match 18650 pack size to bms?

[haroldj1956]

Hi, I am new and building my first packs. I have an MPPsolar hybrid split phase setup (2 inverters parallel for 220V). I am just about ready to turn on the first inverter and start testing. I will be using 18650 diy packs in 7s configuration. I have been working on checking, testing, sorting, and building for quite awhile now. I have a7s60p pack almost ready and have 2 100A BMS modules. I started thinking (which usually gets me in trouble) that the weakest cells I used were 2200mah, and I want to run them only about .5C, that I should be able to pull 1amp/cell. My 7s pack has 420 cells, am I wasting capacity by making 60p packs? Could I still get 100A with a 40P pack?
Remember that this is just for the first 2.4Kw inverter so 2400watts X 1.15/24V=115A max. I will be adding more packs as I get them built, but wondering if .5C can be achieved with fewer cells? Or if I should have a bigger BMS?
Thanks in advance, eventually I will figure out the amps vs watts vs capacity stuff. Any links to good explanations appreciated.

 

[haroldj1956]

Oops, just figured out that 60 cells would be 1.9A/cell at 24V so a 100A Bms should work for now, I will just be pushing the cells a little harder than I was trying for but still under 1C. Is that correct?

 

[snoobler]

60P7S is the more common terminology. This means 60P cells in a group and 7 of those groups in series. 7S60P implies 60 70S batteries in parallel.

Since your not running LFP, you won't get the the cycle life of LFP. You'll need to be running 3.5-4.05V to keep between 20 and 80% SoC for about a 2K cycle life.

I trust you're matching cells such that all 60P modules have identical capacities. If you plan to parallel BMS, better to go with 2X 30P7S batteries, each with their own BMS. You do not want to run a BMS higher than its rated capacity. Depending on the BMS, you may want to run LOWER than the rated as cheap Chinese stuff may not be up to the task.

Your 1.9A calculation is correct (divide total current by # of parallel cells).

Glad to hear you're taking the time to test them. Hopefully, you've assessed internal resistance as well.

 

[haroldj1956]

Yes, and used repaker to match cells. I am only going to run 1500wattsof solar panels to start on each inverter and running just lighting circuits so I will have time to build more packs. Just anxious to see it up and running