LIPO Capacity and C rating

[DaveG28]

Hi all,

I'm sure this is probably covered elsewhere(?) but I am looking at LIPO's and want to see if my assumptions are right:

Higher capacity (eg 3600, 4800 etc) equals higher runtime?

Higher C rating equals more "punch"?

But if I am trying to estiate runtime, do I need to look at both the above, or ust the nominal capacity?

Dave

[glypo]

All lithium (polymer and manganese are the common 2 for RC) cells are pretty much the same as NiCad and NiMH when it comes to the basic specs. A larger capacity in mAh will mean a longer runtime.

The C rate is the only different thing. The bigger C will mean potentially a bit more punch, using like for like capacity. 1C being the capacity of the battery. So a 4000mAh cell, i.e. 4Ah, with a 1C rating would be able to discharge at 4A. So the greater the C the greater the discharge rate. However with LiPo cells you don't need to worry an awful lot, simply because the motor and ESC will only draw what it wants from the cells, and nearly always this is less then the cells can supply.

For example you might have 20C 3200mAh LiPo, which means the cells can discharge at 20x3.2 = 64amps. However the ESC might only draw 30 amps max, so as long as your C rating is high enough, it doesn't matter too much. Obviously being a C rating, it makes your capacity dependent. Most cells come in ranges, where the C rating is constant. Like Kokam for example offer a range of different capacity 20C cells. But obviously a 20C 2000mAh cell (40 amps discharge) would be different to a 20C 4800mAh cell (96 amps discharge).

For runtime, you need only to be concerned with capacity, simply as it's the only measurement that is relative to time. Obviously a high C might allow a little more punch, but average over a run it really won't consume an awful lot more then a low C, simply because it's what's the system draws rather then the maximum the cells can supply.

I hope that makes some kind of sense, if not here's a simple summary:

Higher capacity (eg 3600, 4800 etc) equals higher runtime?
Yes

Higher C rating equals more "punch"?
To an extent

But if I am trying to estiate runtime, do I need to look at both the above, or ust the nominal capacity?
Only capacity is of concern