+254 796 488 826info@outrightsolar.co.ke Prime time Hardware Building, Southlands Langata. Near Langata High School

Solar batteries, Lead-Acid Vs Lithium-Ion Batteries

Ronny

Categories Hybrid solar power systems, Off-Grid Solar power systems No Responses

When it comes to solar batteries, there are two types of batteries that are most commonly used: lead-acid, which has been around for over 100 years, and lithium-ion, which gained popularity in the 1990s. Over recent years lithium-ion batteries have surged in popularity and overtaken lead-acid batteries as the preferred option for solar storage systems. But this does not mean the lead-acid battery is dead! Lead-acid battery technology is still advancing, with the release of high performance tubular Gel batteries and advanced lead-carbon batteries. In this review we compare two of the popular lithium-ion (LFP) batteries against deep-cycle lead-acid and lead-carbon batteries.

Best Battery Type For Solar Storage

Lithium-ion batteries are now widely regarded as the best battery type for general energy storage applications due to the high energy density and very high efficiency.

Battery Capacity – Depth Of Discharge

Battery capacity is measured in either Amp Hours (Ah) or kilowatt hours (kWh). The amount of energy used, known as the depth-of-discharge or DOD is taken as a percentage % of total battery capacity, refer to the diagram below.
To convert Ah to kWh simply multiply the battery Ah rating by the total battery bank voltage. For example a 24V lead-acid battery bank made up with 12 x single cell (2v) 600Ah batteries: 12 x 2V x 600Ah = 14,400Wh. This can then be divided by 1000 to convert to kilowatt hours – 14.4kWh total capacity.

Battery usable capacity

As a general guide, lithium (LFP) batteries are designed to be discharged up to 90% total capacity (10% SOC) while the traditional lead-acid (gel & AGM) batteries are generally not discharged more than 30-40% on a daily basis, unless in emergency backup situations.
Maximum daily depth of discharge (DoD) allowed

  • Lithium-ion = 80 to 90%

  • Lead-acid AGM = 15 to 30%

  • Lead-acid Gel = 20 to 40%

  • Lead-Carbon = 20 to 50%

Advanced tubular gel and lead-carbon batteries are more durable than traditional gel and AGM batteries and can sustain much greater depth-of-discharge, with up to 70% available (in backup situations). However, battery cycle life will be severely reduced if deep discharges occur on a daily basis.
It is recommended to set a maximum depth of discharge to no more than 40% for lead-acid and lead-carbon batteries, and a maximum of 70% in backup situations. Lithium batteries on the other hand can generally be discharged to 70-80% on a daily basis, and up to 95-100% in backup situations*

Always refer to manufacturers specifications

Lead-Acid Batteries

Lead-acid battery technology has been around for over 100 years so it’s a very well proven technology as well as being very safe and reliable when sized and installed correctly. For larger off-grid systems, lead-acid batteries are still one of the most trusted options and a correctly designed system can typically last 10-15 years or even longer (if shallow cycled and temperature controlled). Unlike lithium batteries, lead-acid battery banks do not have a specific cut-off point at a certain depth of discharge, so in an emergency situation they can continue to provide power until the voltage reaches the inverter cut-off, however this will reduce battery life to some degree

Types

Flooded lead acid (FLA) batteries

  • Flooded lead acid (FLA) batteries: The least complicated and least expensive kinds of deep-cycle batteries. Designed with cylindrical lead plates submerged in an electrolyte bath of water and acid.

  • FLA batteries may be inexpensive, but they’re also complicated to care for, because the electrolyte inside the battery case evaporates over time, requiring distilled water at be added at regular intervals. If not added the battery’s lifespan will be greatly decreased. 

  • Because they are built to be opened, FLA batteries need to be stored upright and given adequate ventilation to prevent a buildup of the hydrogen gas that is released when the battery discharges. This means keeping a bank of deep cycle FLA batteries suitable for home energy storage can take up a lot of space, as shown in the image above. 

  • If properly cared for and discharged to no more than half of their capacity on a regular basis, FLA batteries can last from 5 to 8 years in a home energy storage setup.

Valve-Regulated Lead-Acid (VRLA) / Sealed Lead-Acid Batteries

VRLA batteries are commonly known as sealed lead-acid batteries or maintenance free batteries. As the term ‘valve regulated’ implies, these batteries have a safety valve, which allows the release of gases produced. VRLA can be distinguished into two types:

Absorbed Glass Mat (AGM)

  • The mat in this case is made of a fine fiberglass mesh that kind of looks like a gauze pad, and is placed between the negative and positive plates in each of the cells of the battery.

  • Because an AGM battery traps nearly all of the electrolyte inside the battery, it can be discharged to a greater depth while allowing the sulfate to recombine with the free hydrogen. 

  • Some AGM batteries can be discharged to 80% of their capacity over hundreds of cycles, and typically last 4 to 6 years in a home energy storage setup. Discharging to no more than 60% is still recommended for maximum battery life. 

  • These advantages come at a price, though, and AGM batteries typically cost 1.5 to 2 times as much per kilowatt-hour (kWh) of energy storage. AGM batteries also take up slightly more space per kWh, but again, they can be stacked on their side in order to save space in a home storage setup.

  • Choose AGM batteries for solar energy storage if you prefer not to maintain a strict schedule of testing and watering FLA batteries, you want versatile mounting options and long life, and you’re willing to pay for it.

Absorbed Glass Mat (AGM)

  • Like AGM batteries, gel batteries are designed to keep the electrolyte inside the battery and prevent it from evaporating or spilling. Sulfuric acid is suspended in a silica gel, which makes the battery very stable and has a low self-discharge rate over long periods of time. 

  • Gel batteries are the safest lead acid batteries because they release very little hydrogen gas from their vent valves. They perform well in places where high temperatures are a concern, and can be discharged below 50% and retain their lifespan.

  • The drawbacks of gel batteries are that they store less energy in the same space as other lead acid battery types, and the gel doesn’t perform well in low temperatures.

  • Also, gel batteries require careful charging with specific smart chargers that are designed to limit the voltage used to charge the batteries and prevent overcharging. Failing to use these chargers can result in a battery that dies years before its time.

Advantages

  • Compatibility with most inverters.

  • Proven and trusted technology.

  • Safe, very low risk (sealed Gel/AGM).

  • Battery will not cutout at low SOC or low voltage**

  • Long life (if not over discharged) upto 15 years.

  • Easily recycled

Disadvantages

  • Lower efficiency – around 80%

  • Low energy density – Very heavy.

  • Usable capacity limited – Max 40% DoD on regular basis.

  • Not modular – Fixed size once installed.

  • Cannot sustain partial state of charge for long periods.

  • High temperatures can drastically reduce battery life.

How to choose


Choose AGM batteries for solar energy storage if you prefer not to maintain a strict schedule of testing and watering FLA batteries, you want versatile mounting options and long life, and you’re willing to pay for it.

Choose gel batteries for solar energy storage if you live in a hot climate and can’t store your batteries somewhere cool or well-ventilated, and also if you can absolutely 100% make sure they’re never charged at voltages outside their specific range.

Lithium-ion Batteries

More recently lithium-ion battery systems have become extremely popular due to the high efficiency (92% to 98%), compact size, lightweight and scalability. In contrast, lead-acid battery banks have a fixed size or capacity whereas lithium systems do not suffer this limitation. This flexible sizing allows for additional capacity to be added at a later stage, which is a real bonus for both installers and customers alike. Lithium batteries have a much higher energy density compared to lead-acid and are therefore lighter and more compact. A huge advantage of lithium is the ability to sustain a low state of charge (partial state of charge) for a prolonged amount of time without any negative effects such as sulfation which is a common problem with lead-acid batteries. Also, extremely high charge rates can be achieved using lithium with charging times up to 70% faster than lead-acid.

Advantages

  • Very high efficiency – Approx 97%.

  • Very high energy density – Compact and Lightweight.

  • High charge and discharge rates allowed.

  • No degradation issues with partial state of charge.

  • Modular and scalable systems (upgradable).

  • Safe and low risk (if charged correctly)

  • Most lithium batteries come with a 10 year warranty

Disadvantages

  • Can shutdown at high temperatures (45+ degC).

  • Can shutdown at low temperatures (below 5 degC).

  • Can ‘trip off’ under continuous high surge loads.

  • More difficult to recycle at end of life.

  • May not function without a compatible inverter.

Leave a Reply

Your email address will not be published. Required fields are marked *