Lithium batteries are cheap!! - comparing LiFePO4 & lead-acid
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.
- 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.
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 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.
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:
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).
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.
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.
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.
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