I got a good deal on some LG MJ1 batteries. There were a bit old, but the price was too good to pass on.

I had enough to make a 7S20P pack which gives a 72V battery with 24.5Ah. Very respectable at just over 7kg. I don’t full build pics for this battery and I ended up re-building it once.

The first time I built the battery, I was using a commercially available battery tab welder and it wasn’t very good. I was using a single layer of 0.1mm nickel to join the cells which limits the current it can safely deliver. After making the Ziva style battery welder, I re-did all the welds on the battery and added a second layer of 0.15mm nickel on top. That made the wiring match the cells capabilities. After that I was comfortable pulling 60A from the pack. I never pushed it past that. My motor wasn’t that powerful anyway.

The one problem I came to when building this pack, and all 18650 packs really, is how do you connect the final group of cells to the wire? I have in the past, soldered wire directly to the nickel strips. For this to be effective you have to solder the wire all the way along the nickel strip. If you only attach it at one place, the max safe current is limited to the cross sectional area of the nickel at that point.

The best way that I’ve found to do it, is to solder thick copper strips to the nickel and use that as flat wire to get the terminal to where you want the wire to exit the pack. Hard to explain but I think it’s more clear in the pictures.

Attaching the BMS and balancing wires is straight forward enough. On this pack I did not let the BMS control the max output current. I only used it for charging and balancing. All output protection is done by the motor controller. If you output the current through the BMS and have the motor controller with safety settings for current, you have two levels of protection. I chose one and am comfortable with that choice. My motor can’t draw that much current, I trust the motor controller, and this particular BMS doesn’t have a high enough rated output current anyway.

With all things related to choices in safety, it’s your choice. You are the one riding this vehicle and you must take into account your own safety and also how your choices affect the public in general. Whenever you build a vehicle, especially one with a bit of power, you are betting your life, health and safety on your choices and skills. If you don’t understand what you are doing, get some help or advice. You will have the live with the consequences. My point is that no matter what choices you make some people will agree with you, some won’t but at the end of the day it’s your choice, your life, your ride. Be safe. That was more of a rant than I intended but I know there are people cringing that my battery isn’t using the BMS for controlling max current output. So be it.

Here is a gallery of build pics from re-welding the pack.

The original welds were pretty bad. The welder was terrible. If you’ve never used a bad welder, they pop holes, blow electrodes and are just generally a pain. Using that welder you had to hold the whole pack up to the welder to weld. The spring force in the electrode holder wasn’t great and if the contact wasn’t great it would blow the tip off the electrode and make a mess. It was a constant struggle and there was lost of electrode grinding going on. It was a nightmare.

When fixing bad welds, the strip is already damaged. It’s tough to find enough good places to add good welds. Even after adding a second layer, since the underneath has issues, like holes and bumps, you can’t even get a beautiful looking set of welds on the top. They are overall decent but it’s not as pretty as I’d like. It is however well bonded and it was the best I could do. It’s like most of my welding, functional but not beautiful.

That’s about it for that build. The battery works great and a good welder makes all the difference.