I thought I'd follow up on my status with respect to fechter's method. My 3S graphene Turnigy 5000 mAh battery arrived and in preparation for a trial with my delivery system (based upon a small engine starter relay) I wanted to get an estimate of where my current would be at relative to the manufacturer's stated "burst" rating of 130C = 650A for a 5Ah pack. At 11.39VDC charge, I connected the pack to my delivery system using the XT90 connector supplied such that all connections and cabling remain the same as in the welding scenario, including the use if nickle strip as the electrode contact surface to the load. For a load I used a 3.2 Ohm 25 watt resistor.fechter wrote: ↑Dec 25, 2017 9:50 pmThese are Turnigy RC Lipo batteries, mine is a pair of 2.2Ahr in parallel. It is 3s. These don't have CCA rating, but are rated for 130C discharge rate. Not cheap, but very lightweight and small.
My electrodes are semi-rounded and have about a 2mm dia contact area.
Open circuit voltage before and after the test was 11.39 and 11.38VDC, respectively. Using a separate 12VDC source to activate the solenoid, I measured voltage across the resistor load at 5 and 10 second intervals at 11.32 and 11.31VDC respectively. So with the 3.56A current under load, the combined drop for internal R, cables, and contact resistance was 0.07V yielding a combined "rig-resistance" of about 20 mOhm and an associated maximum burst current during spot welding of 578A, about 10% lower than the burst rating of 650A.
I'm now proceeding to evaluate via scope the relationship between Arduino interval setting and the actual pulse interval delivered by the Turnigy battery which will likely involve a latency due to its use in powering the solenoid as well. For the car battery this was a constant 8 ms -- delivery of 60 ms pulse required the Arduino interval to be 68 ms, for example. Once I get that, I'll do a test starting at 8ms and see what happens. After thinking about it, fechter's comment makes sense -- lower current requires longer pulse time and due to thermal conductivity of the strip and substrate this results in more total heat transfer to the cell. The higher the amperage, the shorter the pulse and the more quickly melting temperature is achieved at the weld contact area, reducing total heat transfer.
Results: Pulse width testing showed the same 8 ms difference between Arduino setting and delivered pulse. I think this is probably a function of the solenoid coils and would have to be evaluated for each specific set of parts. So with the info on likely current limit, I finally decided to try it. It makes very nice welds at 8 ms delivered pulse (16 ms Arduino setting) without sparking. For about $50 for the graphene lipo, not to mention its small form factor and weight as fechter mentioned, this looks like it could be a viable option for spot welding. For testing purposes, I've been using a steel paint tray, probably nickel coated, of about the same thickness of the bottom of an 18650 can with a wood support under the weld location and 0.1 mm pure Ni strip. Previously I was seeing relatively huge black char spots along with smoke, and had to turn the tip on my soldering station up to 700 - 800°F to duplicate it. With this current source, the discoloration of the wood under the weld spots is only barely visible light brown, indicating a very substantial reduction in heat and resulting temperature versus the 325A LA battery and 30 / 60 ms double pulse minimum required for a good weld.
I'm still trying to get the Vruzend system optimized by changing out the stainless steel across the board with phosphor bronze spring temper, brass bolts and nuts, and copper parallel bus bars. But if that doesn't pan out, I now have an alternative to further explore.
Thank you, Mr. fecter !!
RISK WARNING: In the event anyone decides to attempt something like this, they should be thoroughly familiar with the potentially dangerous, life/property-threatening consequences associated with short-circuiting lipo packs; which this could easily become depending upon the system used to deliver and control the pulse to the weld areas. I think the risk is considerably higher using a graphene lipo, even though it is "rated" for 130C burst current, than it would be with a lead-acid battery as the current source in this regard. From my research on this topic, it appears there are no recognized and accepted criteria for burst current rating and many maintain that every excursion beyond continuous use current rating causes additive damage to a pack and could be catastrophic. Proceed at your own risk !!!