Domestic batteries replacement

[Poey50]

Why must it be AIC requirements for LFP will exceed AGM - what am I missing?

Ohm's Law. This states that the current flowing in a circuit is directly proportional to the applied voltage and inversely proportional to the resistance of the circuit, provided the temperature remains constant.

As said, LFP sits at a higher resting voltage than lead acids for most states of charge. It also has much lower internal resistance. Both those are established facts. Therefore the current flow into a dead short must be higher for LFP than AGMS.

 

[geem]

? But the above suggests pssc for lifepo4 battery is 6C = 600A for a 100ah lifepo4.
My 100AH AGM specs say short circuit current = 2550.....equivalent to 25C.

Why must it be AIC requirements for LFP will exceed AGM - what am I missing?

Internal resistance of LFP is a fraction of that of lead. The internal resistance limits current flow. The potential current on LFP is so much higher.

 

[ckris]

Ohm's Law. This states that the current flowing in a circuit is directly proportional to the applied voltage and inversely proportional to the resistance of the circuit, provided the temperature remains constant.

As said, LFP sits at a higher resting voltage than lead acids for most states of charge. It also has much lower internal resistance. Both those are established facts. Therefore the current flow into a dead short must be higher for LFP than AGMS.

Still not with you.

AGM battery at 12.8v and IR 4.8 milliohms = short circuit current 2666a.
LifePO4 drop at 13.6v and IR 20 milliohms = short circuit current 680a.

( I know AGM specs said short circuit current 2555a but assume they used a mid voltage rather than highest)

I understand an individual lifepo4 cell has low IR, but when assembled as a battery the IR of the whole assembly is higher which means potential short circuit current coming out of the box is lower. Isn't that what the links migs found are indicating?

 

[Poey50]

There's a lesser known law - 'Battery manufacturers are notorious liars.' I wouldn't trust manufacturers figures.

If you think that LFP packs have higher internal resistance than AGMs, how do you account for the established facts that LFP takes charge very rapidly whereas lead acid needs a lengthy absorption stage?

 

[ckris]

There's a lesser known law - 'Battery manufacturers are notorious liars.' I wouldn't trust manufacturers figures.

And the research paper linked to by migs above?

If you think that LFP packs have higher internal resistance than AGMs, how do you account for the established facts that LFP takes charge very rapidly whereas lead acid needs a lengthy absorption stage?

I dunno, I am just a bloke on the internet trying to have more fun sailing.

I would offer that AGM IR reduces as it reaches higher SOC......but it doesn't start charging faster so there is more to charge acceptance than IR.

I also suggest there is a big difference between what happens at an individual LifePo4 cell level and what happens at the terminals of an assembled battery. Quite possible the IR of an individual cell can be low but a pack containing wiring, BMS etc can be more than the sum of the cells.

 

[Poey50]

OK, I'm done. Good luck with it.

 

[geem]

And the research paper linked to by migs above?


I dunno, I am just a bloke on the internet trying to have more fun sailing.

I would offer that AGM IR reduces as it reaches higher SOC......but it doesn't start charging faster so there is more to charge acceptance than IR.

I also suggest there is a big difference between what happens at an individual LifePo4 cell level and what happens at the terminals of an assembled battery. Quite possible the IR of an individual cell can be low but a pack containing wiring, BMS etc can be more than the sum of the cells.

There is no difference at cell level. As you add cells the voltage of the pack increases so ohms law holds true. A full pack of lithium cells in series will be virtually identical resistance as a single cell

 

[PaulRainbow]

OK, I'm done. Good luck with it.

Might i refer you to post #20

 

[Poey50]

Might i refer you to post #20

I'm a slow learner.

 

[ckris]

There is no difference at cell level. As you add cells the voltage of the pack increases so ohms law holds true. A full pack of lithium cells in series will be virtually identical resistance as a single cell

Sorry, I know this might be tiresome and way off the topic of this thread but I am really trying to be sure I have not misunderstood this.

If a single Lifepo4 cell is fully charged at 3.4v and has internal resistance of 0.5 milliohms using I = v/r gives a potential short circuit current of 7200a.

Put 4 cells in series and you sum both V and R so you now have 13.6v with an IR of 2.0 milliohms.

Now add BMS and internal wiring resistance of, for example, 15 milliohms.

Using I = V/R gives I = 13.6/(2.0+15.0) = 800a.

Isn't that a plausible explanation of why the short circuit current of a Lifepo4 drop-in is much less than that of an individual cell and much less than an equivalent sized AGM battery?

 

[HughClayton]

Sorry, I know this might be tiresome and way off the topic of this thread but I am really trying to be sure I have not misunderstood this.

If a single Lifepo4 cell is fully charged at 3.4v and has internal resistance of 0.5 milliohms using I = v/r gives a potential short circuit current of 7200a.

Put 4 cells in series and you sum both V and R so you now have 13.6v with an IR of 2.0 milliohms.

Now add BMS and internal wiring resistance of, for example, 15 milliohms.

Using I = V/R gives I = 13.6/(2.0+15.0) = 800a.

Isn't that a plausible explanation of why the short circuit current of a Lifepo4 drop-in is much less than that of an individual cell and much less than an equivalent sized AGM battery?

Using the idealized model of a single resistance representing the internal resistance of a cell doesn’t represent the actual electrochemistry of a short-circuited cell. Ie you can’t determine battery short-circuit currents by simple calcs and, in reality, they are determined experimentally.

 

[geem]

Sorry, I know this might be tiresome and way off the topic of this thread but I am really trying to be sure I have not misunderstood this.

If a single Lifepo4 cell is fully charged at 3.4v and has internal resistance of 0.5 milliohms using I = v/r gives a potential short circuit current of 7200a.

Put 4 cells in series and you sum both V and R so you now have 13.6v with an IR of 2.0 milliohms.

Now add BMS and internal wiring resistance of, for example, 15 milliohms.

Using I = V/R gives I = 13.6/(2.0+15.0) = 800a.

Isn't that a plausible explanation of why the short circuit current of a Lifepo4 drop-in is much less than that of an individual cell and much less than an equivalent sized AGM battery?

Fully charged would be 3.65v. Internal resistance of a cell typically 0.25 mOhms. A direct short would suggest 14,600 Amps without the BMS.

 

[ckris]

Using the idealized model of a single resistance representing the internal resistance of a cell doesn’t represent the actual electrochemistry of a short-circuited cell. Ie you can’t determine battery short-circuit currents by simple calcs and, in reality, they are determined experimentally.

Yes - but we were (well I was) trying to understand why such experiments as linked in post #56 indicate short circuit currents of a drop in lifepo4 battery are much lower than an equivalent size AGM which is counter-intuitive to LFP cells having low IR, very high discharge currents and faster charging.

The example calculation I gave seems to explain why both can be true.

 

[ckris]

Fully charged would be 3.65v. Internal resistance of a cell typically 0.25 mA. A direct short would suggest 14,600 Amps without the BMS.

OK, we can plug in slightly different numbers but when you add in the BMS and internal cabling I still think this explains why the short circuit current from a drop-in LFP may be much smaller than you might intuitively expect......which practically matters to determine the AIC rating of any fuse you might choose to put next to the battery.

 

[geem]

OK, we can plug in slightly different numbers but when you add in the BMS and internal cabling I still think this explains why the short circuit current from a drop-in LFP may be much smaller than you might intuitively expect......which practically matters to determine the AIC rating of any fuse you might choose to put next to the battery.

My LFP cells are quoted at 0.18mOhms. I chose EVE cells at 0.25mOhms for the purposes of a quick calc. Your quoting 0.5mOhms which I think is incorrect. However you look at it, with no BMS you are in the territory of class T fuses. With a failure at the BMS ( and they do happen) I don't know the likely additional resistance in a short. For me it always better to play it safe so I purchased class T fuses. At some time I will need to get an electrical survey done of the boat for insurance purposes. I don't want to be arguing the point with an electrician about class T fuses.

 

[migs]

I'm not sure that it’s possible to estimate a cell’s PSSC by dividing its open circuit voltage by its AC impedance. The standard approach for estimating battery PSSCs is the delta-v method shown in the second example of my previous post (and here is some more information about this method):

Battery Internal Resistance & Short Circuit Current.

However, the most accurate results must come from actual testing (such as the research paper cited in my post). Nevertheless, both the standard method and real tests show that large LiFePO4 cells have significantly lower PSSCs than the figures derived from a cell’s AC impedance.

 

[Nina Lucia]

Just bought two new batteries and they arrived! 12v 150ah Expedition Plus Agm Deep Cycle Leisure Battery (EXP12-150) | eBay

Is there easy version step by step how to change settings when you replace new batteries, we have Victron BMV-712, Victron MPPT 100/20 smart charger controller, Blue Smart charger 12/30/ 3 output ?
We had wet lead batteries before and now AGM

I guess : 9172-Manual_BMV_and_SmartShunt-pdf-en.pdf ?