100AH lithium battery holds MORE power than 100AH lead acid??

[Chotu]

This one is for the electrical engineers.

Talking about lithium batteries in a different thread, my mind wandered to something.


A 100AH lead acid battery is anywhere from 13.6 to 12.2V in the normal usage range, delivering 50AH over this discharge cycle.

A lithium battery is at 13+ the whole time delivering 50AH during the same discharge cycle.

Putting aside the ability to more deeply discharge a lithium battery for a moment….

If I’m getting 10 amps over 5 hours out of a lead acid battery and half of that is in the 12V range, while I’m getting the same 10 amps over 5 hours from a lithium but it’s all over 13V, isn’t the lithium actually delivering (and storing) more power by definition?

After all, that definition is Watts(power) = VOLTS x amps.

So given the same 100ah of power in both battery types, the one returning the higher voltage through that discharge is actually returning more watts (power) for the same discharge in AH we usually talk about.

I think that makes a 100 amp hour lead acid battery and a 100 amp hour lithium battery not equivalent in terms of their ability to return energy. We never talk about this. We talk about amp hours.

Yes, we say we can discharge a lithium more deeply, but we don’t talk about the fact that it’s returning more power even in the same cycle the lead acid is in.

Yes? Comments?

[tanglewood]

Originally Posted by Chotu

This one is for the electrical engineers.

Talking about lithium batteries in a different thread, my mind wandered to something.


A 100AH lead acid battery is anywhere from 13.6 to 12.2V in the normal usage range, delivering 50AH over this discharge cycle.

A lithium battery is at 13+ the whole time delivering 50AH during the same discharge cycle.

Putting aside the ability to more deeply discharge a lithium battery for a moment….

If I’m getting 10 amps over 5 hours out of a lead acid battery and half of that is in the 12V range, while I’m getting the same 10 amps over 5 hours from a lithium but it’s all over 13V, isn’t the lithium actually delivering (and storing) more power by definition?

After all, that definition is Watts(power) = VOLTS x amps.

So given the same 100ah of power in both battery types, the one returning the higher voltage through that discharge is actually returning more watts (power) for the same discharge in AH we usually talk about.

Yes? Comments?

Yes, I think so. I'm not sure what the rating conventions or standards are, or even if there are any, but in practice I agree that a 100Ah LFP battery will have more stored energy than a 100Ah lead battery. The stored energy would be expressed in Kwh.


I think there are 3 things that favor LFP:


1) A higher overall discharge voltage


2) A discharge voltage that drops much less over the SOC range.


3) Zero Peukert effect in the discharge current ranges that we care about.

[smac999]

it depends if the load draws less amps at 13v then at 12v. if the load current is the same you don't gain anything. and still consumes the same ah.

some loads probalby will and some won't

either way it's still 100ah. but it might have more wh

[Bellinghamster]

But wait, there's more! Lead acid batteries have higher internal resistance so more power is "lost" to heat in the battery while charging and discharging. Energy efficiency is 70-80% (what you can get out vs. what you put in), compared to 96-99% for LiFePO4. Also, lead acid AH rating is typically for C/20, or 20 hour discharge rate (i.e. 100AH battery should give rated capacity if discharged at 5 amps for 20 hours). If you discharge at a higher rate you'll get less AH back out, like at 1C discharge rate (for a 100AH battery), you'll only get about 50AH out of it. Crazy, eh?

[SaylorMade]

Yes
Plus no Peukert effect.

[Tupaia]

Lithiums are rated in Wh (watt hours), essentially the area under the discharge curve. A 12v 100Ah Lithium would be typically 1280Wh. If you did the same for a LA battery the curve as a comparison it would be limited to when the voltage dropped below say 10.5v at 50% discharge.


So to compare apples with apples you should really compare the area under the curve for a 200Ah (100%-50%) LA with a 100Ah (100%-0%) Lithium.


Obviously this doesn't account for cycle degradation.

[Chotu]

Originally Posted by Bellinghamster

But wait, there's more! Lead acid batteries have higher internal resistance so more power is "lost" to heat in the battery while charging and discharging. Energy efficiency is 70-80% (what you can get out vs. what you put in), compared to 96-99% for LiFePO4. Also, lead acid AH rating is typically for C/20, or 20 hour discharge rate (i.e. 100AH battery should give rated capacity if discharged at 5 amps for 20 hours). If you discharge at a higher rate you'll get less AH back out, like at 1C discharge rate (for a 100AH battery), you'll only get about 50AH out of it. Crazy, eh?

It really is. And I am so glad this thread came back to life.

This is something that we don’t talk about normally and it really needs to be out front and center when people are trying to decide which battery technologies to use.

I wouldn’t have hesitated at all knowing about the higher efficiency and higher energy storage the lithium bank has over anything lead-based.

I think it’s important just for this discussion to note that even comparing the in the 50% discharge range, the lithium is storing and returning quite a bit more power. Absolutely mind blowing.

It’s an incredible feature that needs to be more front and center in the “which battery technology should I use” discussions.

[Tupaia]

Originally Posted by Chotu

It’s an incredible feature that needs to be more front and center in the “which battery technology should I use” discussions.

Agree totally. I replaced 6 x 110ah AGMs with 4 x 100ah LPO's that has effectively doubled my useable capacity.


LA - 6 x 110 * 30% = 198ah
LPO - 4 x 100 * 50% = 200ah plus the ability to go 100% discharge.



After AGM's watching the SOC drop below 50% can be a little disconcerting.

[Chotu]

Originally Posted by Tupaia

Agree totally. I replaced 6 x 110ah AGMs with 4 x 100ah LPO's that has effectively doubled my useable capacity.


LA - 6 x 110 * 30% = 198ah
LPO - 4 x 100 * 50% = 200ah plus the ability to go 100% discharge.



After AGM's watching the SOC drop below 50% can be a little disconcerting.

But the real thing here is not about being able to more deeply discharge lithium batteries. That’s a given. Everybody knows that part. It’s great. One of the primary selling points.

The incredible part is that even discharging 50% of total published AH capacity, comparing apples to apples, you are storing and retrieving more watts (power) due to the higher voltage maintained by lithiums throughout the discharge cycle.

That’s huge, and never discussed

Main point: AHs are NOT the same. A lithium AH holds more energy than a lead AH. That’s incredible based on our typical battery discussions.

[Tupaia]

Originally Posted by Chotu

But the real thing here is not about being able to more deeply discharge lithium batteries. That’s a given. Everybody knows that part. It’s great. One of the primary selling points.

The incredible part is that even discharging 50% of total published AH capacity, comparing apples to apples, you are storing and retrieving more watts (power) due to the higher voltage maintained by lithiums throughout the discharge cycle.

That’s huge, and never discussed

Main point: AHs are NOT the same. A lithium AH holds more energy than a lead AH. That’s incredible based on our typical battery discussions.

Like I said it is the area under the discharge curve Wh (watt hours). These are never quoted for LA.

[CrispyCringle]

Im no expert, but Im willing to bet an AH is an AH. There is no lead AH or Lithium AH. I see what you are saying Chotu, but I dont think the math is right. But its to early in the morning for me to prove it. And even when fully awake I may not be able to. But I dont think the claim is correct.

[Chotu]

Originally Posted by CrispyCringle

Im no expert, but Im willing to bet an AH is an AH. There is no lead AH or Lithium AH. I see what you are saying Chotu, but I dont think the math is right. But its to early in the morning for me to prove it. And even when fully awake I may not be able to. But I dont think the claim is correct.

I’ll take that bet. How much? Dead serious. Let’s make the bet. (:

The math couldn’t be more straightforward.

W=V x A

Watts are the power. Power is dependent on Voltage, not just amps per hour.

100% serious. $100 good? We can use cash app or something.

I’ll write a more formal proof and we can have the resident electrical engineers (retired ok) and physicists (excluding myself) decide if it holds water.

[team karst]

Originally Posted by Tupaia

Like I said it is the area under the discharge curve Wh (watt hours). These are never quoted for LA.

Yes.

One needs even a basic skill at integral calculus to fully estimate the difference.

[Chotu]

Originally Posted by team karst

Yes.

One needs even a basic skill at integral calculus to fully estimate the difference.

To quantify it, yes. You’d need to integrate the find the area under the curve.

Although I doubt we have an equation for the discharge curve to integrate at all. It would involve numerical integration methods if you wanted to quantify it.

But the evidence is already there in W=VA. It’s a non zero difference, observable at the battery monitor.

A=a constant with both battery types

V ranges from 13.6 to 12 in a safely discharged lead acid battery during discharge to 50% of stated capacity

V stays well above 13V throughout the entire 50% discharge with lithium (50% used to compare apples to apples)

So… VA or power out is higher with lithium over the same 50% discharge

[sailingharry]

From a technical standpoint, as others up thread have said, you are 100% correct. The average discharge voltage for an LFP is probably one volt higher than the average discharge voltage for an AGM. Assuming for the argument 13.5 and 12.5, the difference is 13.5/12.5 = 1.08, or 8%. Frankly, that number starts off in the statistical noise.

But it gets worse.

Some loads, like incandescent bulbs, drop off fairly fast in power. The voltage drops the current drops and the power drops. So an hour of run time on an AGM is less power than an hour of run time on an lfp. But we run it for an hour either way.

Some loads, like a windless, drop off fast in power. The voltage drops so the current drops so the power drops and The windlass turns slower. But you need to recover the entire anchor, and so it takes longer. But the total power consumed probably remains constant.

Some loads, like your inverter, are smart. They draw a constant power. The voltage drops, and the current rises. More ah comes out in the same length of time.

But I'm not sure that all of this has a practical meaning. As I consider what size LFP bank to install, I am considering something between 600 and 800 Ah. The 8% difference doesn't factor into that discussion. My overnight draw is someplace between 90 and 125 Ah. I don't know if I would notice if those numbers went down by 8%.

But regardless of the real world relevance, you are 100% correct. And 8% is a bonus no matter how you look at it!