Lithium-Iron-Phosphate vs. Lead Acid Batteries……. Is the extra cost worth it?

The question I keep asking myself and keep getting asked is? How much more expensive is it to go Lithium Iron vs. Lead Acid Batteries and is it worth it.

As with all things it depends. On the application, budget environment and many other variables. However, for this example, I am going to use our Smart Power Box 5000 Q product. In essence a smart box the changes your generator into a Hybrid Power solution by adding Batteries, Inverter, Smart Controls and the ability to add Solar.

The spec sheet for our smart inverters can be found here.

Our Smart Power Box range is designed to be portable, rapidly assembled and to work in Africa. Therefore being safe, strong and reliable and able to deal with the heat. Heat and temperature are crucial in your battery selection, but I will come back to this shortly.

For our example, I am going to be comparing a Freedom Won LifePO4 lite 10/7 and standard lead batteries

The key criteria for our choice of a Lithium-Iron-Phosphate battery over a Lead Acid Battery are:

Robust Design

Temperature: Africa is hot, and temperature largely determines the efficiency of your system. Lead acid batteries rapidly degrade when used at temperatures above 25 deg c. In most of our country that is every day of the year. Additionally, the difference in round trip efficiency of 75-80% for lead iron vs 92% for the Freedom Won LifePO4 battery results in further heat production as a consequence of the additional charging required. The alternative allowing for the use of lead acid batteries is to include for air conditioning/cooling. However, this further reduces the efficiency of a lead acid battery system….. For what appears to be a small difference in efficiency and temperature sensitivity quick grows to a large difference as a result of the compounding effect of incremental betterment.

 

Design Life/cycle life: Lead acid barriers if well maintained will last 3-5 years and 500 cycles vs 10-13 years 3500 cycles for Freedom Won LifePO4 batteries. When considering the cost per kWh it is clear that it is worth the larger initial outlay for the longer life and number of cycles. Additionally, this cost comparison does not consider the cost, time and reliability issues associated with replacing lead acid batteries.

Battery maintenance: Lead acid batteries need a lot of maintenance. They need to be kept charged and be fully charged to ensure that they do not fail prematurely due to sulfation. Freedom Won LifePO4 batteries do not need to be fully charged and do not required  a detailed charging cycle that is hard to maintain when the batteries are left standing.

 

Size and Weight: Freedom Won LifePO4 batteries are 70% smaller and 300% lighter than lead-acid batteries

Charging: Lead acid batteries can be charged quickly to 80% charged. After this point the rate that they can be charged at gradually drops. This means that getting a battery fully charged takes us into the area of really poor efficiency! We cope with this by stopping the automated generator charge on our lead acid battery systems at 80% leave the remaining charge to be undertaken through solar generation. In this way, we maximise the fuel efficiency of the generator. This means that we are only using the battery between 50% and 80% in the main. This optimises battery life and generator/fuel efficiency – but also means that we are only really using 30% of the stated capacity of a lead acid battery.

With a Freedom Won LifePO4 batteries can be charged all the way very quickly to 100% charged with no loss in efficiency.

The cumulative effect of these incremental differences is a dramatic difference in the battery sizing due to the greater efficiency cycle of the Freedom Won LifePO4 batteries combined with the larger Depth of Discharge.

If we take a 1,000 Ah lead acid battery as a reference we can calculate the following:

  • To optimise battery life and generator loading we operate the battery between 50 and 80% – effectively we use only 300 Ah of capacity.
  • Of this 300 Ah of capacity 20% is lost through the charge-discharge We can either view this as lost energy in or a further reduction in battery capacity to 240Ah.

We compare a 1,000Ah Freedom Won LifePO4

  • 70% of the DoD issued – effectively we only use 700 Ah of capacity
  • Of this 700 Ah capacity, 8% is lost through the charge-discharge We can either view this as lost energy in or a further reduction in battery capacity to 644 Ah.

This means that from a 1000 Ah Freedom Won LifePO4 battery you will get an effective 644 Ah of energy vs. an effective 240 Ah for a Lead acid battery this is a stark 268% more Ah.

 

Description Lead Acid batteries Lithium-Iron-Phosphate batteries
10# Trojan T125 240Ah 6V Deep Cycle Battery Freedom Won LifePO4 lite 10/7
Rated power 14.4kWh 10kWh
Usable power 7.2kWh 7kWh
Usable depth of discharge 50% 70%
Cost  R30631.60 (ex Vat) R 8,3061:00
Life cycles and DoD 500 cycles 3500 cycles
Design Life 3-5 years 10-3 years
Cost / Kwh R8.50/kWh R3.39/kWh
Effects of Temperature

 

The optimum operating temperature for the lead-acid battery is 25°C (77°F). As a guideline, every 8°C (15°F) rise in temperature will cut the battery life in half.

 

Lithium-Iron-Phosphate batteries can comfortable be charged 0°C to 45°C and discharged –20°C to 60°C
Effects of not fully charging or allowed to stand partial charged or empty. Lead Acid batteries will fail prematurely due to sulfation. A LifePO4 battery does not need to be fully charged….

 

Efficiency (round trip 100% charge to 0% charge back to 100% charge) for our off grid application of the Smart Box 5000Q The typical energy efficiency (energy that can be taken out of the battery compared to energy required to re charge) for lead acid batteries is 70% – 80% The typical energy efficiency (energy that can be taken out of the battery compared to energy required to re charge) for lead acid batteries is 92%
The final 20% charge for a lead acid battery is particularly inefficient with efficiencies of 50% and can take a very long time for the battery to become completely charged. This is important to note as the recommended DOD is 50% and the last 20% of the battery is very inefficient to charge. The result being that you have circa 30% of usable power from a lead-acid battery. Additionally, you need to continually fully charge a lead acid batter to prevent sulfation
Size 10 x 264 x 181 x 284mm = 0.135m3

167% larger

896x626x146mm = 0.081m3

 

Weight 10 x 30kg = 300kg

258% heavier

116kg

 

Charging Lead Acid batteries have some constraints with regards to the start of charge of each battery and the voltage of the charge A LifePO4 battery can be charged with a voltage that varies between 14-16v (as long as no cell is subject to a charge greater than 4.2V. They do not need to be fully charged.
Battery Management Systems (BMS) It is vital that the correct battery management system (BMS) be used to control the battery charging. This is important to actively balance the individual cells that make up the battery and prevent under or over voltage which can otherwise destroy the battery

 

 

Reference

Victron: https://www.victronenergy.com/upload/documents/Datasheet-BMS-12-200-EN.pdf

Batteries http://prod.sandia.gov/techlib/access-control.cgi/2004/043149.pdf

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