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Baldyslapnut

Max Amps from a Battery

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Baldyslapnut

We have had a reduction in the minimum weight of the flight pack for F5B this year.

So I want to ask how do you work out what is the maximum number of Amps a pack will produce.

So currently we use circa 10s or 9s 1,800mah packs. These give pack weights of about 450g.

We can now go down to 400g. There are a number of good 1,350 packs out there. However, are they powerful enough.

Any suggestions on how to calculate max amps released would be gratefully received.

Regards

 

Greg

 

 

 

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paulj

Wouldn't this be the C rating of the pack? For example, a 1300 mAh pack with a 40C rating should be able to deliver 40 * 1.3 A = 52 Amps? Looks like you need to consider the nominal discharge rate and the burst discharge rate:

http://multicopter.forestblue.nl/lipo_need_calculator.html

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wookman

"The maximum recommended continuous discharge current of a Lithium Polymer battery is the Battery Capacity (mAhr) x the 'C' Rating, i.e. a 1000mAhr pack with a 25C rating would have a maximum continuous discharge rating of 1000 x 25 = 25,000ma (25Amps). The 'C' rating is a reflection of the internal resistance of the battery and its ability to satisfy high current demands. Unfortunately not all batteries live up to the 'C' rating on the label."

This is lifted from Stan Yeo's Phoenix Model Products website, full article here. https://www.phoenixmp.com/articles/cratingandtimetocharge.htm

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mikef

I would be inclined to take any battery label claims with a pinch of salt.  I think you need to test examples to see what they give in practice, remembering that the more you abuse the pack, the more often you'll be buying a new one.  Don't forget that the voltage will drop when you pull a high current, this may cause other problems.

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wookman

Absolutely Mike, lots of the C rate claims are quite optimistic and there is definitely a trade off in longevity of the cells if you operate up towards the limit.

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Baldyslapnut

Thank you for the advice.

I have some new packs to try. So it will be interesting to see if the 70c 1,800 pack we are using currently is better than the smaller 100c 1,350 pack I will be testing.

Regards

 

Greg

 

 

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Feefo
On 18/05/2019 at 11:39, Baldyslapnut said:

We have had a reduction in the minimum weight of the flight pack for F5B this year.

So I want to ask how do you work out what is the maximum number of Amps a pack will produce.

So currently we use circa 10s or 9s 1,800mah packs. These give pack weights of about 450g.

We can now go down to 400g. There are a number of good 1,350 packs out there. However, are they powerful enough.

Any suggestions on how to calculate max amps released would be gratefully received.

Regards

Greg

Maybe asking Alan Flockhart or JJmouris about this would be an idea. As far as I know C rating doesn't really apply, F5B is burst flying and not continuous discharge from your packs. I remember a few years ago the Turnigy 30c packs were used as they performed very well due to lower internal resistance than some of the higher C rated packs, I use the  4s 2200 30c versions in my X Plane, motor is a 16-35-1 on a KPG 5.2 and a 15x16 RFM with a reflashed HK 200a esc that JJ used to do, amps are around 120-130a  on 4s so I'm well over the 'recommended' burst rating and the packs perform well. Occasionally I'll run 5s on same prop using an old 1800mah 40c and amps are closer to 150a, shorter motor run time of around 3-4s, but my 5s packs are past their best tbh

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Baldyslapnut

Sounds like a good set up. Thank you for the advice. 

Happy landings 

 

Greg

 

 

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Graham Woods

Hi Greg,

I think the recognised way is measure the Internal Resistance of the individual cells; the lower the better. This is determined by the chemistry and manufacture of the cells. Some of the better chargers already do this measurement and I see Hobby King sells an ESR meter which will do this for you. As I say it needs to be low as possible, in your case less than 5 milli-ohms per cell I should imagine. Ideally, all the cells should be matched. The resistance of the whole system (cables, connectors, esc) is as important and should be as low as possible to get the most current flowing through the motor with the least amount of current drop across interfaces, so short, thick wires and gold plated plugs and sockets are in order as one would expect.

David and I were talking about this a few weeks ago and came to the conclusion that the one obvious weak spot with LiPo batteries is the use of the white,  rubbish JST-XH balance plugs and sockets used by many LiPo batteries. Sometimes just waggling a plug in its socket can result in a different internal resistance showing on the charger. While this may not be detrimental to club fliers, for competition work it seems much more important. The reason being that during the charging and balancing the charger my 'see' a different voltage and either put more or less charge on that particular cell or even trip out the whole pack before it being properly charged. Our simple solution would perhaps be to replace said JST-XH plugs with a soldered gold plated MPX type, not the cheaper versions available.

Never overcharging or discharging is good practice too and I read warmed batteries (20-40ºC), pre-charge, may get a bit more capacity into the cells.

ESR Meter:

https://hobbyking.com/en_us/wayne-giles-designed-universal-esr-meter-for-100-10000mah-1-6-cell-lipo.html?countrycode=GB&gclid=EAIaIQobChMI146blN3r4wIVBLTtCh3MGATVEAQYAiABEgJhgvD_BwE&gclsrc=aw.ds&___store=en_us

Gold plated MPX plugs:

https://www.ebay.co.uk/itm/253817716808

 

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Baldyslapnut

Hi Graham,

Good feedback and thank you.

I have an iCharger Duo and measure the IR on every charge for each cell. I also try to charge at a similar temperature each time. I am using mylipo.de packs in F5B and they suggest 45 degrees C as the optimum operating temperature.

The reason for the question revolved around getting packs that fitted my current competition plane and enough capacity to give me the burst amps I needed plus also having low IR at high temperatures.

As you know when you fly in a competitions in mainland Europe the  ambient temperature can be much higher than the UK.

Luckily I have found a pack which does the job.

Hope to see you soon on the slope.

Regards

 

Greg

 

 

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