Building a pack of Lithium Iron-Phosphate cells. Need help with balance theory/charging.

Discussion in 'Power Electronics' started by Tony Kuban, Dec 29, 2016.

  1. Tony Kuban

    Thread Starter New Member

    Dec 29, 2016
    Hello gents, new to the forum, been searching for a place to find some smarter individuals on this matter.

    I'll start here. I'm building a home PV system. Iv'e made my own solar panels, and since grid tie is very difficult thanks to WE energies, i'm forced to use a battery storage system. Iv'e looked all around at different types of batteries and landed on LifePO4. They are safe, have a high energy density, can discharge pretty deep, i read 80%. Also the cycle life should be a lot better than LA. Specifics on goals below.

    So here it is, i have 160 3.2v 5500mAh lifePO4 cylindrical cells for a grand total of around 2.8kWh of lithium cell. Not bad for $300. My goals are to create a pack, power a 24/7 draw, whilst charging the pack on a 6 hour cycle through the day with the solar. Questions on solar i can pass along to my solar buddies, what i really need help with is the charging system/battery management system, and the overall design of the pack.

    Battery ????'s

    1)With 160 cells... do I want to parallel as many as I can, and series those parallel lines to achieve my desired voltage?
    i.e. Wire 4 rows of 40 parallel cells to achieve 3.2v@ 220 amps, and then series those 4 chains to attain 12.8v @ 220amps?

    2) Choosing a voltage. Since i make my own panels.... voltage is not really any special requirement. I don't need a 12.8v system. It could be a 24v or 48v system. I would assume higher voltages are just better for power transmission... but the transmission distance to my AC inverter is only going to be a few feet anyways. Although using life batteries... I can see finding a charge controller that can balance charge 16 cells might be tricky. More or less i need help verifying my thought process on this.

    3)Balance charging
    Is it necessary with LifePO4? I see some say its absolutely critical... and others who say Life can balance itself much like Lead acid due to its larger than Lipo overcharge tolerance.

    4)Balance charging with MPPT. Do MPPT chargers have this ability? Would it be desirable? Otherwise on the charging front i see hobby chargers work very well. Its just a hobby charger wouldn't work with a solar application. I need some form of MPPT that knows how many cells i have much like a hobby charger, and with the abilitiy to balance those cells.

    5)BMS iv'e heard good and bad things about BMS's. Some have fried and destroyed packs. Others have prolonged battery life to 5 years. My question here is; would the answer to my solution be to run a standard 'off the shelf' MPPT to a BMS that manages the cells? I feel that's where i'm headed.

    6)BMS/MPPT charger ratings.

    When i see a battery management system with an amperage rating, is that rating intended to be above the value of the battery? Or is it more or less the max discharge/charge rate? Same question on MPPT's. I assume the latter and more specifically it would pertain to the max charge rate of the battery or the max drain rate, which my load will depend on as well.

    That's all i can think of for now! Any help super appreciated guys!
  2. Tony Kuban

    Thread Starter New Member

    Dec 29, 2016
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    Sorry but a 6-question post of 2000 words is beyond my attention span.
  4. Tony Kuban

    Thread Starter New Member

    Dec 29, 2016

    That's the response of a "AAC Fanatic!" lmao... I guess i should reduce my expectations regarding the knowledge here quite a bit already.