What is the point of a high discharge rate Li-ion battery if the wire gauge is too low?

Question

On my "50C" 3300mAh Li-ion battery, the wire gauge is only 10 (30 amps max) while the calculated max current draw is 50 * 3.3 = 165 amps. So if the realistic max amp draw is only 30 amps, why would anyone spend more money on a higher C rating battery if that current draw could never be utilized?

For example, if 2 batteries both have a 30 amp max current draw due to low wire gauge, what is the advantage of a 2.2Ah 60C battery (132 amps), over a 2.2Ah 50C battery (110 amps) if they both already exceed the maximum current draw through the 10 gauge wire?

Answer 1

AWG 10 is 1 Ω per 1,000 feet. So, 1 foot of wire (305 mm) will be 1 mΩ (that's 1 milli ohm). I don't know how much wire is needed to wire up a battery pack but it's going to be less than 305 mm.

So, with 30 amps flowing, the power dissipation of the wire is about 0.9 watts and, for something that is 305 mm long that is not going to be a problem.

In one of my designs, I used 100 mm of the equivalent of AWG 10 wire for a transformer core primary. The wire was 5 square mm in area but, reshaped to 1 mm x 5 mm and, I'm intermittently pushing 300 amps RMS through it with no problem.

OK, it's a 100 kW converter (in total) that operates very intermittently but, the point is that the current is 10 times greater than what you found on the web. It's all about power dissipation and heat removal; if you have decent heat removal (or low duty cycle), then "big current" can flow.

Image from HyperPhysics.

Answer 2

As others have explained, current capacity of a given gauge of wire is based on temperature rise, insulation rating, and conditions that add to or remove the heat generated. There is also the I^2t rating, which determines the wattage and the temperature that is reached in a period of time. The common current ratings are based on continuous operation, and may also take into account the thermal conductivity of the insulation and surrounding material.

The peak current rating of lithium cells may be overly optimistic, and probably also dangerous if actually achieved, but it is likely based on the cell's internal resistance and the short circuit current.

I made a video showing that #16 AWG magnet wire nominally rated at 7.5 amps can handle 25 amps for a full minute without getting hot enough (in free air) to feel painfully warm, and can handle 185 amps (about 25x) for about 1 second without damage. That's about 185 watts in a piece of wire about 6 inches long.

https://www.youtube.com/watch?v=HPll6fdUjnY

Answer 3

The fusing current for bare copper wire in free air is about 333 A. This can be higher or lower depending on how the air flows. This is a melting temperature of 1084^oC

Most wires that we use are insulated so that they can be bundled and routed without shorting together or to other objects. Polyethylene (PE) melts at about 120^oC. The thermal resistance from copper to air through the PE is much higher than from copper directly to air, so the temperature rises to a higher value for the same current flow.

Other insulating materials allow higher temperatures and so may allow higher currents.

For maximum safety from touching a too hot wire, the electrical codes often limit the max temperature to 60^oC. This generally assumes an ambient temperature of about 25^oC. Under these conditions the max current is about 30A dc or rms.

In addition to thermal resistance there is thermal capacitance. Together they form a time constant similar to the electrical time constant. Thus it takes time for heating and cooling to occur in response to a step change in current. So then it is the average current that determines the heating. So intermittently, large currents may be drawn as long as the average is ok and the pulse is short enough.

what is the advantage of a 2.2Ah 60C battery (132 amps), over a 2.2Ah 50C battery (110 amps) if they both already exceed the maximum current draw through the 10 gauge wire?

Well, let me say:

1. Determine the load current draw. (average and peak)
2. Select the battery,
3. Select the wire gauge to suit the current draw, charging requirements and the temperature requirements.

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Wikipedia source for American Wire Gauges.

Answer 4

As others have said, the ampacity of a wire is typically based on how hot the insulation can get. One thing to keep in mind is that when you discharge a battery at 50 C, by definition, the discharge time will be extremely short. Because of this transient nature, the wire can sustain quite a bit of overload. Also, the only place where I see such high discharge rates in in the world of radio control (RC), where silicone jacketed wire is used with a very high temperature insulation. So there is less chance of melting the insulation compared to automotive wire or household wire.

There is a good possibility that the battery ratings are exaggerated, also. But since I am not active in RC stuff, I am not sure. Maybe some batteries really can be discharged completely in 1 minute or so. I think I would need to see it to believe it.

I have personally tested a bunch of name brand Japanese and Korean 18650s and none of the ones I tested could be discharged that rapidly without overheating.

Answer 5

Because the "50C" rating is utter and complete rubbish. It would mean discharging the battery in 72 seconds. No Li-ion cell can survive being fully discharged that fast without some serious damage. That's not the discharge rate that the cell designers intended. That's what the company that uses those cells marked the package of the battery.

As they say: there's liars, damn liars, and battery companies. (Especially those that cater to hobbyists.)