Advanced level, Batteries, DC, Lithium, Medium level -

Lithium batteries are cheap!! - comparing LiFePO4 & lead-acid

(Read more about Lithium batteries here)

 

People often claim that Lithium batteries are expensive, and although initial costs are higher, Lithium batteries are cheaper over a longer period. In this article, we will try and compare batteries that use Lithium Iron Phosphate (LiFePO4) chemistry with Lead-Acid batteries, in terms of capacity, longevity and price over an extended period of time. We will conclude that LiFePO4 batteries are about 40% cheaper in the long run, next to having a range of other benefits. We will look at discharge rate, the Peukert effect, longevity, warranty and other aspects.

Let's compare the following two batteries:

 

        • I-G3N 5.6 kWh i-Node LiFePO4 48V, Nominal capacity: 110 Ah, Current price, roughly ZAR 60,000 (including VAT & Shipping), click here.
I-G3N i-Node LiFePO4 Lithium battery
        • A popular larger model Lead-acid deep cycle battery for solar PV, 240Ah 12V Sealed Lead-Acid Battery, priced on average around ZAR 9,200 (including VAT & Shipping). Several of these 12V batteries are needed, in order to be able to compare with the capacity of the 48 V LiFePO4 battery.

 Lead Acid battery

 

Discharge rates

First of all, Lead-Acid batteries can be discharged at a maximum of 50%, while LiFePO4 batteries can be discharged up to 100%. However, as the i-G3N battery warranty is for a maximum discharge rate of 80%, we’ll work with a discharge rate of 80%. This means that the useable capacity of a Lead-acid battery is 50%, and for the LiFePO4 battery, this is 80%.

This means that you need 80% / 50% = 8/5 = 1.6 x the Lead-Acid battery capacity to have the same capacity of the LiFePO4 battery.

 

The i-G3N battery at 80% discharge has 0.8 x 110 Ah = 88 Ah capacity.
The Lead-Acid battery at 50% discharge has 0.5 x 240 Ah = 120 Ah capacity.

Lithium Iron Phosphate

 

Peukert effect
The German scientist Wilhelm Peukert determined in 1897 that with increasing discharge rates, the available capacity of Lead-Acid batteries decreases, as the resistance increases. This is called Peukert’s law. Lithium batteries do not experience the same effect.

 

Wilhelm Peukert

 

Current ratings for Lead-Acid batteries are given at a discharge rate over 20 hours. However, in a real-life scenario, shorter discharge rates are much more realistic. Many households will discharge their battery, e.g. during load shedding in about 5 hours, but let's assume an 8 hour discharge rate is used. Looking at the spec-sheets of our Lead-Acid battery, the capacity at 8 hours discharge is around 180 Ah. If we include our 50% discharge rate that we discussed above, this means that we are remaining with 90 Ah capacity for the Lead-Acid battery.

Taking the Peukert effect and discharge rates into consideration, we are left with the following capacities:

The i-G3N battery = 88 Ah capacity.
The Lead-Acid battery = 90 Ah capacity.
 
Please note that if bigger appliances are used or if the battery bank is small, and the Lead Acid battery is discharged at a much quicker rate, e.g. 3 or 5 hours, that the capacity of the Lead Acid battery will drop dramatically.

 

Comparable battery bank
If we want to compare the i-G3N battery to the Lead-Acid battery, we need to have a similar Ah and Voltages. The Ah is about the same, but we would need to increase the voltage. We would, therefore, require 4 Lead Acid batteries in series to get the same voltage (4 x 12 V = 48 V).

 

Click here for ‘how to connect batteries (series and parallel)’

Batteries series connection

 

Warranty

The Lead-Acid comes with a 1-year warranty, while the i-G3N battery comes with a 10-year warranty.

 

Other aspects to consider

The i-G3N battery can be monitored via Bluetooth and an app, up to cell level, e.g. discharge rates. To monitor Lead-Acid batteries, you would need to buy additional equipment, like a battery monitor (see, e.g. here). The addition of this monitor would add another ZAR 2,500, including installation.

The weight of the LiFePO4 battery is 50 kg; the 4 Lead-Acid batteries weigh together 4 x 62.5 = 250 kg. The Lead-Acid batteries need a lot more space and would need a lot more cables and lugs to connect.

 

Longevity

How long will these batteries last? Many people have experienced that in general Lead-Acid batteries will last no more than three years, based on the how the battery is used, while the LiFePO4 will last you ten years.

Let's look at the life-cycle of the batteries, e.g. the number of times the batteries can be discharged and charged back up. The i-G3N battery is rated for over 4,000 cycles (10 years if cycled once a day), while the Lead-Acid battery is rated for 2,000 cycles at 50% discharge.

This would mean that you need around two times the number of Lead-Acid batteries compared to the LiFePO4 battery over ten years.

 

Final cost comparison

The four Lead-Acid batteries in this example are comparable to one I-G3N LiFePO4 battery. We would have to replace the Lead-Acid batteries two times over ten years. Taking into account an inflation of 3% per year, the Lead-Acid batteries will initially cost 4 x ZAR 9,200 = 36,800. The second time it will cost you 42,661 (we added five years of 3% inflation). This means you will spend ZAR 79,461 over ten years on the Lead Acid batteries.

Buying and installing a new Lead-Acid battery bank twice over ten years, also means additional expenses in installation, transport, etc. For this, we included a lump sum estimate of ZAR 4,000. This means the total for the Lead Acid batteries comes to 79,461 + 2,500 (monitor) + 4,000 for the 2nd installation.

 

1 x LiFePO4 battery is ZAR 60,000
8 x Lead-Acid batteries are ZAR 85,961

 

Conclusions

Comparing the LiFePO4 battery to Lead-Acid, we can come to the following conclusions (over ten years):

 

      • cheaper by 40%
      • 5 x lighter
      • 3 x longer warranty (3 x 1 year compared to 10 years)
      • No need for additional monitoring equipment
      • Requires fewer cable connections
      • Takes up less space

 

If you can afford the initial costs, you are much better off buying the LiFePO4 batteries. If you cannot afford the initial cost, you can look for a financial solution, and some companies offer rent-to-own solutions, that become available more and more for solar products and installations.

 

For Sunstore's range of batteries, click here

 

  
Read more: 
CoC - Certificate of Compliance, why do you need it?

Solar panels, Mono or Poly?
Solar panels - What to Look for when Buying Panels 
Wiring solar panels: Series or parallel?
SunStore Solar System Sizing Calculator
9 Easy Steps towards installing Solar
What DC Wire Sizes to use for your Solar PV System?
What size of inverter do I need?
What are the elements of a Solar PV system?
Iron or Ion ???? Lithium batteries explained...

 


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