18V Portable Backpack battery?

[DIY]

So a few power tool makes portable backpack battery to extend runtime and put the weight of the batteries on your back.
Ex 1: Makita PDC1200 https://www.makita.co.nz/products/model/PDC1200

Ex 2: Husqvarna BLi520X 36-Volt 14.4AH 520Wh
Ex 3: STIHL AR 1000 36-volt Backpack Battery

Ex 4: Hitachi BL36200 21-AMP Lithium Ion Backpack Battery, 36-Volt

Ex 5: EGO Power+ BH1001 Backpack Link

Looking to DIY my own version for 18v tools, specifically my Milwaukee, Ryobi tools. I have a few power hungry tools that would benefit from longer run times (e.g. angle grinder, blower, mitersaw).
I guess as long as my final voltage is in the 15-21v range (since 18v tools are just a 5S configuration), I should be find making a battery pack in that range.

What would be some battery recommendations if I want to match that Makita version with 1200Wh?

My thinking would be a 6S configuration using CALB CA100 3.2V 100AH LiFePO4.
With a nominal voltage of 3.2V, using 6 of them I would be at 19.2V.
Do they make a 6S BMS for LiFePO4?

In regards to the battery to tool adapter, I would just gut some old battery (I have a few 1.5Ah that I would remove the cells and solder some wires directly).

Thoughts on this approach?

PS, if Milwaukee or Ryobi made one, I would much rather buy it. But I doubt they ever will.

-------- EDIT

After searching YouTube, it looks like a few people have DIY their own for 18V battery backpack
1. Dewalt 20V 50Ah backpack

2. 5S configuration, but max voltage is only 18.5V

 

[Substrate]

Instead of a bms, consider a balance-charger. (Slang term - lipo charger). These do a variety of chemistries, including LiFePO4. (usually seen as LIFE) in the charger.

In fact, higher powered 10A units like the HTRC C150 balance charger might be suitable for this job. It has a 6S limitation. The balance leads run from the batt to the charger instead of a bms. This way, when a runaway cell is detected, instead of disconnecting, it throttles back the charge current.

Too much to go into here (such as using it as a single-cell 1S charger with no balance leads for initial top-balance if you want to go that way).

Sure, it looks like a toy, but it does the job as long as one isn't afraid of initial menu setup. (like changing the chemistry from lipo to LIFE) !