by Lennert van den Berg

The Soluxio is one of the most advanced solar-powered streetlights in the market. Technologically speaking, the light post will work with any type of battery chemistry. Other solar street light manufacturers sell and produce their products with lead/VRLA/AGM batteries by default. However, the true cost of ownership (TCO) of these types of batteries is much higher than that of lithium batteries – even though lithium batteries may appear to be more expensive at first glance. FlexSol Solutions distinguishes itself by exclusively working with lithium batteries in our solar lighting products.

Both the Soluxio street lighting and NxT outdoor lighting product ranges operate exclusively on lithium-ion battery chemistry. We highly recommend integrating lithium batteries in your solar lighting products, since it greatly improves the return on investment (ROI) of your purchase. In this article, I will go into every detail about the advantages of the lithium batteries powering the Soluxio solar light post and explain why the use of lithium batteries actually results in a lower Total Cost of Ownership (TCO).

Why do we recommend lithium batteries?

Over the last decade, lithium has been proven to be an extremely reliable battery technology. Lithium batteries are widely used in electric cars, cell phones, cameras, laptops and other mobile devices. Lithium batteries are even used for energy storage in the electrical grid. So it seems like there would be a lot of advantages of lithium batteries over the more traditional types of batteries such as lead, don’t you think?

To start with, I will summarize the most important benefits of lithium batteries compared to lead batteries. Lead batteries in this context refers to all the different chemistries of lead batteries and even the most modern types of lead batteries, like VRLA, deep cycle sealed (VRLA) AGM batteries or deep cycle sealed (VRLA) gel batteries. In addition, this article will give a detailed explanation of what the differences between lithium and lead batteries mean for the performance and durability of a solar light pole.

Lead Lithium
High temperature tolerance 20oC 45oC
Low temperature tolerance >0oC <0oC
Turnaround cycle efficiency 75% 98%
Charge rate 0.2C >5C
DOD tolerance 50% 95%
Discharge cycles @ 80% DOD 400-500 2000
Discharge cycles @ 30% DOD 1500 7000
Weight (vs Lead) 100% ±25%
Replacement timeframe 1.5-3 years 8-10 years
Complexity Easy Complicated
Environmental impact Higher Lower
Total cost of ownership Higher Lower

High temperature tolerance

Batteries do not like high temperatures, as they reduce service life. However, increased temperatures have a much larger impact on lead batteries than they do on lithium batteries. Temperatures above 20ºC reduce the lifetime of lead batteries rapidly, whereas lithium batteries tolerate temperatures up to 45ºC. When the temperature rises from 20ºC to 30ºC, the service life of lead batteries (e.g. AGM/GEL) will shorten by a factor of 2.

The graphs below clearly show this, with the capacity retention plotted against the number of charge cycles for both 20ºC and 33ºC. In the Middle East for example, solar street lighting with lead batteries will likely have to be replaced within 2 years (or 750 cycles). When temperatures increase from 20ºC to 40ºC, the service life of lead batteries will decrease by a whopping factor of 4. As for lithium-based battery technologies, temperatures of 40ºC do not have any impact on battery lifetime at all. Especially considering the fact that solar street lighting is most common in warm, sunny regions (where ambient temperatures often rise above 20ºC) the use of lithium batteries is strongly advised.

Lithium and lead batteries cycles at 33 degrees Celsius
Lithium and lead batteries cycles at 20 degrees celsius

Low temperature tolerance

Both lithium and lead batteries lose useful energy capacity in cold weather conditions. However, for lead batteries the effect is much more severe. At temperatures of -20ºC, the useful energy capacity of lead batteries is reduced to 30%, as shown in the graph below. Under the same discharging conditions, lithium batteries retain 82% of their energy capacity. Lithium batteries will not only last longer in countries with high temperatures but also in regions with very cold conditions, like mountainous regions.

Lithium-ion battery capacity versus temperature

Turnaround cycle efficiency

The turnaround cycle efficiency shows how efficient the battery is during a complete charge and discharge cycle. For lead batteries this is usually around 75%. This means that if you charge a lead battery with 1000Wh you will only get 750Wh back for actually powering your device. So you lose 25% system efficiency in the batteries alone! In the case of a solar streetlight (or any other solar-powered system) this infers that you need to have at least 25% more solar panels to power the same load. The system will inevitably turn out to be more expensive under these conditions (or perform worse for the same configuration). For lithium batteries however, the turnaround cycle efficiency is roughly 98%. No wonder we are seeing a rise in the use of this battery technology.

Depth of Discharge tolerance and discharge cycles

The Depth Of Discharge (DOD) relates to how deep you drain the battery in every cycle. The deeper you discharge the battery, the fewer discharge cycles it can make, thus the shorter the service life of the battery will be. Lithium batteries can be easily discharged up to 95% whereas lead batteries are limited to 50%. The graph below shows how the depth of discharge is related to the number of cycles for both lead batteries and lithium batteries.

If we want to use the batteries for around 2000 cycles (2000/365 days = 5,5 years), you can only discharge lead batteries between 25-35%, while for lithium this is around 80%. This means you need to have at least 4x the capacity of a lithium battery to get the same battery life for a lead battery! Or to put it otherwise: if you want to use 80% o