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

Thread Starter

Tony Kuban

Joined Dec 29, 2016
3
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
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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!
 

splud

Joined Jun 30, 2013
16
Okay, this is a really stale question, but AAC highlighted the thread while I was reading something else, which begs viewing it, and finding NOTHING ever answered.

One of the tasks a BMS has is cell balancing. A LOT of series cells is going to be near impossible, if not simply impractical to manage. That's issue one. If a BMS is killing batteries, it's either the wrong BMS for the chemistry, or just a poor BMS.

Ideally, if the end goal is to invert and to a higher voltage, you want the system at as high a voltage as you can manage - if you're pulling an amp at 120Vrms, you've got to provide over 10A at 12V DC (conversion losses, peak, etc), and the effect of resistance in the wires is going to lead to greater losses: a drop of 1V at 12V is significant. A drop of 1V at 60V is much less so, but all things being equal, you'd experience a drop of 0.2V at 60V for the same _power_ delivery because you'll be pulling less current.

However, higher voltage still means more series cells, so you'd want to strike a balance. Also, as the voltage goes up, you have more safety issues, esp with DC.

Your solar modules should be outputting some voltage at MPPT, and that, or a couple volts or so below it, is ideally about where you might want to engineer a battery pack, because you'll lose less to conversion inefficiencies.

Investing in higher capacity LiFePO4 cells would in general be easier than a slew of smaller ones. I have a bunch of 50Ah cells, with threaded studs (no tack welding to make connections!). just eight of them are good for circa 1.3KHw, at 28.8V or so. Not so many connections to make (or go wrong), few enough to manage with a BMS.
 
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