Quick Question about C Rating of LiPos

[Chris King 3]

I hadn't really done the maths with my LiPos. I worked out that C Rating is maximum disharge rate and therefore bought the highest that seemed reasonable to buy.

Now I'm struggling to get the same so I've done the maths and I think (but want to check) that I can use something with a much lower rating.

I tested the brushless/30A ESC/Prop combination and was drawing 27A. The previous LiPos were 2200 3S 60Cs. Given the 27A max current draw I wouldnt actually need more than a 20C.

Is that correct or am I missing something?

I do understand that it's worth getting higher ratings for future models but I dont think I'll ever need the 60C.

[Brian Dorricott 1]

Your math looks ok there Chris 2200 20c good for 44amps but you might want to increase the ESC rating , most people have a bit more headroom in case of heat or Chinese slant on specs maybe a 40 amp ESC ? 60c seems a bit much but downright lies are often told on spec sheets .

[Richard Wills 2]

C ratings are mostly very exaggerated, to the point of being complete lies. I once had a small foam pylon racer with a brand new 60c 2200 3s which going of the c rating should have given 132A. Voltage crashed at 65A. Other than a guide to a higher C rating being better than a lower one the numbers mean very little. A 30-40c pack I find is fine in most models.

Edited By Richard Wills 2 on 14/12/2020 17:12:56

[buster prop]

It's a good idea to choose batteries with a C rating well above the most you will need. This so that the voltage doesn't drop too much under load which will give less power; Watts = Amps x Volts. 60C is probably a bit excessive if you are only drawing 27 Amps. A 2.2 battery x 60 is 132 Amps, about 5 times over what you need! I generally go for a C rating at least twice as much as the maximum current draw. So for your 2.2 battery, 30c will give a theoretical maximum of 66Amps. Plenty. Don't push your batteries too hard, they will get hot and it will shorten their life as well.

[Chris King 3]

Posted by Brian Dorricott 1 on 14/12/2020 17:06:24:

Your math looks ok there Chris 2200 20c good for 44amps but you might want to increase the ESC rating , most people have a bit more headroom in case of heat or Chinese slant on specs maybe a 40 amp ESC ? 60c seems a bit much but downright lies are often told on spec sheets .

Thanks for all the responses. I know I'm a little close to the ESC max but it's actually a modified Lazy Bee and it's going vertically upwards when flat out so it doesnt happen often!

[Martin Harris - Moderator]

I've never confirmed it personally but perceived wisdom is that ESCs work harder at part throttle as there's more switching involved, meaning that specifying an ESC should reflect the potential full throttle demands rather than simply assuming that you won't use full performance much or even at all.

[Geoff S]

That's largely true, Martin, but full throttle doesn't necessarily mean maximum current for the esc rating. Full throttle (which is probably the most efficient state) can mean a much lower current than the esc's maximum if the system is setup suitably eg with a smaller prop or a higher voltage LiPo (4S rather than 3S, say)

In any case a higher rating esc is physically bigger and able to dissipate heat better. So, all other things being equal (ie space, weight and cost) I always choose the biggest I can accommodate.

Geoff

[Martin Harris - Moderator]

Yes - perhaps I should have been a bit more specific. It's a shame they don't give them a wattage rating but it does remind me that we also need to consider cooling air flow as part of the selection process.

This was graphically illustrated a few years ago when I was test flying a friend's model and it went dead stick (yes, it can happen with electric!) and on touching down, I lost all communication with the model. After removing the hatch, we were greeted by the sight of the battery wires hanging loose after unsoldering themselves from the rather hot speed controller! As it was operating from the inbuilt uBEC, my friend was very lucky not to lose the model as well as the ESC.

A new ESC and some improved cooling was the answer.

Edited By Martin Harris on 14/12/2020 20:09:37

[Geoff S]

That must have been quite on the warmish side! When oven testing a circuit (a telenetry transmitter IIRC) many, many yeas ago it stopped working and the wires had all fallen off due to the solder's melting ... and it was only supposed to be at 125 deg C ie mil spec max at the time.

Certainly LiPo C ratings are rarely accurate. I have a Wayne Giles ESR (effective series resistance) meter which also indicates C rating of LiPos and I've never had one I've checked that matched the manufacturer's claimed spec. I'm not too fussed about it usually because I'm rarely pushing the current ratings on my LiPos. I use a lot of 4AH 4s packs and even a C rating of 25 equates to 100 amps when I very rarely exceed 60 and that for only a few seconds on a climb out or vertical.

I think the models that really need the high currents are EDFs and they don't interest me - I'm more a biplane man

Geoff

[Simon Chaddock]

I have raised this before but some ESCs have thermal protection if they get too hot. They do this by reducing the power which rather contradicts the 'part power creates more heat' theory.

I believe the heat generated is not only as a result of the switching process but also the current being switched so part throttle can mean less heat.

[Chris Walby]

C ratings are grossly overrated, buy the best you can afford and don't push them that hard....My funfighter used to pull 48A on a 3S2200 and in the end it would not pull anymore current as the voltage rolled off so the watts did not increase anyway.

On the subject of ESC's I spoke to a very knowledgeable chap a day ago on this subject and apparently the current is a bit of a red herring. The heat dissipated relates to the efficiency of the MOSFET devices (switches) and the fact that at low throttle settings they are much less efficient, so generate more heat.

This begs the question of assuming that grossly over specifying ESC might work against the most efficient operating region of the device?

Tin hat on

[Geoff S]

The only time heat is generated is if the switching of the MOSFETS isn't ifinitely fast which it never is. That applies to all digital circuits - they're ideally either on or off - true or false - but that can never be the case in the real world. The design of semiconductors is way over my experience. I just used them. But they must be a lot better than when I retired 25 years ago - everythng else is! The faster you switch the hotter things get because the device in the intermediate state for longer, which is why a good heatsink with a fan is used on modern fast processors.

Just don't overwork anything and it'll be happier. Set the highest speed/current to be well under the esc maximum if at all possible. The current flow when everything is at maximum will be safe. If you have an 80 amp esc that passes no more than 40 amps it's unlikely to overheat at any throttle setting which Simon implies - correctly IMO.

Geoff