Crude Balancing

Hello, i've been lurking here for over a year and this is my first post. I've found a lot of helpful advice here so thank you all for everything!

I've repaired 3 gen 2 battery packs successfully without balancing. Each pack had one or two bad cells, replaced them and they've been working fine for months. Now I have people that want to pay me to repair their packs. So if i'm getting paid I want to do things right, or at least the best I can. I'm working on a gen 1 pack, my plan is to replace with all gen 2 modules from 2 different packs. I want to balance the cells and im trying to figure out a simple way that isn't too time consuming.

What im thinking is discharging the cells to the lowest common charge that would still be within spec for the pack. So if I have 38 cells that are within 7.4-7.8v discharge each cell individually down to 7.4v. Then simply reassemble the pack and reinstall. Then by running and driving the car the cells can charge back up and discharge. If all the cells are at 7.4v to start wouldn't they stay pretty close when charged back up by the vehicle? I know this is not the same as conditioning the cells through charge/discharge with an appropriate smart charger. But could this be a more crude way to balance a pack or does it have to done by discharging down and charging back up to the same voltage?

 

The gen 1 pack I worked on today is fixed. All modules tested at 7.8v. I had pulled a P3009 code before starting the job which is a high voltage leak detected internal battery. I cleaned all the busbars, plastic holders, terminals, sides of battery modules. I then reassembled the pack and installed into car, drove 60 miles so far tonight and it's working great. These gen 1 packs have all that sealant gunk toyota put on it, a real pita but I got most of it cleaned off.

As far as experience I do have a certificate from Cleantech Institute funded by California Energy Commision. I've been a mobile mechanic for 9 years, never had any serious issues, I always triple check my work.

I just started doing more work with prius and camrys. I plan on getting the right equipment soon. I do have gloves rated to 10,000v with leather protectors, face shield, all the good stuff.

Thank you all for your input. I think the grid charging is what I'll do for now. I have another gen 1 pack I'm picking up tomorrow, I may do the same thing if all cells are good, or I'm gonna replace whole pack with used gen 2 cells and balancing will be important.

 

I'm glad you brought this up I forgot to mention that one of those ring terminals broke because of corrosion, I had to clean and solder it back together. The pack I worked on today had such bad corrosion from excessive electrolyte leakage I decided to rebuild with used gen 2 modules. I'll be working on it tomorrow, I cleaned all the busbars and plastic, tomorrow I'm gonna clean the rest of the pack when I take out the modules. This pack had a broken sensor terminal from corrosion as well.

 

The gen 1 pack I reassembled with gen 2 cells has been working good for over 2 weeks. I didn't need to balance them because I got lucky and all the cells were reading 7.3-7.4v most were like 7.34v even though they were from 2 different packs. Those wire looms are delicate, if I can get them from Toyota I might start doing that because i've seen ones that were literally disintegrating. What do you guys think about the method of hooking up 3 modules in series and charging with 2 12v chargers (for 24v) there's a video on youtube of a guy doing this, should work, he had little jumper wires to connect modules that were close in voltage and then charged.