LiFe cell internal resistance

Suntrekker

After testing this battery bank (24V 280AH), it should meet impulse drive. https://youtu.be/sFGmhvx1elk

Suntrekker

Bill, I’m interested in operating the 24V battery pack in SOC range of 10% to 90%.  ...looking at different LiFePO4 charts, I see different voltages but in general 10% = 24V and 90% = 26.6V.  What do you think?  Thnx

BB.

ST,

I am not any sort of expert on batteries or Li Ion chemistries... I suggest following the manufacturer's specifications.

Also, remember that Li Ion cells are very voltage sensitive (over/under voltage). And besides the overall bank voltage of ~24 volts--You have the per cell voltage issue (balance vs unbalanced cells).

Cell balancing when done manually and no BMS--You have the +/1 xx% min/max SOC for the overall bank voltage limits, which are really based on the per cell limits. And without a BMS, you generally want a larger "buffer" in min/max SOC voltage to prevent one or two cells from going out of spec (voltage wise).

If you have a BMS--You have the (more or less) Passive type that simply monitor per cell voltage and alarm/possibly shut down charging/discharging (open relay, short out fuse)--Or integrate with charging/discharging sources and shut those down.

And you have active BMS that also balance cell voltages....

Personally, I always like to have a bit of a margin (such as errors in voltage measurement or out of balanced cells) -- Because I don't want a BMS that cuts out the battery bank. At best, that leaves you "in the dark" until you get the system reset... At worst, many charging sources are not "voltage regulators" in the sense that most people expect... They do not (for example) hold 26.6 volts when the battery is disconnected--But they can easily over voltage the DC bus (by some or a lot) at least for a few seconds before the secondary charger shutdowns can take place (and/or an inductive kickback as current to battery goes from max charge rate to zero amps). And during that time, your DC loads can be over-voltaged and damaged/ruined.

So, while running at 10%-90% of rated SoC/bus voltage is certainly within specifications... looking at the system as a whole (including BMS) will give you a better answer (such as an "alarm" at 20%-90%, and battery cutoff at 0-10% and 95-100%).

Others here have much better experience than I--So perhaps they will chime in here.

-Bill

Suntrekker

Great info!  

And I’ll install an 8S active equalization balancer for the no BMS system.  ...should you see a mushroom cloud from Northern AZ that’s me ; )

Suntrekker

Well my enthusiasm of balancing cells in short order has faded after countless passes.  And when I say balance I mean 8 cells remain at the same millivolt, no change overtime.  As I passive balance (in parallel), an equal millivolt reading ticks upward every 24 hrs.  Hopefully they’ll come to rest soon and “truly” be balanced.  It’s patience and laying a solid foundation.  ...we’ll see.

Suntrekker

After passive balancing for 4 days, it seems 8 cells will settle around 2.632 volts (began at 2.585V).  My hope is all cells will be near equal voltage at 90% SOC...in other words no rogue cells.  The idea may be faulty but worth testing. 

If tests are successful, the battery can manage itself, little monitoring. 

Suntrekker

Bottom voltage balance:  The latest is eight 280AH cells are still (passive) equalizing after a week but getting close, thereafter all cells are inline (no BMS necessary imo).  That said, ‘active’ top or bottom balancing may require a BMS due to residual voltage between cells.  ...that’s my observation.

Suntrekker

Passive balance began at 2.585V per cell, now at 2.658V.  Once cells equalize, I can’t see issues down the road. 

Suntrekker

...still passive balancing 8 cells, I think complete soon.  The bank voltage hasn’t changed today (so far) but definitely time consuming.  What’s interesting is it began at 2.585V now at at 2.680V...a huge change!  I’m banking on once all cells are ‘in sink’, they’ll breeze through the SOC range (10-90%) without spikes. 

Suntrekker

And I see no need to bottom balance ‘in sink’ cells to 2.5V (x8=20V) if minimum SOC is approx. 24V (10%).  IMO 

I feel good about the end result though could be a bust.  Anyway parallel voltage is steady at 2.680V!  

Suntrekker

Passive balance is complete and I learned a lot.  I started at 2.585V per cell and ended with 2.705V...a huge upswing.  Now that the cells are balanced, I’ll connect them in series (24V) and charge to 90%.  They’re in sync and shouldn’t deviate much in voltage (charge/discharge), that’s my guess.

Suntrekker

After passive balance, charge the 24V battery bank.  Cells are sandwiched between aluminum plates (separated by thin plexiglass to reduce friction)...no sign of bulging.

Suntrekker

Charging fast.  Target is 26.60V (90% SOC).

Suntrekker

Charging has slowed down considerably as the battery nears 26.60V.  ...continuous monitoring.

Horsefly

The charging curve is - by the nature of it - different than the discharge curve. 26.6V (3.325V/cell) is probably not 90%. Remember that once you stop charging the voltage will settle back down to its "resting" voltage. Most tables you will find are showing the SoC at rest.

The charging voltage at any particular SoC will be higher if the charging current is higher. Here's one chart showing an example of how it can change:

Based on this chart, I would think that 3.325V/cell during charging is probably closer to70% SoC.  I assume you are using 3.325V/cell = 90% based on resting voltage. I'm just suggesting you may want to rethink that.

Also, if you take it off the charger as soon as the target voltage is achieved, you will be even farther from 90%. The lower the charging target voltage, the longer you need to keep the cells at that "absorption" voltage. Best wait until the tail current drops down to 1 or 2 amps.

I know you are going without a BMS, and working off a bottom balance. So I have very little in common with you. I would never do what you are doing, but I guess you would never do what I'm doing. 

Suntrekker

Thnx Horsefly, any information is valuable!  “I assume you are using 3.325V/cell = 90% based on resting voltage. I'm just suggesting you may want to rethink that”.  Ok thnx for explaining the difference between SoC (at rest) and charge/discharge charts.  I didn’t know.

And I’m charging Constant Current at 10 amps, I’ll switch to Constant Voltage mode w/less amps and note the voltage drop at rest. 

Suntrekker

The battery (at rest overnight) is 26.50V.  The voltage difference between cells is 2mV.   I’ll charge to 26.60 (90% SOC) and install this week, test system and hope a go.  ...definitely a learning experience and I appreciate members help!
Dispersed camping (4-6 months) begins in 3 weeks!

Suntrekker

Battery charge has stabilized around 90% SoC and cells aren’t bloated, that’s good.

Suntrekker

After 12 hour rest the battery is about 90%. The cell voltages are; 3.325, 3.326, 3.326, 3.327, 3.324, 3.324, 3.326, 3.326.  This is my charge limit without a BMS.

Suntrekker

I’ll finish wiring solar system this week, lithium battery connected and charged (90% SOC), test, boom or bust ; )

Suntrekker

The solar install is complete, ready to test! The 640W/280AH system occupies a 2.5' x 3' space.

BB.

Nobody ever likes to talk about it... But what about fire?

For telephone systems that I helped design decades ago--We had to show that there was access to "burning components" with a fire extinguisher... Having a punch out/cut out to access the electronics box vs needing to unbolt access panels/remove bedding/seat covers/etc. to access the are--Are things that I worry about. In an RV, rear access doors--But backed up against a wall/vehicle parked behind that limits access to area....

-Bill "ignore me" B.

Suntrekker

BB. said:

Nobody ever likes to talk about it... But what about fire?....

Good point.   My bunk is above the system so I need to be especially aware of the possibility.  The entry door is unobstructed and there’s a little smoke alarm above the battery.  Maybe add hinges or clips to the air vent for quick removal in case of fire. 

BB.

Then the question of dry chemical or CO2 (or similar).

Dry chemical is like spraying baking soda powder everywhere. Messy to clean up and probably ruin electronics.

CO2 nothing to clean up.... Not great for wood& paper fires. Much more expensive vs typical dry chemical.

Maybe one each?  Overkill or over thinking..

Bill

mike95490

The Li battery should have a recommended fire extinguisher type to be used.  Are these LFP / LiPo4 cells or some other chemistry?

Note: Fire extinguisher

• Water, carbon dioxide, dry chemical powder and foam are the most effective means to extinguish a Lithium

Ferrous Phosphate (LFP) battery fire

• Use ABC Fire extinguisher, if the fire is not from battery and has not spread to it yet.

Suntrekker

I have a BC extinguisher but will check out something for lithium, thnx! 

Battery monitoring (Bluetooth) showing solar system at idle.  I need to taylor the app for my system. 

Suntrekker

Solar has signs of life but I need to learn the battery monitoring app.  

https://youtu.be/QuoKNZjr8_U  LOL

Suntrekker

Solar online, adjusted the SCC parameters.

Suntrekker

I’ll turn-on the inverter next.  And because the system is somewhat oversized (inverter, converter, SCC, wire), l haven’t noticed any heat dissipation.  Of course I’m testing and not in the real world.

Suntrekker

All AC/DC systems are a go. Battery is strong and solar charges quickly.