pack will hold much steadier sustained voltage and give a much higher power output than a lower C pack.

 

Of course, it's all about balancing the economics as well.

[Tim Mackey]

Posted by Ross Clarkson on 15/08/2011 17:18:24:

Hi Tim,

 

As you know, i am soon going to be getting an electric model and 2 x Overlander Sport 2200 3S 11.1v 25C batteries used with an EFL 30A Switch-Mode BEC BL and 15-size 950Kv Brushless motor.

 

I already have an Imax 6 charger.

 

So, as i am new to Lipos.

 

I should charge at no more than 2.2A yeah?

 

I will obviously take a look at my charger and read up on it, because i have only used Nimh's so far, but when do i need to balance them, or is that automatically done with my charger/balancer?

 

Also, can i use my Imax 6 charger at the field along with my 12v battery to charge them?

 

Thanks,

Edited By Ross Clarkson on 15/08/2011 17:32:16

 

1) Yup, unless they state that they are chargeable at more than 1C . personally, I have had no issues at all fast charging at 5C some of the "Tipple" 5C chargeable packs.

2) Balance as often as you like...but at least every 5 cycles or so. If not in rush, best to balance charge every time, as the charger can then monitor each cell for safety during the charge.

3) Yes - but the charger will likely need higher input voltage to work at maximum potential...but basically - yes, thats fine .

[Ross Clarkson]

Thanks Tim,

 

So, if mine said 5C, then theoretically i could charge it 11A??

[Tim Mackey]

Yes mate .

Of course, the limit then is both the maximum wattage of the charger, and the PSU driving it. I have some 4000mah 4s 5c packs. This would require a charger capable of outputting

...heres the maths.....

1) 4s lipo = 4 x 4.2v = 16.8V

2) 4000ma ( charge at 1c) X 5(c) = 20Amps

3) 16.8 X 20 = 336 Watts. Now my best charger currently can output 10A maximum, and is 300 Watt limited, so frankly, 5C is just not possible......yet.

Unfortunately, many so called high power chargers are a bit of a con - they may well claim charge rates of say 10A, but are limited to a total of a measly 50Watts.

[Ross Clarkson]

Cool, thanks mate. Wouldn't have had a clue if it wasn't for all your awesome electric threads.

[Rex Keene]

Thanks for a very informative thread!

I can now see I've been charging at 2C, when I should be charging at 1C. Thanks for that, I hadn't really understood "C" before.

Now, how much charge have I taken out? And how much should I? I understand Lipo's shouldn't be run flat, and that one should land well before the power goes down. In the absence of telemetry, judging available charge can only be reactive. One sets one's Tx timer, lands, charges and sees how much charge has been consumed.

Using a 3 cell 11.1 v 30/70 C battery ( why two "C" ratings?.), how many mah should I see as the total delivered charge current?

At the moment, if I set my timer to Tx timer to 6 minutes, I'm seeing around 850 - 900 mah supplied to full charge, if I fly for 9 minutes I'm seeing around 1400 mah supplied for a full charge. This is doing half throttle figure of 8's on a Foamie Wot4.

I think ( thanks to this thread) I've got my head around the charging side, I'm concerned about the discharge side of things. I'm suspicious that I'm losing flight time by landing early, but I'm wary of draining the battery pack too far and having it " balloon" on me.

thx.

[Steve Hargreaves - Moderator]

Well to answer the easy question first there are two C ratings, "continuous" & "burst".....as you might guess "continuous" means the continuous discharge rate the battery will support. "Burst" means the short duration maximum current.

How far should you discharge your Lipo? Personally I try & not go beyond 85% in the hope that my batteries will reward me with a long life.....so, assuming you have a 2200mAh battery this means putting around 1800-1900mAh back... obviously how much capacity you use depends on how you fly....gentle figure 8s take much less power than flat out "stick banging" so its a bit of a "How long is a piece of string ?" question really.....based on your figures above I think you would get an easy 11, maybe 12 minutes of figure 8's but I'd think you might only get around 6-7 minutes of "stick banging"

Your ESC will have an LVC (low voltage cutoff) that will stop you discharging the battery too much but its not a good idea to fly until the lvc cuts in....it might mean you find the plane low & slow in an awkward position & it also means your batteries will die sooner rather than later....

Hope that helps...

[Rex Keene]

Thanks for a great answer!

That's exactly the info I needed. How was it calculated? I need the same info for my UMX Beast, but that's got tiny little batteries....

[Steve Hargreaves - Moderator]

Rex I used a very technical method known as "guessing"......admittedly it was based of your figures plus a bit of experience but it is just a guess........it really is impossible to say because it depends on your flying style & this can vary depending on how you are feeling & even the weather....you will probably use more throttle if its windy for example.

The Wot 4 E is a very popular model.....loads of owners on this forum.....maybe post up a new thread asking about duration & start from there.

Its a very in-exact science though.....I set my timer for 10 mins on my electric Extra but I still get caught out if my flying has been particularly...er...exuberent....

[Peter Beeney]

Rex - just to ‘add on’ some rubbish to Steve’s answer, there are some simple calculations you can apply to get some very approximate ideas on flying times and currents used, etc.

We could say that the basic Flying Time, FT, is equal to the Battery Capacity, BC, divided by the Motor Current, MC. So FT = BC/MC. Or you can say BC = FT by MC, or MC = BC/FT, always giving you one value in terms of the other two.

The trick is to always operate in terms of the same unit, battery capacities are invariably given in milliamp hours, whilst flying times are usually quoted in minutes, so it always needs some adjustment. An example might be that we have a battery of 2,000 mAh and a motor current of 2 amperes. So we can either call our battery 2 ampere hours or our motor current 2,000 mA’s, whichever we do, the answer will be 1, that’s 1 hour, multiplying by 60 it becomes 60 minutes. A more realistic motor current of 20 amps then becomes 2 Ah/20 = 0.1 hour, then multiply by 60 = 6 minutes, or one tenth of an hour.

In your case of flying for 9 minutes on 1,400 mAh, the 1,400 mAh will be the battery capacity in this case, then ft = bc/mc. We want to know the motor current, so mc = bc/ft, or mc = 1,400 mAh/9 minutes. Standardising the units, then we have 1.4 Ah by 60 = 84 ampere minutes. Then 84/9 = 9.33 A. So your average motor current is 9.33 amps. To confirm, you fly for 6 minutes on 900 mAh, or 0.9 Ah, therefore 0.9 Ah by 60 = 54. Then 54/6 = 9 amps.
If your battery is indeed a 2,200 mAh pack then 2.2 by 60 = 132 Am divided 9 = 14.6, so in theory you could fly for 14 minutes and 40 seconds.

It doesn’t matter how obtuse the figures get, that makes no difference to a calculator. And as Steve says, there will always be some variation due to throttle use etc. But at least if you have some of the figures you might be able to get some idea of how much current you need or how long the battery will last.

Hope this may be of some interest…

PB

[Heaton Craggs]

I bought an Ultramat 16 charger about three years ago. Shortly after purchase its internal PSU 'packed a terminal sad' - maybe due to a mains supply power surge.

If I remember rightly, the Graupner replacement was prohibitively expensive, but an 'electo-guru' pal suggested that a Maplin's equivalent PSU would be fine to use. I bought one at a fraction of the cost of the Graupner replacement item and, now, two and a bit years later my Ultramat 16 is still percolating 'like a bought one'!

However, just recently, when I'm charging NiCads/NiMh's, at varying times I am getting a 'contact error' message. This occurs at any time of the charging cycle, yet when I press the red button to activate the charger, the charging cycle resumes, sometimes until the 'full charge' buzzer sounds.

Could this be a problem with either of the banana leads to the battery connector socket? How can I test if it is?

Would be much obliged for advice!

[Steve Hargreaves - Moderator]

Difficult to know Heaton.....do you have a multimeter you could put across the lead & flex it to look for a break? A similar thing happened to me recently & I thought my charger was on the blink but it turned out to be a dodgy connection within the lead somehere......I cut the lead back & fitted new banana plugs & all was well....

[Heaton Craggs]

Thanks for that advice, Steve - I'll give it a go!

Does anyone have advice re. the cheapest but reliable way of converting my Graupner MC 19 to 2.4Ghz, please?

Also, I need to replace the battery. The 3300NiMh replacement is over fifty quid - I don't want to spend that if there's a Lipo or Fe suitable - any advice most welcome!

[riverlandgirl.]

Great thread, learning heaps! I'm just starting to get into 'lecky, being a died in the wool ic flier, a lot more to get my head around! Any way I now have a little rtf "firebird" lots of fun. I also have an old speed 400 motor and esc from a little flutter years back, 6 mintue flights on a 7 cell nicad brick, as aposed to the firebirds 30 minutes plus! My question is, is there any reason why I can't use the 2 cell 1300 mah li po from the rtf with the speed 400 and esc, to get another model flying?

Thanks.

[Tim Mackey]

Not really, other than the ESC will not activate the LVC correctly as the voltages are higher, and the motor will likely die a little sooner than it would have done on a diet of nickel.

Just be sure to land as soon as you notice a slight drop off in revs.

[riverlandgirl.]

Thanks Tim, shall give it a try...

[Bruce Austin]

Hi guys

Maybe off topic, and please redirect this newbie query.

With all this talk about batteries in parallel, i have the following questions

1 Where can i find out about battery wiring sizes needed for making up Y connections for two batteries into one controller? (I already have the needed XT 60 connectors.)

2 Where can I buy say 1 meter each of red and black silicone wire?

Go With the Flow

Bruce

[TinPot Pilot]

Can't help with wire sizes, although I would suggest you use the same gauge as the existing battery wires. Wire is available by the meter here

[Peter Hill]

I have recently purchased a 500w charger and have been thinking of trying parallel charging of lipo's. There is lots of info on the internet regarding the subject.....most of which I understand! However,I am struggling when they say you can parallel charge packs of different capacities. For example; if I charge 2@ 3s 2300mah packs and 2@ 3s 1300mah packs together, I add the capacities together and set the charge rate at 7.2amps. So does this mean each pack gets charged at 1.8amps or am I missing something here? I normally charge packs at 1C so in this example the 2300mah packs charge at less than 1C and the 1300mah greater than 1C. I only have a basic understanding of electricity so if anyone would like to comment on the above I would be gratefull!

Pete

[Biggles' Elder Brother - Moderator]

Peter,

you might want to look that this thread - or this one.

BEB

[Peter Hill]

Thankyou for the quick reply and links BEB!

Pete