Best way to install BMS's

I recently built my first set of batteries for an Ebike I purchased which came without batteries.

Based on space available, I chose to build 4 - 12V batteries, 24 cells, 8P X 3S each, using the HX-3S-FL25A BMS Module.


I had quite a bit of difficulty. The welded parts were fine, but I chose to solder on the BMS's and even though I have been soldering since around 1955, my skill level at this point in my life seems to be on the decline. I had numerous cold solder joints and I seem to have over heated my BMS modules to the point of destruction. So after replacing about 3 out of 4 BMS's and reflowing practically all of my solder joints, I'm finally done. This is my first battery build project and hopefully I'll get better. :lol:

In review, it seems that most of my problems related to my method or technique of attaching or mounting the BMS's. Even as my soldering is improving with practice, it won't hurt to get some advice on the matter.

What methods or techniques do you folks implement that you have successfully deployed and recommend?

Thanks, Jim

Start with good gear. I have a Weller 8200 solder gun. It's great. I also have a Harbor Freight gun, which was junk, until I put Weller tips in it. I also have a Weller solder station, with a dial for temperature control. for finer soldering. Good solder also matters, While industry has gone to lead free, we at home need the old fashioned 60-40 rosin core solder. Melts at lower temperature. Cools nice and shiny. Also get a tub of flux to keep your tips clean.

I confess that my good solder and good flux were filched from work 25 years ago. The stuff I have bought in the past 5 years isn't as good. Probably safer, but not as good. The little tubes of Kestor solder sold at Home Depot are lead-free. I can't use that stuff,

I've noted that the BMS I buy must use lead free solder. It is harder to get the base layer to melt on the P-, C-, and P+ pads.

By the way, I've got these coming in. NCR cells and a BMS. I will reconfigure them as a 3S-3P. It's not the cheapest way to build a battery, but I couldn't find a 12V battery on aliexpress that looked like it could run an ebike. I plan to put the 3S modules in series with my 10S packs to run a (small) 48V motor.

I always use BMSes that have a multi-pin plug for the balance wires. Easier that way. I never solder the balance wires directly to the cells, but always to the tabs, connecting strips, etc.

As for the heavy wires-- first I quickly but heavily tin the pads on the PCB, trying to leave little pillows of solder on them. Then I tin the wire ends, after shaping them first into a spatula if necessary. To attach the wires, I heat up the ends until the solder is fully melted, then apply them to the pads until the pads flow, and get out of there as quickly as possible.

I use a cheap regulated 60W soldering iron, set to about 550F. For attaching heavy wires to PCBs, I use a tip like the one on the right:.

These are 3S, so a harness isn't necessary.