LifePo4 Voltage at .5C - .6C Discharge

BFL13 wrote:


The AH thing is just baffling. In that one video the German guy says you can be down 50% in AH but more than half way down in Energy. Huh?

The whole idea is that amps is "power" and AH is "energy". Are there are two kinds of "Energy" ? Just bizarre.

...

You lost me after the first two statements.

I am pretty sure we have discussed this all before and it is pretty basic physics: Power = volts * amps.
Energy = Power * time.

With a battery the first half of the Ah drawn from the battery are at a higher voltage than the second half, so yes at 50% Ah you have used more than 50% of the energy in the battery.

Amps are not 'power', they are current or the rate of flow of charge. Ah is not energy, it is charge.

From there on you loose me...

But at any rate, this would probably make much more sense to you if you brushed up on physics 101 first.

After BMS cut-off you can charge the batt and eventually it will come back on. Until it comes back on you have no 12v in the RV from that batt.

If you have an inverter/charger, which needs 12v to even be on so you can use its charger, how are you going to use the inverter/charger? It is dead.

You can use your converter as long as you have a gen to run it with. The converter can also then supply 12v to the RV while you wait for the battery to come back to life.

If your installed gen needs that batt to start it, your gen can't be used to run the converter. A portable gen would start.

Solar could be used unless this happens at night.

So you need a way to deal with it in your RV. Too late if it happens and you never thought of that.

StirCrazy wrote:

FWC wrote:

StirCrazy wrote:

BFL13 wrote:
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.

wow, man is it good thing we have chargers built into our RV's or we would be totaly screwed..

actualy it would because a BMS cutoff point is above 5V, if I was setting up my own it would cut off at 11.9 but even if you have it cut off at 10.1V that is still above 5V.

Steve

This depends on the battery and the charger. Some batteries have a reset, either a physical button, or a switch in the bluetooth app (what mine has) to reset the BMS. Some BMS will also still show a voltage on the battery terminals, even after cutoff, but won't provide any current - this is often enough to satisfy a 'smart charger' without a lithium mode.

Most converters don't care if there is a battery present and will reset your BMS no problems, and many modern smart chargers (either solar or plug in) have a lithium mode, which will provide voltage even if the charger doesn't see a battery. There is a sub-set of older pre-lithium 'smart chargers' that will have an issue with this, but there is usually a way around this, either through solar, or using the vehicle alternator, or even a jump pack.

all the BMS I have been looking at have automatic reset once the battery charge rises a spicific amount it will turn the discharge function back on. they usaly show disconect at 10.8 and reconect at 11.3 type thing. but they are customizable so you can put them where ever you want. I can see where the older "smart chargers" would be an issue, but if some one is building a functional electrical system on a RV using LFP as a battery, why would you have a smart charger instead of a LFP compatable converter? I imagin we would find issues with any type of set up but a older smart charger would be some one conceniously pick older stuff and takes newer stuff out instead of building the system properly. I believe this is why people have so many issues with LFP, because a lot of systems seam to be throwen togeather haphazardly with out any forthought.

Steve

Caviar wants on a peanut butter & jelly budget.

3 tons wrote:
Uh-oh.... FLA Passports may soon be required?? Unsupervised, overcharged LFP erupts into a flaming inferno ruining camper’s holiday - Campgrounds to consider ban of RV’s with LFP’s - WT#O??

April fools!! (sorry, I couldn’t resist - just found myself caught up in all the grifting - lol :) )

3 tons

dont laugh, while it is a small hazard and highly unlikly with LiFePo4, people who are using old packs out of tesla's and nickel cad and such it is a real concern..

Steve

FWC wrote:

StirCrazy wrote:

BFL13 wrote:
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.

wow, man is it good thing we have chargers built into our RV's or we would be totaly screwed..

actualy it would because a BMS cutoff point is above 5V, if I was setting up my own it would cut off at 11.9 but even if you have it cut off at 10.1V that is still above 5V.

Steve

This depends on the battery and the charger. Some batteries have a reset, either a physical button, or a switch in the bluetooth app (what mine has) to reset the BMS. Some BMS will also still show a voltage on the battery terminals, even after cutoff, but won't provide any current - this is often enough to satisfy a 'smart charger' without a lithium mode.

Most converters don't care if there is a battery present and will reset your BMS no problems, and many modern smart chargers (either solar or plug in) have a lithium mode, which will provide voltage even if the charger doesn't see a battery. There is a sub-set of older pre-lithium 'smart chargers' that will have an issue with this, but there is usually a way around this, either through solar, or using the vehicle alternator, or even a jump pack.

all the BMS I have been looking at have automatic reset once the battery charge rises a spicific amount it will turn the discharge function back on. they usaly show disconect at 10.8 and reconect at 11.3 type thing. but they are customizable so you can put them where ever you want. I can see where the older "smart chargers" would be an issue, but if some one is building a functional electrical system on a RV using LFP as a battery, why would you have a smart charger instead of a LFP compatable converter? I imagin we would find issues with any type of set up but a older smart charger would be some one conceniously pick older stuff and takes newer stuff out instead of building the system properly. I believe this is why people have so many issues with LFP, because a lot of systems seam to be throwen togeather haphazardly with out any forthought.

Steve

The AH thing is just baffling. In that one video the German guy says you can be down 50% in AH but more than half way down in Energy. Huh?

Yup..To much info lol

FWC wrote:
Clearly they derate their battery. I am sure their actual customers don't complain about their batteries exceeding the nominal specifications.

Thanks for posting a reasonably complete data sheet from a reputable manufacturer. Hopefully this helps dispel some of the mythology about lithium batteries - that they should only be used down to 80% SOC and they can't be charged below freezing.
----

The AH thing is just baffling. In that one video the German guy says you can be down 50% in AH but more than half way down in Energy. Huh?

The whole idea is that amps is "power" and AH is "energy". Are there are two kinds of "Energy" ? Just bizarre.

When 2 + 2= 4 we believe that both 2s are the same size. It is just bonkers to say that the second 2 is smaller than the first 2.

Meanwhile, the other video guy is showing how they do coulomb counting integration to get AH. I am not at all clear that they are using the "20 hr rate" for saying the batt has 100AH. Something really funny is going on.

I am used to measuring the capacity of a battery. First you need to adjust for temperature and also for age, use the 20 hr rate for 10 hrs (or less time if the batt is lower in capacity than expected) and get to your 50% marker (SG in the case of FLA is the best marker for 50%. You cross-check that marker with the resting voltage after it settles once you stop the discharge. You have an AH count from your AH counter so it all works out.

Say your 100AH as rated batt is 90AH at the temp. and it is two years old, so you would allow 5% or so for age, you could be at 85AH = full, but your AH count at 50% marker is 40AH, so that means capacity is 80AH.

Ok , you are down more than you expected so it is sulphated more than you thought. Now you can try to desulphate it and try again.

So-- those BMS videos where they try to juggle age, temperature, matching voltage/SOC with AH count is nothing new at all. Been doing that for years. It is not unique to LFPs.

We even had arguments on here 10 years ago and further back whether it was correct to say your AH down from 10 hrs could just be doubled to get the capacity. Same issue-- 2 + 2= 4 and both 2s are the same size. If it is 3 + 1 then the 50% marker is in the wrong place.

You can then cross check again by recharging and see if it takes back the 40AH (allowing for charging efficiency factor in the AH counter)

I am a 2 + 2= 4 guy and if the LFP crowd say they are special so 2+2= 4 but their 4 is not the same as my 4, or both 2s are not the same size, then they all belong in a rubber room IMO.

Obviously ,they must know that, so what's wrong is they are not explaining what they mean by "AH" and where they are using the 20 hr rate or whatever they are using instead so their AH comes out the same as if they used the 20 hr rate.

Uh-oh.... FLA Passports may soon be required?? Unsupervised, overcharged LFP erupts into a flaming inferno ruining camper’s holiday - Campgrounds to consider ban of RV’s with LFP’s - WT#O??

April fools!! (sorry, I couldn’t resist - just found myself caught up in all the grifting - lol :) )

3 tons

Clearly they derate their battery. I am sure their actual customers don't complain about their batteries exceeding the nominal specifications.

Thanks for posting a reasonably complete data sheet from a reputable manufacturer. Hopefully this helps dispel some of the mythology about lithium batteries - that they should only be used down to 80% SOC and they can't be charged below freezing.

BFL13 wrote:
The graph here showing the G27 discharge curves at various amps (scroll down) relates to the OP's question.

https://www.trojanbattery.com/pdf/datasheets/27_Lithium_12.8-110.pdf

( It also shows what FWC was saying about how quickly voltage falls as soon as any size load is applied)

Another thing showing there is how the AH capacity varies around the 110AH rating. I don't understand how the 9a line can end above 110AH, when the 20 hr rate is 5.5 amps. Anything over 5.5 amps should have a lower capacity.

I was asking before about how they can use the 20 hr rate, which ends at 10.5v when the battery is rated to end at 10v. Something is going on with AH and LFP, but I don't know what.

The capacity vs temp graph is a reminder that below 25C, there is less capacity as shown. But SOC percentages are of the actual capacity.

So when saying the 100AH battery has 15AH at 15% SOC means I was wrong except at 25C.

Running the 100a load for 10 minutes is still 16.7AH, but what % of capacity? My 32% SOC marker might not apply at 10C--I have not done the math on that, but anybody who might use such a marker should.

Also that 100a load is a higher C rate discharge when temps are lower, so it would be like one of the lower amp lines in the G27 graph.

BFL13 wrote:
The graph here showing the G27 discharge curves at various amps (scroll down) relates to the OP's question.

https://www.trojanbattery.com/pdf/datasheets/27_Lithium_12.8-110.pdf

( It also shows what FWC was saying about how quickly voltage falls as soon as any size load is applied)

Another thing showing there is how the AH capacity varies around the 110AH rating. I don't understand how the 9a line can end above 110AH, when the 20 hr rate is 5.5 amps. Anything over 5.5 amps should have a lower capacity.

I was asking before about how they can use the 20 hr rate, which ends at 10.5v when the battery is rated to end at 10v. Something is going on with AH and LFP, but I don't know what.

The capacity vs temp graph is a reminder that below 25C, there is less capacity as shown. But SOC percentages are of the actual capacity.

So when saying the 100AH battery has 15AH at 15% SOC means I was wrong except at 25C.

Running the 100a load for 10 minutes is still 16.7AH, but what % of capacity? My 32% SOC marker might not apply at 10C--I have not done the math on that, but anybody who might use such a marker should.

Also that 100a load is a higher C rate discharge when temps are lower, so it would be like one of the lower amp lines in the G27 graph.

Perhaps you missed the below link that provides a far deeper understanding, else it’s become apparent that your multi-thread confusion tends to grow Legs....

https://www.youtube.com/watch?v=rOwcxFErcvQ

3 tons .....to those yet to be initiated, ‘Caution, Discernment Required, Now Entering the Rabbit-Hole Zone’