Advice for Building a 72V Battery

[mabdelwanis]

Hello!

I hope this message finds you well.

I am in the process of building a 72V battery for my BBSHD with a VESC controller, and I currently have four Hilti 36V 9Ah batteries. My plan is to combine them, achieving a total of 72V and 18Ah. The cells contained within are the US18650 VTC6, which has a continuous discharge rate of 30A. However, I have some uncertainty regarding the built-in BMS of these batteries.

Would you recommend that I sell these batteries and pursue a 72V 25Ah option instead? If so, I would appreciate any suggestions for the best cells to consider for my setup. Additionally, if you know of any reliable suppliers from whom I could order these cells or a ready-made battery, I would be grateful for that information. I have seen many options from Chinese manufacturers, but I want to ensure quality and avoid any potential issues with cloned versions of Samsung or Melonia cells.

Thank you very much for your assistance!

 

[docw009]

When you put two batteries in series, the BMS transistors need to withstand the combined voltage of both batteries, or 84 volts, That's a design criteria. How do you know? You get the part numbers of the MOSFET's on the BMS and look up its data sheet.

For example, this is a $12USD 36V BMS for an ebike battery. The transistors are RU6888R, which are rated for a max voltage of 68V. Could not put two of these in series safely. Maybe your Hilti is better. Plenty of people have probably put two tool packs together, Just pointing out what a design engineer must consider. There was a fellow here a few years ago that put two 48V packs in series. Now you have 110 volts. One of his batteries caught on fire while riding the bike..



Aside from that criteria, it would seem that four tool pack batteries will take up a lot more space than a single battery. You have to figure out a mounting system, etc. You will be charging them separately? On the plus side, they are a commercial battery pack and probably better built than an ebike battery,

 

[mabdelwanis]

When you put two batteries in series, the BMS transistors need to withstand the combined voltage of both batteries, or 84 volts, That's a design criteria. How do you know? You get the part numbers of the MOSFET's on the BMS and look up its data sheet.

For example, this is a $12USD 36V BMS for an ebike battery. The transistors are RU6888R, which are rated for a max voltage of 68V. Could not put two of these in series safely. Maybe your Hilti is better. Plenty of people have probably put two tool packs together, Just pointing out what a design engineer must consider. There was a fellow here a few years ago that put two 48V packs in series. Now you have 110 volts. One of his batteries caught on fire while riding the bike..

View attachment 370052View attachment 370051

Aside from that criteria, it would seem that four tool pack batteries will take up a lot more space than a single battery. You have to figure out a mounting system, etc. You will be charging them separately? On the plus side, they are a commercial battery pack and probably better built than an ebike battery,

Thank you @docw009 Unfortunately, the BMS was filled with epoxy, so I can't see any components to analyze its behavior. I will order a ready-made 72V 30Ah battery and additionally sell the Hilti battery. For reference, I have attached some photos of the Hilti 36V/9Ah battery from the Inside, the cell inside is Murata/Sony US18650VTC6 I hope someone finds them helpful.

 

[E-HP]

It looks like the balancing wires are accessible. Maybe you can place two packs in series, and use a single port 20s BMS with the existing balancing leads?