LiPo instead of a capacitor for micro spot welder ?

[rob77]

Hi Guys,

going to build myself a micro spot welder, but considering to replace the main capacitor bank with a LiPo battery. many of those LiPos for RC models are made for 35C discharge currents continuous and 50C pulsed... so a 4000mAh pack should be able to spit out 200Amp pulses without any issues. the internal resistance of those packs is in the few miliohm range, so no issue there as well.
did anyone experimented with LiPos in such application ? looks like it would be both smaller and cheaper and obviously holding more energy  compared to a 20x47000uF capacitor bank. and as a bonus, there would be no need for charging during operation because the lipo would hold enough energy for hundreds of spot welds.
20 x 47mF is 0.94F - charged to 8V would be ~ 30Joules
2cell LiPo  7,4V 4000mAh is approx 28Wh  ~ 100kJ

[cowana]

One of the benefits of the capacitor is you never have to switch the large currents - you charge them up, touch the electrodes of the spot welder to the metal, and *zap* - they discharge through it forming the spot weld.

If you use a lithium battery, you need a way to interrupt the circuit once the current starts to flow - if you have any more than a quick pulse, you're just going to melt a hole in your sheet material you're trying to weld. For interrupting a 200A current, you'd need some pretty bulky silicon - the inductive voltage spike will need high power diodes, and the main current high power MOSFETs.

[wraper]

If you want to burn down your place, then sure, do it.

[rob77]

even with capacitor you need to switch off the high currents... the micro spot welding is usually a few milisecond pulse or few consecutive short pulses. so from this perspective it's very similar.
ok the capacitor bank has the advantage of limited energy to be dissipated during a fault (30J vs. 100kJ) but i can put a 50 or 100Amp fuse in series with the battery to help with that (the short pulses will not blow the 50Amp fuse).

[wraper]

[rob77]

If you want to burn down your place, then sure, do it.

yes i'm aware of the fire hazard

but again those cells are designed for 130Amp continuous and 200Amp pulsed , 2 cells is 8V fully charged - to get 200Amps you need a load of 40miliohms. 5-10miliohms is the battery's internal resistance approx 10miliohms will be the bank of parallel mosfets, few milliohms for the wires , some miliohms for the welding contacts , few miliohms for the fuse in series with the battery and suddeenly we have  more than 40miliohms and therefore can't even exceed the rated current of the battery... during a fault (shorted mosfet) the fuse will blow and protect the battery.

[Marco]

the inductive voltage spike will need high power diodes

Almost all the power would be burned in a snubber resistor (with parallel capacitor). So it doesn't see any real power, just a short high current pulse.