LifePo4 Voltage at .5C - .6C Discharge

otrfun wrote:
Our two GC2's (with 100% charge) drop below 12v while under a 100a - 125a load. When they reach 50% (at 100a - 125a), voltage drops into the low 11's while under-load.

What kind of under-load voltage can I expect with two 100ah LifePo4's (paralleled) with a 100a - 125a load? At 100% charge? At 10-20% charge?

To some degree it kinda depends on the nature of the load your battery is subject to - not all loads of the same amperage react in the same way - inductive loads are the most severe...

Having said that, given the LFP scenario of which you describe you should have no problem... With only one Aims 100a/hr LFP, I’ve seen about a 95 dc amp air conditioner load (Coleman Mach 1 p.s.) down to an amazingly uber low 6% SOC - but for the record this was inadvertent, not done purposely and with concurrent solar...

Along with a faster harvest recovery, this is another attribute of LFP’s...FWIW, my former set-up was with two GC’s in series, and now with just one 200a/h Aims LFP..

3 tons

FWC wrote:

otrfun wrote:
Any idea what the under-load voltage during this flat period would be?

I would hope LifePo4's could easily maintain at least 12.0v while under .5C - .6C discharge down to 20% SOC.

I hear lots of discussion about LifePo4 high charge rates at a given charge voltage, but very little discussion about discharge voltage while under heavy load. Got to have both to make the battery productive.

To answer this question - yes they should be able to maintain 12V under a < 1C load down ~10% SOC. Here is a typical discharge curve (multiply by 4 for a 12V battery):

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.

Even with no change in the R from wiring and battery, you still do better for staying above inverter alarm at 11v by going to LFP because the voltage/SOC voltages are higher as your starting voltage, for the wiring part.

Your wiring might have a 1 volt drop at 100 amps. Your full 6s start at 12.7 so you see 11.7 as you said in the OP. The full LFP is 13.6v so your 1 volt drop in the wiring will be showing 12.6.

You get the same effect going to AGMs where they have a 0.2v higher v/SOC, so you get an extra 0.2v wiggle room before hitting the 11v inverter alarm. LFP gives you way more wiggle room than that.

Not being mentioned is the battery bank AH, where within the same type of battery, adding more AH to the bank reduces R, so voltage drop is reduced with four 6s instead of two, eg.

I suppose there might be some factor there going from 230AH to 200AH that would normally increase your voltage drop, but could be trivial in the overall improvement from using higher voltage batteries.

Thank you, FWC! Very helpful. Been looking for a chart like this but couldn't find one.

otrfun wrote:
Any idea what the under-load voltage during this flat period would be?

I would hope LifePo4's could easily maintain at least 12.0v while under .5C - .6C discharge down to 20% SOC.

I hear lots of discussion about LifePo4 high charge rates at a given charge voltage, but very little discussion about discharge voltage while under heavy load. Got to have both to make the battery productive.

To answer this question - yes they should be able to maintain 12V under a < 1C load down ~10% SOC. Here is a typical discharge curve (multiply by 4 for a 12V battery):

Where are you measuring the voltage, at the battery or at the load, and what is between them?

Remind us of which batteries you are using?

Any idea what the under-load voltage during this flat period would be?

I would hope LifePo4's could easily maintain at least 12.0v while under .5C - .6C discharge down to 20% SOC.

I hear lots of discussion about LifePo4 high charge rates at a given charge voltage, but very little discussion about discharge voltage while under heavy load. Got to have both to make the battery productive.

That voltage drop is from a combination of wiring and battery. How much of the drop is due to the wiring? When you go to LFP that will help reduce the battery part of the drop AFAIK, but is there any change to the wiring when you swap them?

Besides the wiring to charging and loads, on the battery bank now there is the one series link this is part of the loss. The LFP cells have low R, but are connected with lots of wiring. Apparently some LFPs have better wiring than others that could affect voltage drop. You might have to specify which LFP to make an accurate comparison--not sure--that came up in the video where the guy rook apart an LFP and was commenting on its wiring.

They're going to be very flat down to about the 20% SoC level and then quickly dive.