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

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?

• Where do you get the 30 A max number from? What assumptions are being made in that calculation? Commented Nov 26, 2022 at 19:50
• There are a number of assumptions you have to make to determine what the actual current carrying capacity of a wire is. I promise you 30 A is not the wire's fusing current, where it actually breaks--more likely, that's some building code somewhere (most likely US) that says 10 AWG is good for up to a 30 A circuit, but building codes are going to use extremely conservative ratings, because the cooling in conduit is not great, there might be other wires in the conduit, and the consequences of too much current in that setting is a house fire (very bad). Commented Nov 26, 2022 at 19:59
• No, it definitely can. It'll get really hot, and you'll have a high voltage drop, but it won't melt. Commented Nov 26, 2022 at 20:03
• Well, the insulation might melt. The copper itself, though, won't. Commented Nov 26, 2022 at 20:04
• @luekbaja The battery is like you: Just because two people can walk continuously doesn't mean they can sprint the same, and no one is expected to be able to sprint continuously. But the ability to sprint can make you snappier when you need to be. Similarly, the battery is not meant to be run at 50C continuously. Commented Nov 27, 2022 at 3:48

5 Answers

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.

• Slight nitpick; it's not so much the length that matters - more length means more heat, but also more wire to absorb (short term) and dissipate (long term) that heat. They cancel out. More importantly, the battery simply can't supply high currents long enough to heat the wire to the point of danger. Commented Nov 30, 2022 at 8:58
• @SomeoneSomewhereSupportsMonica my answer implies 0.9 watts dissipated per 305 mm for 30 amp current flow. I never said length mattered. In fact, apart from this comment, you are the only person to mention the word "length" on this web page. So, it seems you might be best advised raising a new answer rather than making a comment about some "perceived-by-you" minor failing in my answer. In short, I don't know what you are talking about. Commented Nov 30, 2022 at 9:05
• It seems (to me) that the comment and post are essentially in agreement. The post used 1 foot as an example. The comment is "an aside" which does no harm. Commented Nov 30, 2022 at 11:55
• @SomeoneSomewhereSupportsMonica can you please explain why you believe your comment is a "nitpick" (with the implication that I may be giving incomplete or wrong or misleading information)? Commented Nov 30, 2022 at 14:31

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

• Nice video. The mounting terminals for your wire will cool the wire somewhat. So a higher current than Preece predicts should be expected. I don't have a high current source so it is nice to see some experimental data. Commented Nov 27, 2022 at 0:29

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 1084oC

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 120oC. 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 60oC. This generally assumes an ambient temperature of about 25oC. 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.

=======================

Wikipedia source for American Wire Gauges.

• Is your fusing current rating of 333 amps for the #10 AWG wire in the question? And if so, how long would it take to reach the melting point of copper in free air? Based on my tests with #16 AWG, it should handle 25 times 30 or 750 amps for at least one second without damage, so based on I^2t it should handle 333 amps for at least 5 seconds. Commented Nov 26, 2022 at 23:08
• @PStechPaul: The wire in the question is in a constrained environment and may not reach 333A. Wikipedia quotes for Preece 333A for approximately 10 sec. Onderdonk equations predict 1600A for 1 sec. I will add this reference to my answer. Commented Nov 27, 2022 at 0:34

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.

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.)

• It may, however, be possible to get pulses of that much current out of it. Definitely not for more than a few microseconds though. Commented Nov 26, 2022 at 19:55
• People are discharging RC lipo batteries at very high rates. I am not sure if 50 C is attainable but 45 C is attainable. I don't follow RC closely. I saw one report of 45 C discharge and 4.5 C charge and > 1000 cycles. rcgroups.com/forums/… Commented Nov 27, 2022 at 0:17
• In other words, I think you have overstated and should probably do a little research before you say that the 50 C rating is utter and complete rubbish. It may just be that you haven't been exposed to this segment of the battery market. Commented Nov 27, 2022 at 0:19
• This is non responsive. The same issue the OP is asking about holds even for a 10C battery. Commented Nov 27, 2022 at 1:20
• This (about the battery ratings) is not necessarily true. I have a 3Ah, 12V LiFePO4 battery (3S) that is perfectly capable of cranking a 4-liter car engine (a honest 300A draw when cold). This is something like 100C and it works every time. Commented Nov 27, 2022 at 13:28