LifePo4 Voltage at .5C - .6C Discharge

otrfun wrote:
Our two Costco GC2's are rated at approx. 200+ ah. However, I'd guess-estimate only about 30-35% (60-70ah) of that is accessible with a 125a load (true since new). The *battery* voltage drop while under this much load is just too high. Our inverter goes into low-voltage shutdown around 10.9v. FWIW, we only have a .15v - .20v voltage drop at 125a (battery terminal to inverter terminal).

It would appear to be a very different story with the LifePo4's. For folks like us, who need access to 125a on a regular basis, we should be able to easily access 160ah (or 80% of the rated 200ah) of two paralleled 100 ah LifePo4 batteries while easily maintaining at least 12.0v while under a 125a load. If the graph that you posted is to be believed, it would seem two 100ah LifePo4's (i.e., Battleborn or Lifeblue) offer a huge performance advantage over our two GC2's.

The discharge curve for LiFePO4 is well established and verified. But one thing to note - this is for LiFePO4 cells. If you are using a 'drop in lithium' then there is also a BMS in the box, and how that handles high currents is function of how the battery assembler implemented the BMS.

As the drop in batteries have a single set of terminals for both charge and discharge, the BMS has to use transistors (MOSFETs) for the charge/discharge disconnect. Depending on how it is implemented, these can add significant resistance to the battery. This would be the reason BB limits the charge/discharge to 0.5 or 1C. The cells them selves are fine with 3C day-in day-out, but the BMS will overheat due to the resistance.

I am sure others who are actually using large banks will chime in, but if you are planning on using large loads, a bank of prismatic cells with a standalone BMS which uses contactors maybe a better option.

You will have seen that there are two "phases" to running the MW on inverter off the battery bank.
-initial voltage drop, say 1 volt as observed in the OP. This drop is the same whether you start at 90% SOC or at 50% SOC, same 1 volt drop.

-- gradual drop in voltage from then on as the load is running until you stop or the inverter quits. The flatter voltage curve for LFP helps there.

You might also get an advantage in running time in the second "phase" with the LFP compared with the same AH worth of FLA. Makes no sense to me, but I am still struggling with that notion. It is all this Power /Energy thing

You would have to compare the batteries' Discharge Rate Constant Current tables to see if the times are longer for LFP than for your 6s.

As to my previous (i.e. air cond to 6% SOC), I should clarify that this was with a single Aims 100a drop-in (with built-in BMS) and never got a inverter low voltage alarm...This alarm ‘non-event’ may have been due to the concurrent solar (??), but either way (though unintentional...) proved to be a substantial, non-theoretical test for the abilities of single 100a/hr cylindrical cell drop-in !!

FWIW, I would also agree that ‘phases talk’ in this stated context makes little sense...

3 tons

Sounds like Aims may use a lower resistance BMS, which is a plus.

If it is a problem, another option would be to lower the inverter alarm point, or turn it off completely. The BMS will save you if you run things down too far.

If you did have BMS cut-off, don't forget you will need a way to get the battery bank going again by recharging it. BB suggests you have a charger that can do that. EG my Vector smart charger will not do anything until the battery voltage is above about 5 volts, so it would not work on a cut-off LFP.

So first you would need to use a "dumb" charger. I think the converter would work, not sure. it would need power from a portable gen. Your installed gen needs a starting battery, but the LFP is cut-off and it might be the gen's batt where they use the House batt for that. Your inverter/charger would not work. It needs some battery voltage to operate at all, so its charger would not work. Solar if it happens in the daytime. Jumper cables from another good battery is a way.

This is just one reason why I finally chucked my Vector charger (labeled B&D) into the garbage can - it would always dump a code to find a reason not to work regardless of battery type - BD service was a waste of time...For some strange reason, I found myself feeling much better after tossing it in the garbage... No more B&D for me!!

otrfun,

Using a 12 volt form factor rather than twin 6 volt batteries improves greatly on voltage drop under load, simply because there are twice as many cells.

Some LI battery management systems limit discharge rate to 1 C.

There are a host of other hoops to jump through with LiFePo4.

That said, the LI chemistries behave much like the old Nicads--where they supported the voltage well and then dropped off a cliff.

My next battery bank will be SiO2 chemistry because they meet my needs, do not sulphate and there are many fewer hoops to jump through.

Any battery type can be designed to fit the needs of a particular end user.

Please wire your new bank in a balanced manner.

I have 2 110 Ah lithiums, currently at half-charge. With minimal load (propane fridge and a couple USB chargers running), the batteries are at 12.9 volts. My inverter and household toaster draws about 75 amps, pulling the batteries down to 12.2 v. I switched the fridge to 12 v as an experiment, so now drawing 95 amps, and got 11.4 v. In the past, using a Keurig, I’ve seen 11.8 v on the batteries. These days I’m making coffee on the propane stove.

My Keurig draws about 124a from a single 200a/h LFP, but this last for less than 2 min - repeated cups are no problemo. With inverters, proper battery cable sizing is paramount to reduce voltage drop...I went a bit overkill with 0004 aught, 3’ length...

3 tons

Appreciate everybody's inputs. Always learning something new.

Called LifeBlue (lifebluebattery.com) on Mon and inquired about the voltage drop with their 100ah LifePo4 battery while under heavy load. Surprisingly, he offered to test one and get back with me. Received an email yesterday with the results. With a fully charged battery, battery voltage was 12.8v at 80a, and 12.7v at 100a. Pretty impressive. Based on the graph FWC posted, I'd guess-estimate battery voltage while would drop to approx. 12.0v at 15% SOC with a 100a load.