470A 120V AC MOT Spot Welder for Battery

My dad wanted an electric bike. After searching, he realized they are quite expensive. In addition, they aren't that powerful. After searching for a few days, he discovered a mid-drive motor, Bafang. From various reviews, they are quite powerful and capable of climbing hills. As such, he decided to convert his 20+ years old Diamondback bike to an ebike.

The first thing he needed to do was to buy the motor and install it. For installation, there are many youtube video for that. The second part is a battery. A decent battery is around $500+. There is the mini pack for around $300. For a family of 4, that would be around $2,000+. Besides, this is for recreation and not for daily commute to work. Therefore, he decided to build the battery too (since he is an engineer and enjoys building thing). In this article, I will describe my dad experience building a 120VAC microwave over transformer (MOT) battery spot welder along with advise.

From my dad, reading of various articles and experience, here are your option on battery spot welder:

1. Buy a cheap $50 Chinese portable welder
2. Buy hobby Sunkko spot welder
3. Build one from battery
4. Build one from microwave oven transformer (MOT)

Those cheap portable sport welder (around $50) don't cut it. They can't really make a good weld. With ebike battery, it won't work in the long term. Besides, after a few weld, you have to charge again.

Those Sunkko isn't bad but they aren't that powerful. But they do work from reviews and youtube. Though people mod them and have issue with 110VAC version. My dad recommend this option if you don't want to build one.

There are various youtube videos on building a 12V battery spot welder from car battery. My dad did build one but as mentioned by other youtuber, they need to be charged. And over time, they get weak. Besides, it actually cost more. You need a battery as well as a charger. In addition, those car starter relay isn't that reliable. Depending on which one you buy, they don't function correctly. My dad tried 3 car starter relays. If you want to build battery based, make sure that you use the same relay they used - not all relay are reliable. If you want to build one on the cheap, go to a car junk yard to get your battery and relay starter. It would be cheaper even though you still need to buy a battery charger. Also, make sure you get yourself a timer too (that can go as low as 10ms resolution). There is noo point in building a timer when a timer costs around $10.

Now on the MOT (Microwave oven transformer) spot welder, this is the rest of this DIY article.

Here are the material required:

1. Fuse board (Free from microwave oven)

2. 9-12VAC lower voltage transformer (Free or ebay)

3. MOT (Free or ebay)

4. Wire (Free from microwave oven)

5. 100A spot welder module $ 24 (https://www.amazon.com/Control-Aideepen-Digital-Transformer-Controller/dp/B08DV6W67H/ref=sr_1_3?dchild=1&keywords=100A%2Bspot%2Bwelder&qid=1611206933&s=hi&sr=1-3&th=1)

6. Welding Cable Panel Connector Socket Set DKJ10-25 & DKZ10-25 Quick Fitting Dinse-Style $ 10 (https://www.amazon.com/gp/product/B07MZ3GNMW/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1)

7. 4 AWG wire 5 feet $ 21 (https://www.amazon.com/gp/product/B07KBCZLLN/ref=ppx_yo_dt_b_asin_title_o06_s02?ie=UTF8&psc=1)

8. 1 AWG cable $ 6 (Go to car junk and get from car with battery in the trunk)

9. 2 AWG lug $ 11 (https://www.amazon.com/gp/product/B07HKRHY7G/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1)

10. 4 AWG Ring Terminal Lug Bare Copper Uninsulated Gauge 3/8" Hole $8 - https://www.ebay.com/itm/25-Lot-4-AWG-Ring-Termin...

11. Spot welder pen $22 (https://www.amazon.com/gp/product/B085FY4VHY/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1)

12. Power cable (My dad has many of these around)

13. Tool to drill hole

14. A box (can be make of wood and screw together)

Here are the optional materials:

1. Optional Power Socket Plug Module $ 7 (https://www.amazon.com/GFORTUN-Rocker-Socket-Module-Switch/dp/B06XJGSDNW/ref=asc_df_B06XJGSDNW/?tag=hyprod-20&linkCode=df0&hvadid=216499414469&hvpos=&hvnetw=g&hvrand=13995230276889903065&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9032189&hvtargid=pla-350150226864&psc=1)

2. Optional wire spade crimp $ 24 (https://www.amazon.com/Terminals-Qibaok-Insulated-Ratcheting-Connectors/dp/B07Y7YRTKT/ref=sr_1_8?crid=2J2R3NNNVBB3G&dchild=1&keywords=wire+crimp+tool&qid=1611207085&s=hi&sprefix=wire+crimp+too%2Ctools%2C211&sr=1-8)

3. Optional wire crimp $30 (https://www.amazon.com/gp/product/B079NCMNK5/ref=ppx_yo_dt_b_asin_title_o08_s00?ie=UTF8&psc=1)

To get a MOT on the cheap or free, go to Craiglist. Then search for microwave oven that are $10 or less. There are people who will give microwave oven away for free (used or broken). My dad got two free microwave ovens from Craiglist. Once you have a microwave oven, search for various videos on youtube on how to take it apart. Make sure you follow the instructions. We are talking about high voltage here.

After you took apart the MOT, also remove the fuse board as well as the low power transformer too. The fuse board is used to add a fuse to protect you from short. The lower power transformer is used for powering the electronic if the voltage is around 9 to 12VAC. Otherwise, you will need to buy one from ebay or savage from a power adapter.

For the electronic 100A spot welder module, just buy one from Aliexpress, Bangood, or Amazon.

The Welding cable panel connector socket set is nice and allows you to change the cable size from 4 AWG to 1 AWG cable.

To get 1 AWG cable on the cheap, go to a car junk yard and removal it from a Dodge or Chrysler car model where its battery is located at the trunk. The following car models have the battery in the trunk:

Dodge Magnum
Dodge Charger
Chrysler 300

For the wire, use the power cord from the microwave oven and slice it to get the wire. They are 14 gauge.

There are a lot of videos on youtube that teach you how to cut and re-wind the MOT. Just google "MOT spot welder".

For the wire and number of wind:

1. Use a 4 AWG (4 gauge wire) 100% copper with length around 4 feet or longer if you choose option A below

2. You want 4 winds or else you won't have enough voltage for low welding time (10 to 60 ms). With 4 winds, it is around 3.5VAC.

As for the length from the MOT to the welder pen, you have two options:

Option A: Wire directly from the MOT to the pen.

Option B: Connect the wire from the MOT to dinse panel connector.

My dad recommends Option B as this allows you to change the cable size to a larger gauge wire and have more current. For this, use the "Welding Cable Panel Connector Socket Set DKJ10-25 & DKZ10-25 Quick Fitting Dinse-Style". In addition, the length for the wire from the MOT to these panel connector should be short (around 4 inches).

Crimp the MOT wire end to 4 gauge lug. You can crimp or hammer it.

To mount the panel connector, you need a plastic plate and drill two 1 1/4" hole. See the attached 3D file below.

The total length of the wire from the box to the pen should be around 24". The cable length from the dinse connector to the pen is 18". The length is important. If it is too long, you will have voltage drop. You want a voltage of 3.5VAC.

Instead of building one, I would recommend that you just buy one for around $15 - $25. Aliexpress is around $15 shipped.

https://www.amazon.com/gp/product/B085FY4VHY/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

To connect the MOT wire to the spot welder pen, you need to crimp the cable to the lug. Here are your option for the cable:

1. Use 4 AWG wire with 4 AWG lug

2. Use 1 AWG wire with 2 AWG lug

The 4 AWG wire works, but it needs long timer setting. My dad recommends using the 1 AWG wire.

Please note that the 1 AWG wire is a bit larger for the dinse 10-25 connector. Therefore, you will need to cut a bit as shown in above picture. The large dinse connector 35-50 is way too large. Just stick to the dinse connector 10-25 and trim a bit as shown above.

You will need to house this spot welder. Others have built out of these:

1. Wood box (or make yourself) - 14 x 6.5 x 6.5"
2. Old computer case
3. Or what ever you can found that can be of size 14 x 6.5 x 6.5"

For the spot welder electronic, just buy the "100A spot welder module". It is simple and better than building one yourself.

For complete instruction, the "100A spot welder module" provides instruction. Below is a overview.

1. Start with the outlet socket first. Cut out a rectangle that fits the outlet socket and screw it in.

2. Mount the fuse board, it needs some stand off. My dad 3D printed them. But any stand off will work as long as it fits.

3. Mount the low power transformer to the base of the box.

5. Mount the 100A spot welder module. Again use some stand off. Optionally, you can add a heat sink to the FET. For heat sink, an old CPU heat sink will work. Just drill it to two 2mm holes. Cut a rectangle for the display board wire. If you want a case for the display board, print the 3D file above. Then screw two 3mm screws through the hole.

6. Mount the MOT. Screw it down to the wood base.

7. Connect all the wire. The 100A spot welder module provides instruction. You can slice the microwave oven power cable for the wire. Then, crimp them accordingly.

8. Mount the dinse connector. For this, you need a plate. Print the 3D file above. Or cut an.appropriate hole.

9. Connect the MOT wire to the dinse connector.

Connect the pen and set to lowest setting. Make sure that your "INT" is set to 1.

I would suggest that you start with:

T1: 1 C1: 85
T2: 2 C1: 85
INT: 1

The above setting can easily handle 0.15 pure nickel strip. You can slowly increase the setting. With regards to tripping your home circuit breaker, this depends on the outlet length. It seems to be okay with some outlets but not other ones. Try various outlets in the house. At the end, my dad uses the laundry room outlet - 120VAC @ 15A.

As for current measured:

1. At above setting, around 300A

2. At T1 and T2 higher, around 500A and max at 600A

This is very good for welding 0.15mm pure nickel strip. Though for 0.2mm pure nickel strip, it welded on the battery. But between two 0.2 strips, they can be pulled apart by hand. My suggestion is that you use two 0.15 strips or try increasing the timing of T1 and T2.

Here is the Youtube video - https://www.youtube.com/watch?v=rdNXDKsMAbM.