The Battery Powered Arc Welder
No electricity? No Problem!
The first thing most people will ask on seeing this is 'why?'. The answer is partly 'because I can' and partly just because we needed to weld with limited or no electricity and no gas welding gear was available. This was while working on the trike which was built almost entirely on a farm belonging to Larry's uncle. The electric feed to the sheds are/were unreliable and to be honest the house isn't much better. We could run small things (lights, grinders, etc) on long extension leads daisy chained together but obviously such a scheme would not work for welding. Hence the battery powered arc welder was born.
And now onto the 'how': All that is really needed for arc welding is a source of suitably high current and sufficient voltage to strike and maintain an arc. Since lead acid batteries are commonly used to start cars, something requiring several hundred amps it was obvious that they could supply the current. The remaining questions were could they do this for the length of time required without dying/melting/exploding - the obvious way to find out was by experiment. Old batteries were acquired from a friend who works in a boatyard. These were in the 80 Ah range and are usually used to power lights, navigation equipment, etc. These were then connected together with borrowed jump leads and a current limiting device. In this case the current limiting device consisted of several old bike spokes in parallel - when hot (glowing) they provided sufficient resistance to limit the current to an estimated 200A.
I found that at 24V (2 batteries) it was difficult to strike and harder to maintain an arc but with 36V it worked well and I was able to produce some successful welds on test pieces (see the images below). Soon after this the bike spoke current limiter burned out and I had to stop but I had proved the concept was workable. At this point I was using two of the large batteries and one much smaller car battery that was lying around the garage.
Obviously doing this for any length of time would kill the batteries so some means of maintaining their charge between welding was needed. Since there was some power available from the house a mains powered charger capable of delivering about 16 amps was constructed. This was mounted on the grey board that can be seen in the first picture. The overall circuit of the welder is shown in the diagram below but it should be noted that the transformer used was not a single device as shown but instead consisted of two low voltage lighting transformers designed for powering halogen lamps and a total of twelve 12V 50W windings connected in a series/parallel combination to produce a single high current 36V transformer. The cables from the batteries the the terminals consist of many pieces of 2.5mm copper wire braided together and attached to the batteries with G clamps. The terminals themselves are made of 10mm threaded rod for connection to a pair of welding leads and the power resistor is an 8 inch length of very thick iron fence wire strung between two ceramic posts.
It was hoped that in use the 12V bulbs in series with the charger would limit the current delivered by it to a safe value during welding but it was found that the voltage of the batteries dropped so much that significantly more than 12V appeared across the bulbs causing them to blow. In the end we just manually disconnected the charger when we started welding and reconnected it afterwards. At some point I will either find some more powerful bulbs or add a relay circuit to do this (the former is preferred since the charger can then assist in supplying the welding current.)