BatteryMooch
10 kW
Sorry, never tried to calculate it.
What were you considering using it for?
Copper Tape?
- Thread starter YoshiMoshi
- Start date
What were you considering using it for?
YoshiMoshi
10 W
Sorry for the late response.
I'm trying to come up with a generic formula for calculating ampacity. Seems to be reliant on me coming up a value for the heat transfer coefficient.
I'm thinking of just giving in and buying an expensive spot welder, so I can use copper, since it's ampacity is much more well documented.
There's a trick to it, which I used successfully a few years ago, and am about to use again. My spot welder wasn't powerful enough to weld a copper-nickel sandwich of 0.1mm copper, even using the infinite slot method, but by using very short, flat-tipped tungsten welding probes, I can weld 0.1mm copper foil directly, as long as I use the infinite slot method. The resistance of the tungsten provides extra heat necessary to weld the copper, even though the copper itself is too conductive to weld well.
Some tips&tricks:
I bought a pure tungsten (green) 1.6mm TIG rod and dremeled off bits of it for probes.
Use very short tips and clamp them really well in the probes. Tungsten has a lot of resistance, and tips that only stick out 1 mm rather than 7+ mm from the clamp let me drop the welding pulse from 50 ms to 5 ms.
Make sure you hold the tips perpendicular to the battery pole, with good pressure. If you weld used cells, smooth out the nubs from the old spot welds using a dremel grinding wheel (not disc) or similar. You don't need to get rid of all the nickel, you only need things not to stick out. Having a large, smooth contact surface is essential.
Bevel the edges of the tips ever so slightly - sharp edges and a not perfect probe angle will otherwise lead to sticking.
There will be some sticking anyway. Be prepared to grind off copper from the tips and to re-bevel them periodically. A flat dremel cutting wheel works well for this.
Tungsten dust is heavy and annoying. When cutting and grinding, it's best to direct the dust directly into a vacuum cleaner hose as much as possible.
I've been running the 20s17p battery I built using this method for three seasons in my 2.7kW electric moped, with no issues apart from one bad weld, from before I had the process dialed in, and one simply forgotten weld.
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YoshiMoshi
10 W
Is it a good idea to rely on the BMS to shut off current flow based on temp? Some BMSs shut off current flow once a certain temperature is reached, by a thermocouple attached it? I take it that relying on this alone is not a good idea? So some testing and sizing is required for dimensions of the conductors?
Yeah. Fuses, strip and wire sizes, and the BMS current limiter, should all keep conductors from getting too hot. Thermal fuses and thermocouples are backups in case the primary systems should fail, or cells themselves overheat due to internal issues.
BatteryMooch
10 kW
You need to properly address all aspects of good pack design and protection, physically and electrically.
The BMS can certainly be used for over- and under-temp protection (and should) but it will be with a thermistor and multiple ones should be used for larger packs and they must be properly mounted. Otherwise they are pretty useless. Some applications, like esk8, won’t use discharge protection (of any kind) because shutting off current flow will throw the rider off the board. Match the degree and type of protection to the application.
The conductors need to be sized no matter what protection is or isn’t used though. This is for safety and efficiency reasons. It prevents melting of insulation, heating of the cells, ans minimizes voltage drops and power loss.
YoshiMoshi
10 W
Another thing that has been troubling me, is that commercially released battery packs for power tools, seem to be sized with 0.2 mm or 0.3 thick nickel. This is no where close to the ampacity levels for tables found online, to come anywhere close to the power draw requirements for some power tools that you would hook it up to. I think some power tools draw 20 A or even more.
I do not understand how commercial battery packs get away with such thin nickel, but allow users to insert the pack to any power tool they wish that you can draw several amps.
Any ideas?
Educated guess, having only physically had a few toolpacks in hand and apart: generally, lower total voltages, short total series connection, and short high current draw times.
And...a couple of toolpacks I once had were left behind at worksites' garbage piles and given to me because they were deformed from heat enough to not fit in the tool and/or charger anymore.
Whether that was because the *cells* got too hot, or the interconnects, or both, I don't know.
And...a couple of toolpacks I once had were left behind at worksites' garbage piles and given to me because they were deformed from heat enough to not fit in the tool and/or charger anymore.
Whether that was because the *cells* got too hot, or the interconnects, or both, I don't know.
BatteryMooch
10 kW
- Built to a price point and not a performance spec?
- The vast majority of their customers wouldn’t notice the difference if beefier connections were used?
- Most people don’t use their packs hard enough to overheat them?
- The warranty fine print might have enough provisions to keep pack failures from being a financial nightmare for the company?
Just tossing out ideas…
YoshiMoshi
10 W
Right. I'm thinking that the surface temperature is a function of not only current, but time at the said current.Educated guess, having only physically had a few toolpacks in hand and apart: generally, lower total voltages, short total series connection, and short high current draw times.
And...a couple of toolpacks I once had were left behind at worksites' garbage piles and given to me because they were deformed from heat enough to not fit in the tool and/or charger anymore.
Is this idea consistent with your testing? Meaning you send X amps through a nickel strips. It takes a "long" time for the surface temperature to reach a steady state? Are we talking like on the magnitude of a few/many seconds, a few/many minutes?
I'm thinking that for power tools, you pull the trigger for about 10 seconds at most some times. 10 seconds is not long enough for the nickel strip to reach the undersized nickel strip to heat up dangerously high?
I've seen large LiPo batteries tested on YouTube and instantaneous current seems to shut off the battery at about a minute. So if you run current for only 10 seconds it's not a problem to undersize the strips?
I have seen Milwaukee M18 batteries tested on YouTube outputting 70 A for nearly a minute! No way is the nickel sized for 70 A inside those packs. I think it's 0.2 or 0.3 mm nickel. Looking at tables I would need over 2 mm of nickel to support 70 A lol. So just trying to to figure out what's going on. It's almost like power tool battery packs are sized for 10 A continuous =( at 0.2 or 0.3 mm nickel. But some of these batteries are used for lawn mowers and continuously power the lawn mower for 30 minutes or more.
I'm a bit perplexed about what's going on with nickel strip sizing in commercial battery packs for power tools. There's little to no extra room in the battery pack, very difficult to get much more than 0.3 mm thick nickel in there, not enough space.
Also I believe ampacity is a function of width and thickness, the length is not involved. Looking at the actual equation, this isn't true, it plays only a small role in the calculation, but it's so insignificant that most just disregard its impact on ampacity.
YoshiMoshi
10 W
Good point! Only ones I have seen taken apart on YouTube are nickel, 0.2 or 0.3 thick nickel, and looks like 10 mm wide. Never seen copper in power tool pack.
Yet they can support power hungry devices such as angle grinders and circular saws and draw much more current that 0.2 or 0.3 mm thick nickel can handle.
BatteryMooch
10 kW
…and @YoshiMoshi…all copper interconnects (but thin!), more photos (in the tabless testing thread) when I do the teardown to determine what cell is used.
Attachments
YoshiMoshi
10 W
Thanks! Never seen that power tool brand before. Our of curiosity, how thick is the copper and width?
BatteryMooch
10 kW
TBD when I do the teardown.
Bosch is huge though.
DogDipstick
100 kW
Oh yes certainly.
I want a Glitter or.. an American Beauty 1kW.
Fark it Im just gonna buy one.
I used an 80 W Weller iron to build this 22S 1P X2 (in parallel) using these cells Gotion 32135 15Ah 3.2V LFP Lithium-ion cell IFR32135-15Ah and 0.2 x 10 mm copper. I did capacity testing on 2 cells before and after soldering and found no degradation, which gave me confidence to go ahead and solder the rest into packs. Can't justify a Glitter to build just one battery for my scooter. I will be testing it on my scooter for the first time today.
lnanek
10 kW
- Joined
- Sep 26, 2022
- Messages
- 700
Maybe it's nickel plated copper to resist corrosion? Kind of surprising they aren't doing something unreachable by DIY'ers like stir friction welded aluminum, but I suppose pure nickel is cheap as long as the power tool gets given cool down time to make up for the high resistance.
BatteryMooch
10 kW
The laser or ultrasonic welding that’s typically used is pretty unreachable for most IMO.
YoshiMoshi
10 W
@CamLightThe laser or ultrasonic welding that’s typically used is pretty unreachable for most IMO.
Have you found in your testing that "stacking" really doubles the current carrying capacity at the same temperature difference of the nickel strip and ambient temperature?
Meaning a 0.1 mm x 10 mm nickel strip can carry Z amps at 10 DEG C above ambient temp. I stack two of these strips on top of each other. My two strips stack can now carry 2*Z amps with a strip temperature of 10 DEG C above ambient?
Looking at some equations, I'm not seeing a direct one to one correlation.
So I'm wondering if experimental real data shows a one to one correlation, and you really double the current carrying capacity by stacking two identical strips on top of each other.
BatteryMooch
10 kW
It’s not possible.
You cannot stack two strips directly on top of each other, double the current, and measure the same temp rise.
When you stack two strips you decrease the surface area exposed to ambient air. This reduces convective and radiative cooling. Though the latter is minimal with bare metal. You also increase the “local ambient” temp by having two heated surfaces (where they are stacked) affect each other.
All this means that the temp rise of two stacked strips at 2x the current will be higher than a single strip at 1x the current. But, the differences might be tiny, and not of any practical concern, depending on the current levels, strip size, etc. The temp rise difference of stacked strips/single strip at 2A/1A might be almost nothing. At 20A it could be considerable though.
However….
If you just double the width of the strip, or use two strips side by side, then you are barely blocking the cooling of the strips and you should be able use 2x current (or close to it) and get the same temp rise as a single strip with 1x current.
YoshiMoshi
10 W
Thanks for confirming. Stacking two strips giving twice the amount of current seems to be what everyone assumes. I'm trying to come up with a generic equation for ampacity of a single strip to answer some questions. So far I wasn't seeing it be 2x, and intuitively doesn't make sense. Less convection cooling on the largest surface area.
Some estimations have to be made.
Assuming temperature of the air within a battery box is about the same as the ambient air temperature.
Assuming the strip can be any orientation during use, for example a power tool.
Assuming the temperature of the cells is about the same temperature as the strip.
Assuming the temperature of the tape or some other insulation is about the same temperature of the strip.
Seems possible to come up with a generic formula under these estimations.
But now you got me thinking about something I didn't realize, heat transfer via radiation. Everything inside the battery box is transferring heat to everything else in the battery box. Have you found the effects of radiation to be significant? I was hoping to just say it's about zero? Like in experiments if you lay two strips next to each other (with an air gap) and send current through one strip but not the other, does the strip next to it heat up much?
Looking at heat transfer equation via radiation... Forget coming up with a generic formula. Now even the BMS and heat it's at, and it's geometry and every surface area on it has to be considered. So I'm hoping to just say it's about zero. Majority of heat transfer is via convection. And if assumptions listed about are made, don't even have to worry about conduction between strip, cells and insulation.
If the strips are connected at the ends (via welds at the cell, etc) you'll get the heat passing from one to the other via conduction at those points, and it will probably be much faster than radiative or convective transfers. (no idea how to do the math on that, but I bet there's modelling software out there to do this for you).
Whatever the highest source of heat is will feed the materials that are not as hot, so if the cells are not generating as much heat, the strips will heat the cells via conduction. Otherwise, the converse will be true.
Regardless of the transfer method, all the heat contained inside the sealed protected insulated pack will end up in everything that's in the container, so that it all becomes the same temperature. So removing the source of the heat by using the lowest resistances possible in all interconnects, as well as cells that are as much more capable than needed as possible (thus lower Ri) will keep that final temperature down.
BatteryMooch
10 kW
Haven’t tried to quantify it (as a separate cooling mechanism).
You’ll have to do some testing for the applications/conditions you’re focusing on.
YoshiMoshi
10 W
So the widest 0.1 mm thick purely nickel stip I can find is 10 mm. A 21700 cell is at least 20 mm wide. Any thoughts on using 0.1 mm x 8 mm purely nickel strip but do two strips wide. Such that half of one strip is spot welded up to the middle of the cells, then spot weld on the other, the other strip? Essentially having a 16 mm wide strip? I just don't know about spot welding two strips wide in this way onto a single cell. But I don't suppose it would cause any issues, as long as you don't unintentionally to another cell.
Any ideas?
I've tried 0.2 mm x 15 mm to a 21700 cell before without issue. So I presume two strips wide of 0.1 mm x 8 mm should be just fine.
Any ideas?
I've tried 0.2 mm x 15 mm to a 21700 cell before without issue. So I presume two strips wide of 0.1 mm x 8 mm should be just fine.
