Hi guys,

I have a pretty mad idea to build a super high current lead acid battery charging system on the cheap, using a TIG welder as the power source. Yes, this is potentially very dangerous. So lets get down to the engineering.

Intro
I've recently aquired 11x 92 Ah 12 V batteries that I will be using in a UPS backup system of my own design, all of which will be connected in parallel for charging and discharging. Clearly this battery bank requires a significant charging current - 0.08 to 0.1 C is recommended for a "slow" charge, equating to a charge current of 80.96 A to 101.2 A. Lead acid's require a CV / CC combination to charge correctly, meaning I need excellent control of the output of the TIG welder if this project is going to be possible.

Why Parallel Batteries?
It's a lot easier to find high power (3000 W) 12 VDC to 230 VAC inverters than it is ones that accept 130 / 140 VDC inputs. Additionally, it's easier to charge a bank of parallel 12V lead acids than a series stack (balancing for example).

Why a TIG welder?
There are not many sources of such high current power available to the domestic market for a price that is anything like reasonable. However, due to the magic of globalisation and economies of large-scale manufacturing, we can purchase high current TIG welders for a very reasonable price (for example £130 for a 140 A IGBT variant on Amazon - less than £1 per amp!!). On the face of it, these machines are very nicely suited:

• Current: 10 - 140 A
• No Load Voltage: 56 V
• Rated Output Voltage: 25.6 V
• Efficient

Theory being when you strike an arc, the constant current circuitry kicks in and hey presto, instead of melting two bits of steel together you could in fact be charging 11x massive lead acid batteries instead.

How Can it be Done?
I've never attempted to convert a welder to a battery charger before, and I'm not sure if anyone else has either. However - my initial thoughts are as follows (feedback welcome).
1. Measure the voltage & current waveforms from the welder when you simply connect it to a 12 V battery array, set the current limit low and fire (hopefully we get a nice ~12 V current controlled power supply with no spurious voltages and currents on start up. And that it does actually start up)
2. Work out how the welder current control knob on the front panel works and design an interface to it (potentiometer, rotary encoder etc). Include read-back of set current if required.
3. Connect the welder current control knob interface to either an arduino which will manage the charge profile, or use an OTS part such as bq24450 to handle the control. I prefer the bq24450 option as this has all the smarts required a for pre, float and boost charging including ambient temperature adjustments... even though it will take some thinking about how to interface it to the proposed system.
4. Take high current shunt feedback from within welder or external device to the controller
5. Think about safety system (bimetallic thermal cut-outs, overcurrent, overvoltage, etc)

This also sounds like a really fun project for me to get my teeth stuck in to - so much so I may make a YouTube video for my channel on how I built (or failed to build) this absolute monster of a battery charger.

Dan

 

Does a TIG welder really have a duty cycle that can accommodate battery charging?

 

Should be fine - IGBT welders tend to be rated for 100% duty cycle at ~70% max amps (dropping to 40-60% duty cycle at max amps).

Good point though, I will design a dummy load to draw max charge current for 24 hours to verify.

 

Different battery charges use different charging methods depending on the battery chemistry. Even if you could modify the welder, it would be working at the very bottom of the duty cycle range. Also, one slip and there is a terrific chance of detroying the battery with explsive (dangerous) results. It goes withoutout say that none of want to see become a Darwin Award winner. Your personal safety is paramount. This seems like a really terrible 1st project to start with. Checkout Battery University for some battery charger education. It's one of the best sites out there on this subject.

 

Different battery charges use different charging methods depending on the battery chemistry. Even if you colukd modify the welder, it would be working at the very bottom of the duty cycle range. Also, one slip and there is a terrific chance of detroying the battery with explsive (dangerous) results. It goes withoutout say that none of want to see become a Darwin Award winner. Your personal safety is paramount. This seems like a really terrible 1st project to start with.

Thank you but I'm fully aware of the dangers. To be crystal clear this isn't my first electronics project - I'm an electronics design engineer by profession, with lots of time working on HV megawatt power supplies. I just haven't attempted to convert a welder into a battery charger yet (doubt many people have)

Battery university is awesome, some brilliant knowledge on there, cheers!

https://batteryuniversity.com/articles

 

Not nearly the amperage your talking but occasionally I need to get rust off of parts using an electrolysis tank. I was using jumper cables off of a 12v battery connected to a charger. Charger could not keep up. This particular case I was connected to an engine block in the tank. Had a 230v single phase mig welder that seen better days. Added a volt meter and an amp meter to it and connected some 1/0 cable. Installed a switch in place of the gun trigger. I could set my voltage and then monitor my amp draw using the pot that came on the welder. It worked very nicely. Downside is I didn't have anything in place to keep the voltage fixed. So as the rust was removed from the part my current lowered and my voltage increased forcing me to go back to the machine and dial it back. That welder was connected for 8 hours a day for 4 days to complete that job. I for one am anxious to see you convert this tig to a charger.