BMS for a 2p5s pack? MPPT charger flexibility for Li-ion CC/CV thresholds?

[tylerssims]

Hello fellow techies, I am a noobie to power systems design and design in general.

I have been developing a solar power system for a solar robotics project and I wanted to get some feedback on a few aspects of the design that I am unsure about.

The parts and system specs I’m working with:

-Texas Instruments TIDA-010042 MPPT Solar Charge Controller
     -https://www.ti.com/tool/TIDA-010042
     -I've attached the msp430 firmware for it, main.c, in this post

-2p5s Li-Ion battery pack

-18.5V PCM/BMS with cell balancing and equilibrium function. 20Amp limit.           
     -https://www.batteryspace.com/pcmwithequilibriumfunctionfor185vli-ionbatterypack20amplimit.aspx

-Maximum 20V 5.4A primary load (BLDC motor)

-210W 12V solar panel with the following specs
     -22.4V Vmpp
     -9.30A Lmpp

My 3 Questions: 

1) Given that I will be using a 2p5s cell configuration, I would like to use only 1 BMS for the entire pack. From my understanding, if I arrange the cells in parallel BEFORE I series them, I can use one PCM/BMS, but the balance system will be slow to react to imbalance. Can I do this, and use one BMS for the pack, with a connection for each cell pair in parallel (configuration a). Is that the best configuration if I want to use one PCM/BMS? (See attached image).

My specific charge controller is not specifically for Li-Ion batteries, but considering it gives control in the firmware over the CC/CV thresholds, I don’t see why I can’t adapt its charging profile properly for a Li-Ion battery pack. [Apparently the TIDA design is for Li-ions]. Is there any real difference aside from voltage/current settings between normal and Li-ion specific MPPT chargers? Don’t the bulk, absorption, and float stages of charging all happen with a Li-Ion charging cycle as well, just with different voltage thresholds? (I am aware of the max 4.2v*Ncell max for Li-ion cells.)

In my charge controller firmware, it lets you define whether you are going to use a 12/24/48V battery system. Additionally, I can change the following parameters in lines 214-221 of the main.c file:

const float CC_LIMIT           =   20.0;        // Set the maximum charging current

// Typical battery threshold values               //System Voltage | 12V    |  24V    |  48V    ||
const float CC_TO_CV_LIMIT     =        12;         //                | 14.2V  |  28.4V |  56.8V  ||
const float FLOAT_VOLTAGE      =         13.8;       //               | 13.8V  |  27.6V |  55.2V  ||
const float BATTERY_CUTOFF     =        10.2;      //                | 10.2V  |  20.4V |  40.8V  ||
const float BATTERY_RECONNECT  =     11.2;        //              | 11.2V  |  22.4V |  44.8V  ||

2) My question here is, to what precision can I alter the integer and float values? Is there anything stopping me from entering an arbitrary CC_TO_CV limit of 18.5v or 18.3v for my 5s2p Li-ion pack?

I wouldn’t expect it to work with voltage values far greater than or less than the 12-48V range the controller was designed for, and I assume that I can't expect it to do floating-point calculations past one decimal place, but I do want to understand the limitations between the firmware and the hardware.

3) Is the INA240 Ultra-Precise Current Sense Amplifier ultimately what limits the precision of voltage thresholds I can set? I had a quick look at the datasheet and can see that it discusses certain voltage accuracy/ tolerance values.

Cheers and thanks for reading,

Tyler

[Faringdon]

Yes (a) is best. The paralleled cells "take care of each other".
I think you can charge them any how you like , but max charge current should not exceed 1C usually.

Yes i think if you tail off the  charge current as the v(cell) gets high near 4v2, then you can press more charge into the cells...if you want to bother with that.

Also, Vmax to 4.3V as you say.
Dont charge or discharge below -10degC (?)
For a  simple charge management....just stop charging when any cell(pair) reaches 4.3V
And just stop discharging when any cell (pair) reaches 3.2V.

You can balance if you like...the best balancers are an  isolated flyback which takes from the stack and gives to the lowest voltage cell-pair.
Balancers just help you squeeze that last drop from the stack if the cells arent well balanced.
But to be honest, if the cells really badly need balancing, then they are going to get worse and worse...untill eventually the balancer just doesnt cut it.

I dont think you need great accuracy for the voltage and current. InAxxx is fine.
If you want to charge the 5stack to 18V, then i dont see why not.

Sorry if i missed what you are really  asking about.