Lithium Ion versus Lead Acid batteries

In this blog, we will bust the myth that lead-acid batteries are cheaper than lithium ion batteries armed with data. Lithium ion is way cheaper than lead acid batteries and we’re going to use battery studies and data sheets to substantiate the claim. We are going to talk specifically about what makes the Lithium ion battery way cheaper than a lead acid. So the first price determinant factor is going to be

1. THE USABLE CAPACITY

In the lithium ion battery, if it says 100Ah, you get 100Ah. With the lead acid battery, typically, if you look at the data sheet below, you will get a 1000 charge cycles with a high quality AGM @ 50% depth of discharge and that’s very important.

A lot of people typically stick with 50% with the lead acid batteries because you need to charge them up every single day so that they do not damage themselves. Lead acids like to be fully charged all the time. So that means that a 200Ah lead acid battery is the same as a 100Ah LFP battery.

However, we need to consider Coulombic efficiency losses. Most of the AGM lead acids, it will be 90-95% efficient whereas LFP batteries are 99% efficient. With a cheap flooded battery, they have only 70-80% Coulombic efficiency. So instead of a 200Ah lead acid battery being the same as a 100Ah LFP battery, it’s more like 220 to 230Ah because the internal resistance is so high in lead acids. So, long story short, a 100Ah lithium is the same as a 220Ah lead acid and if you’re using a cheap flooded one, you can bump that number to around 250Ah.

2. CHARGE CYCLE LIFE

The next factor is the charge cycle life. This is where lithium ion destroys lead acid. First, let’s take the cheapest available AGM and look at the data sheet (shown below). You can see that you will get only 500 charge cycles at a 50% depth of discharge.

If you buy a high-quality AGM lead acid, you’re going to get 1000 charge cycles at a 50% depth of discharge as shown below.

Now let’s compare the high quality AGM to lithium. Instead of 1000 charge cycles, you get 3000-5000 charge cycles as shown in the specs below:

The fascinating thing is that even after the LFP battery hits 5000 charge cycles, you will still have 80% capacity. You can still use the LFP battery after 5000 charge cycles. You can use the LFP battery after 8000 charge cycles too and some of the LFP batteries are giving warranties up to 10,000 charge cycles (like SimpliPhi below).

So the lithium ion batteries are 5 to 10 times cheaper than the AGM batteries already if you are comparing an affordable LFP to a high-quality AGM lead acid battery. The price comparison is just way off the scales. Typically, LFP batteries will last 5000 charge cycles for solar because of the C rating (or how fast we are charging & discharging this battery).

The lead acid battery vendors make so much money since they are selling the lead acid batteries all the time because the lead acids cost so much already when you factor in just usable capacity and charge cycle life. It is beyond crazy!

It is so easy to hit 5000 charge cycles with the LFP batteries considering the C rate that we’re working with. Even if we have a low C rate for a flooded lead-acid, the Coulombic efficiency losses alone mean that we need to make the capacity so much larger just to even get those charge cycles for our specified application and depth of discharge. It is so hard to get power out of lead acids. It’s very difficult.

The next factor is

3. WEIGHT

Lithium ion are a lot lighter than a lead acid battery. A 100Ah lead acid weighs around 32 kgs whereas a 100Ah LFP weighs 14 kg. However, remember that we need 2.5 times of the lead acids to make one LFP battery to get the same usable capacity. So think about that for a second. We’re talking like 80 kgs of lead acid battery to make the same as a 14kg LFP battery. There is a huge difference in weight.

Now that we understand the usable capacity and the charge cycle life, let’s do a quick cost comparison.

4. COST COMPARISON

So we understand that we need 200 to 220Ah of lead acids to make one 100Ah LFP battery. For a comparative study, we will check the price for both of these battery types on US websites since in India, the price is not published online.

A 100Ah LFP battery costs around $600 whereas an AGM lead acid 12V, 100Ah is $275 on their website right now but we need to double that to give us $550 for same usable capacity – a difference of just $50. So for the price of the lead acid (without Coulombic efficiency or Peukert effect losses), we’re really giving lead acids a hard chance right now. It follows that a $600 LFP is on par with a lead acid if you factor in the usable capacity and efficiency losses. Now if you factor the charge cycle life into this, it makes lead acids 5 to 10 times more expensive over the long term.

I mean there is a huge price difference. Lead acids cost so much more money. Even the expensive lead acids (that last longer than the cheaper ones) cost way more than the LFP. So if you see those cheap lead acids and you calculate how much you get for how many years, you realise that you are spending more than what you bargained for. The lithium batteries that cost more (like Simpliphi Power) – they have a 10,000 charge cycle life guarantee which is incredible and a battery made of lithium titanate can pull 20,000 charge cycles! I would not use lithium titanate for solar right now due to its cost but I’m just trying to make you wrap your mind around the price difference. It is substantial. LFP is so much cheaper overall. It’s a no brainer.

Lead Acid Application

It’s hard for me to even imagine why anybody would use lead acids still but if you are building a system that will only last 2 or 3 years – lead acids can work. The biggest disadvantage with LFP is when we design a battery with LFP (especially a drop-in lead acid replacement), we are limited by the BMS discharge rate. Note that a 100Ah LFP can only discharge 100A continuous. So if you have a 12V, 2000W inverter and you want to run a load that draws 150A, you would need to buy two numbers of 12V, 100Ah and connect them in parallel so that you can pull 150A.

Lead acids can pull larger currents like 500A. So lead acids have great output and they are absolutely incredible for starting cars. For starting cars, lead acid is great for the price and even the size. I have a small 35Ah lead acid for my car and that thing can crank. So lead acids are incredible for starting engines but you must understand that it is a shallow depth of discharge application. You’re only pulling like 2 to 5Ah every time you start your vehicle.

Where lead acids fail is when you have to deeply discharge them. With lithium ion, they don’t care about deep or quick discharge since Peukert’s effect doesn’t apply and you also don’t need to charge them up every single day. You always want to cycle lead acids and charge them up all the way to the top every single day. So that’s why a lot of people need to make a large solar array to make sure that their lead acids stay healthy. With LFP, it doesn’t matter. You can keep LFP at 50% state of charge for months and it will like that. It will actually last longer. So lead acids and LFP have very different thermodynamic properties.

5. PERFORMANCE

The next factor is performance for 12V appliances. If you look at the discharge curve of LFP, it stays around 12.8 to 13.3V. However, the moment you put a load on lead acids and you look at the discharge curve it’s more linear but it is decreased, there is more voltage sag and you will have decreased performance. You have less power available. When lead acid’s are at 50% depth of discharge, their performance will decrease.

With LFP, the internal resistance is so low and the voltage curve is slightly higher but still advantageous for 12V appliances that you will have good performance until the LFP battery is dead.

6. SAFETY

Next factor is safety. Some lithium chemistries are combustible and they are absolutely dangerous. However, LFP and lithium titanate are non-combustible. I can literally shoot LFP batteries with a gun and it will steam a little bit but there will be no fire. Check video below:

Compare that to lead acid – if you overcharge lead acids, it starts gassing and the overpressure valves can’t handle it because this is sealed recombinant system. Lead acids can explode. When lead acid batteries explode, you have sulfuric acid flying all over the place. So I would argue that LFPs are way safer than lead acid batteries.

7. CHARGE ALGORITHM

The next factor is Charge Algorithm.

You can use a constant voltage to charge LFPs. I mean you can set your absorption & float and be done, and it will work perfectly fine. With lead acids, you need multistage charging. You need temperature compensation and you need to do equalization. With AGM, you can technically equalize it but it’s kind of hard to do it right. It’s very easy to over gas and lose some of the electrolyte through the recombinant system. So it is not recommended. You just want to stick to the AGM sealed settings on your controller. LFPs are actually way easier to charge. It’s just constant current for a long time and then constant voltage for absorption and then you just float it. It’s so much simpler to charge LFPs.

Many people will come up and say, “Oh… my solar lead acids have worked for 8 years straight and they’re still going strong”. That is incorrect and if it’s still going strong, it’s because they’re not using them. They are not using the capacity of those lead acid batteries. You cannot fight the laws of thermodynamics. You cannot change the numbers on those data sheets. Physics-wise, you are constrained by lead acids to whatever they can give and if the lead acid battery did last 8 years and internal resistance isn’t much higher due to sulfation, then they’re not using the battery. So please just ignore it when people say that.

8. CHARGING

Next fact is that LFPs can charge a lot faster than lead acids. If you realise how hard it is to push power into lead acids due to internal resistance, you will understand that you need a lot more solar panels on your roof to accomplish it. Another factor is that with the solar system, you have peak sun hours for around five hours a day and that’s when the batteries need to recharge. If you are trying to do an equalize function on lead acids and you’ve been setting the voltage to 14.5 and you trying to equalize those properly and adding water, you’re going to be in for a bad surprise when those lead acids die very quickly. We have seen sites where two sets of lead acids were installed but they died very quickly within 1.5 years. They had extreme decreased performance and capacity. There are some studies and safety bulletins saying to charge the lead acids up to 14.7V because they are chronically under charged but it is very difficult to do when you’ve a solar system to keep your batteries at 14.7 constantly with 12V panels and only 5 hours of sunshine a day.

For a solar lead-acid to last a long time with 50% depth of discharge, you need to cycle them properly to 50% every single day but still you’ll last them only for 3 years max. You might be able to get 5 years out of them if you do 20% depth of discharge. You might get 7 years out of them too but you’re not using them. At that point you’d have to buy a huge bank to be able to do a 20% depth of discharge and support the loads. With LFP, you could replace 4 or 5 of the lead acids with one LFP at that same depth of discharge. So, again, LFPs are way cheaper if you factor that in. You need to factor in all of these things – depth of discharge, and charge cycle life at that depth of discharge. You can’t get around these. These are like the laws of thermodynamics and we are constrained to them.

9. FORM FACTOR

Another thing is form factor size. If you look at an NMC battery (like a 5kW battery pack with the Tesla), it’s around 25kg and it’s small in size. Do you know how much lead acid you would need for a similar 5kW @ 50% DoD? You would need around 275kg. I mean, you need a huge bank of batteries whereas a Tesla battery can fit on a wall. With lithium ion, specific energy is double that of NMC. So form factor wise, LFP is just so much smaller than lead acids.

CONCLUSION

So I hope you understand that lithium ion is way cheaper, way better than lead acids. Just based on lower internal resistance and safety of LFPs, I would choose LFP over lead acid. Once you go lithium, you never go back to lead acid. There is not a single person that has swapped out their lithium batteries for lead acid and there is no person that was unsatisfied with lithium batteries for solar.

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