Discharge rate of LIPO

[Snoopy]

Hi, when choosing LIPO for MX-106 class high toque motors, is there a range of C values that I should be looking for? Is there a threshold C value above which there is no advantage or perhaps even potentially be harmful to the motors? Not sure if too high discharge rate is bad for the motors.

[KurtEck]

I am no expert on this, but my read on this, is you probably can not have too much capacity. The servos will only take as much as they need. Except of course if you do something like short out the battery, in which case it might melt the wires or the like.

Again I could be wrong, but to me, it is sort of like you have a 20amp AC circuit at your house for a plug in, where you are plugging in something maybe that only take 5 amps... But you do need to remember that the wiring for the house needs to be of a high enough gauge as to allow that much current to flow without burning up the wire...

But then again I am a software guy!

[jwatte]

The current capacity of a battery cannot "push" current into a circuit. The circuit draws as much current as it's designed/made to do at the voltage you provide it. If the power source cannot keep up, internal resistance (or limiting circuitry) may drop the voltage, until the circuit draw / battery capacity balance out (which may be in a not-functioning state.)

Having more current capacity is not harmful, but it also doesn't really help.

Also note that C rating is only one-half the equation; the other half if capacity, because the actual amperage capacity is Amp-hours divided by one hour times C. So, 1800 mHh battery with 20 C rating can deliver (1.8*20 ==) 36 Amps "as designed," whereas a 6 Ah battery with 10C rating can delivery (6*10 ==) 60 Amps. Also, these are generally "design suggestion" ratings, not exact cliffs. Being a under max is generally good because it causes less stress on the battery. Also, many batteries will deliver more than the C rating for a very short time, although of course at some amount of voltage drop and extra heat build-up.

[Snoopy]

Thanks KurtEck and jwatte. I am trying to decide between three batteries. They all have the same mHh. One has C=75 but 20-30g heavier. The other two have C=60. At such high C values, although the one with higher C gives higher capacity*C value, I wonder if having an additional 15C matters much in practice.

[jwatte]

The difference between C=60 and C=75 doesn't matter at all.

Also, at those levels, it's very likely the batteries are rated "optimistically" ("this is the largest possible current the battery could physically produce for a short while when conditions are perfect") but, again, the Dynamixel servos won't draw nearly that much, even if you manage to stall sevaral of them at once. (Which you don't want to do.)

[Snoopy]

Thanks jwatte.