LiFe cell internal resistance

Suntrekker

I set Outback charge parameters about 90%. A snapshot of individual cell voltages at rest; 3.347, 3.347, 3.346, 3.347, 3.345, 3.345, 3.346 and 3.346.

Preparing for launch...check electrical, plumbing, load cargo.  On the road, I’ll start a new thread of my travels and solar performance.

Suntrekker

Solar by night, testing the 24V battery.

Suntrekker

Nights are below freezing so I crank the heat in the morning to thaw out the lithium battery (avoid charging below 32 F).  But when traveling, it’ll be room temperatures. 

Suntrekker

Added a lithium battery active balancer to equalize cell voltages for efficiency.

Suntrekker

Battery update - I’m conditioning the 24V battery by slowly discharging (.35A for 2 days) and voltage remains at 26.  Also discharged to 24.2V (10% SoC). 

The active balancer works, cells are within 1-2 mV at rest.

BB.

I love it when a plan comes together. (A-Team movie).

-Bill

Suntrekker

Thnx again Bill for sharing your knowledge and encouragement!

A few hiccups as I learned but the end result is what counts, a happy camper.

I begin a 4-6 month journey this month, test the system in the real world.  …my GS is all excited : )

Suntrekker

A new travel thread next w/drone pics.

BB.

Have fun!

-Bill

Suntrekker

Thnx!!  Loading the RV….

Suntrekker

RV solar update-

System performs beyond expectations on the road. The 24V 280AH LiFePO4 battery has large capacity to power lights, radio, refrigerator, inverter, converter, etc. Only thing I haven’t tested is the new 12 seer AC window unit…shouldn’t be a problem. And the REC 640 watt mono array quickly charges on very cloudy days….

System hardware and wire is somewhat oversized and generates minimal heat. 

…I’m a happy camper : )

Suntrekker

Question about the relationship between charge, rest voltage and SoC.
The chart shows 29.2V charge to achieve 100% SoC, rest at 27.2V.  If 26.6V at rest is 90% SoC, what is the approx. charge voltage?  Any thoughts appreciated!  

Horsefly

Where did you get these numbers?  In general, SoC doesn't translate well to voltage with LiFePO4, so as a rule you shouldn't really use it.  Also, collective wisdom says don't charge to 3.65V. Although I don't know that there is lots of evidence, everyone seems to believe that repeatedly charging to the spec max voltage of 3.65V is bad. A more practical reason is that the cells will diverge quite a bit at the top (3.65V), and if you charge the entire pack to 29.2V (3.65V per cell) you will be likely to have a cell or two that go up close to 4V. Normally a BMS would stop this, but if I recall correctly you are not using a BMS.

So for my two 8S/24V packs, I charge to 28V (3.5V/cell), and I know others that charge to 3.45V per cell. You can still get to 100% SoC at these lower voltages, by just letting your charger stay in absorb for a bit, until the current drops to 1-2% of the C rating for the battery. If you charge to 3.65V the current will drop down to 2% very quickly (like maybe 5 minutes). At 3.5V I see the current drop down still quickly, but more like 10-15 minutes.

Your table's resting voltage for 100% SoC is pretty much correct. After reaching 100% and the charging stops, the voltage will drift down to about 3.4V per cell.

Suntrekker

Yeah I see different LiFePO4 charts online but understand your explanation, very helpful!  Thnx!

Suntrekker

Solar data log for 48 hours - amp production vs. load.  Dip below zero represents load like refrigerator and coffee maker.  Cycle is consistent.

Suntrekker

Non-BMS 640W/280AH system performance update.

After one month, off grid system functions as designed. The LVD, HVD and active cell balancer are important. While charging, cell voltages differ by 10 mV, charge parameters set at 26.8 float and 27.6 bulk/absorb. The coulomb meter recorded 165AH gain on a sunny day! 90% SoC is max without a BMS.

Suntrekker

Data log shows AC cycles and healthy battery voltage.

Suntrekker

Battery performance update - After 4 months the 24V Life battery (w/out BMS) performs well!  I highly recommend a quality active balancer to level individual cell voltages…VM shows within a few mV.  It’s the heart of this system along with a smart battery monitor w/LVD and HVD.