Over the last few years lithium batteries surged in popularity and quickly overtook lead-acid batteries as the preferred option for home energy storage systems, but that 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 from companies such as BAE and Sonnenschein, and advanced lead-carbon batteries from Narada and GS Yuasa.
In this review we selected two of the more popular lithium (LFP) batteries from leading manufacturers Simpliphi and Pylontech against the latest deep-cycle lead-acid batteries. A direct comparison is not easy as each battery type behaves differently depending on the rate of discharge, depth of discharge and cycle life required. The full battery comparison criteria is explained in the detailed complete solar battery review.
Lithium batteries used for home energy storage generally use lithium iron phosphate (LiFePO4 or LFP) cells due to the lower cost and long cycle life, however several well known manufacturer’s such as Tesla and LG chem use Lithium NMC cells which you can read more about here. For this review we will be focusing on Lithium LFP batteries from leading US manufacturer Simpliphi and the popular Chinese manufacturer Pylontech.
Battery Capacity and depth of discharge
Battery capacity is measured in either Amp Hours (Ah) or kilowatt hours (kWh). The amount of energy used is taken as a percentage % of total capacity and known as the depth-of-discharge or DOD, 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 is then divided by 1000 to convert to kilowatt hours - 14.4kWh total capacity.
As a general guide lithium LFP batteries are designed to be discharged up to 90% total capacity while the traditional lead-acid (gel & AGM) batteries are generally not discharged more than 30-40% unless in emergency backup situations. 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, but this can also reduce battery life significantly if deep discharges occur often.
This review does not include unsealed or flooded lead-acid batteries as they require high levels of maintenance and any cost savings can be outweighed by regular maintenance work required throughout the life of the battery. As highlighted in the full solar battery review lithium batteries have many advantages over traditional lead-acid especially in regards to usable capacity, weight, storage space and life cycle performance.
Advantages of Lead-acid batteries
Lead-acid battery technology is a well proven technology as it has been around for over 100 years and is extremely safe and reliable when designed and installed correctly. For larger off-grid systems lead-acid batteries are still one of the most trusted options and a correctly designed system typically can last 10-15 years or even longer. 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.
Deep cycle lead-acid batteries have been around for decades but more recently advanced tubular gel deep cycle batteries have taken lead-acid further with up to 15 year life with no servicing. There are many well established companies producing very high quality tubular gel lead-acid batteries, such as Sonnenschein, Hoppecke and TAB. We selected the well known German BAE solar PVV battery to represent the sealed lead-acid batteries in this comparison.
Advantages of Lithium batteries
The most obvious advantage of lithium batteries is the compact size and weight due to the extremely high energy density. Generally speaking a lithium LFP battery is about 20% of the size and weight of an equivalent lead-acid battery which is helped by the much higher depth-of-discharge available in a lithium battery. Lithium battery systems are generally made up of smaller easy to handle modules of sizes from 2kWh to 4kWh which gives much more flexibility when sizing a system. This also enables additional capacity to be added if needed at a later stage.
When under load the voltage is drop of lithium is very low and will generally not fluctuate more than 1 volt* from nominal even under high load (*48V). This is due to the much lower internal resistance compared to lead-acid batteries. By far the biggest advantage of lithium batteries is the very low losses during charging and discharging resulting is very high round-trip efficiency, this is explained in much more detail in the conclusion at the end of this review.
Simpliphi Power is a US based battery manufacturer which has been a leading developer of Lithium-ion batteries since 2002. Initially developed for durable remote military and mobile power systems the Simpliphi PHI batteries are well known for their extremely high cycle life, safety and stability.
The Simpliphi PHI range of Lithium Ferro Phosphate (also know as Lithium Iron Phosphate or LFP) batteries are rather unique in the home battery market being one of the only fully modular battery systems available. Unlike most residential lithium battery systems which are a fixed capacity or limited to 2-4 modules in parallel the PHI range is fully scalable and only really limited by the inverter and solar array capacity available. With the built in BMS or battery management system in each battery Simpliphi claim the PHI batteries do not require ventilation, cooling or thermal regulation. Considering Lithium LFP is the most stable lithium chemistry available and the proven track record, the PHI range appear to be one of the most robust battery system available on the market and are one of the only 'drop in' replacements for lead-acid batteries.
SimpliPhi also offer the best warranty of any battery on the market with an impressive 10,000 cycle / 10 year warranty (at 80% DOD) and a 3500 cycle / 10 year warranty at 100% DOD. However like all battery warranties there are conditions, in this case they main warranty conditions are the battery operating temperature range (0 - 43 degC) and charge /discharge rate no greater than 1.6kW (PHI 3.4battery) as specified in the warranty document.
3.5kWh 100% usable for 3500 cycles to 80% retained capacity (EOL)
Power output – Continuous = 1.6kW (3 x units = 4.8kW), Maximum Peak = 3.0kW per unit
Round trip efficiency (claimed) = 98%
3.5kWh module = AU$3500 (US$3000) – Cost per usable kWh = US$719
Warranty is 10 years - See warranty certificate for more details.
See full SimpliPhi PHI battery specification data sheet details
The Pylon Technology lithium ferrous phosphate (LFP) batteries were some of the first lithium batteries available which enclosed both the lithium cells and battery management system in a simple rack mounted unit. The first generation Extra2000 was replaced by the US2000B which has many improvements and more robust battery cable connections. The US2000B is the same size 2.4kWh module (previously with a touch screen interface) which allows easy setup and monitoring together with connections for multiple units in parallel. PylonTech also supply higher voltage modules for commercial applications.
The pylontech battery modules are often re-branded and used by several large solar manufactures and distributors including SolaX, Canadian Solar and Redback Technologies.
Lithium ferrous phosphate (LFP) is generally considered the most stable of all the lithium battery chemistries especially at elevated temperatures above 40 degC (common in Australia) which is highlighted by the high battery cut-off temperature of 50 degC. Cycle life is also quite high at a claimed 6,000 cycles at 80% depth of discharge, although it is unknown what the end of life (EOL) retained capacity is after this number of cycles?
The warranty is for 5 years and specifies 60% EOL capacity with the extented 10 year warranty. Note the previous warranty states no more than 360 cycles per year. Based on this the new battery performance will remain the same and we can assume 3,000 cycles at 80% DOD. The battery module operates at the nominal voltage of 48V and now has a higher peak power output of 5kW (previous model was 2.2kW) over a short discharge duration of 60 sec. The updated US2000B battery retails at the same price as the previous Extra2000.
1.92kWh (80%) usable for 3000 cycles to 70% retained capacity (EOL)
Power output – Continuous @ 48V = 2.4kW, Max Peak 4.8kW (1 min)
Round trip efficiency = 94%
US2000B = AU$1250 (US$1100) – Cost per usable kWh = AU$706
US2000B - Warranty: 10 years (after online registration) to 60% retained capacity. Refer to warranty document here
See full detailed PylonTech battery data sheet and manual here
ZHEJIANG Narada Power Source Co. is a Chinese based company primarily focused on power system and energy storage solutions. The REXC series Lead-Carbon based battery technology uses a nano carbon material with high capacitance and highly conductivity on the negative electrode. The company claims this combines the advantages of both lead acid batteries and super capacitors to enable faster recharge. The lead carbon battery technology provides not only a higher energy density, but also high power, rapid charge and discharge, and longer cycle life than traditional lead-acid batteries. However, the batteries are only currently available in large 6V and 12V modules so multiple strings of batteries would be required for usable capacities above 6kWh which is not ideal for large system but suitable for most domestic hybrid setups. Also the rated capacity can only be obtained at a very slow discharge rate of 100 hours (C100) which is not optimal for a hybrid situation with full discharge more likely to be over 12 hours or less. Although the claimed cycle life of 3000 cycles can be obtained at high temperature of 40degC so in theory they should last longer at lower temperatures. Warranty 5 years - See Warranty certificate
48V bank = 4 x 12V 200Ah (C12) = 4.0kWh usable for 3000cycles to 80% retained capacity (EOL)
Power output – Continuous @ 48V bank = 5.0kW, Peak up to 10kW
Round trip efficiency (estimated) = 83% (based on 12 hour discharge rate - C12)
4 x 12V 200Ah batteries = US$2600 (AU$3600) – Cost per usable kWh = US$650
See full battery specification sheets here - Narada Power
BAE Gel Lead-Acid
BAE Gel Secura batteries are high quality German made lead-acid based batteries designed specifically for off-grid and hybrid installations. BAE manufacture a wide range of batteries of various sizes and voltage configurations but for this review we will focus on the Gel Secura PVV solar (tubular Gel) range.
BAE have been manufacturing lead-acid based batteries for more than 100 years and the latest Gel Secura PVV battery design uses an advanced tubular-plate electrode with woven polyester gauntlet and solid grids in a corrosion-resistant lead-calcium alloy. The fully sealed maintenance free battery design utilizes the VRLA technology and have been proven to perform in harsh conditions and elevated temperatures.
The BAE Secura PVV batteries are single 2V cells therefore require battery configurations of either 12 or 24 batteries in series to create a battery bank nominal voltage of 24V and 48V respectively, refer to our battery article for more information. The secura range like most VRLA batteries are available in a vast range of sizes ranging from 140Ah up to a massive 4940Ah which in turn enables a wide variety of battery bank capacities up to a 235kWh or higher. For this review we selected the smaller 770Ah (C25) battery for the comparison (610Ah @ C10). BAE batteries are capable of deep discharges to a maximum of 80% total capacity but for the purpose of this article we will use a DOD of 50% which according the specifications will provide 3000 cycles to end of life (EOL). Warranty: 5 years
48V bank = 24 x 2V 610Ah = 14.6kWh usable for 3000cycles to 80% retained capacity (EOL)
Power output – Continuous @ 48V bank = 5.0kW, Peak 9kW or higher**
Round trip efficiency (estimated) = 82% (based on 10 hr rate - C10)
24 x 2V 350Ah batteries = US$9300 (AU$12800) – Cost per usable kWh = US$635
See full BAE Secura PVV Battery specifications
**Unlike Lithium based batteries Continuous and Peak power output of a lead-acid battery bank depends on battery capacity chosen, up to (235kWh @ 48V) and so power output could be rated as high as 100kW.
Battery comparison Results
Upfront Cost per kWh - usable capacity
Results include all 8 batteries from the complete solar battery review
See the full results in the compete solar battery review article
Battery efficiency known as round trip efficiency is the charging and discharging efficiency or losses during use. Due to the laws of physics the transfer of energy from one source to another results in some energy loss. Generally losses from lead-acid batteries are much higher at around 20% while most new lithium-ion batteries are much lower at 3-5%.
Another problem with lead-acid or lead-carbon battery systems is the long charge time required compared to lithium systems. The Lead-acid charge cycle can take several hours which means during bad or intermittent weather the charge efficiency will be very low and a full charge may not be achieved. In contrast Lithium batteries can charge very quickly (1 - 2 hours) and can efficiency absorb charge at much lower rates.
Deep-cycle lead-acid efficiency = 76 - 82%
Lithium LFP battery efficiency = 94 - 98%
Additionally unlike lead-acid batteries Lithium-ion batteries can tolerate partial state of charge for long periods of time without degradation or sulfation issues which can reduce the life of lead-acid (AGM or Gel) batteries.
Space and weight
Weight isn't always a huge issue for many installations but if space is limited then lead-acid battery banks are extremely heavy and require special racking systems for small areas. Space can be a problem though and lead-acid batteries can take up a considerable amount of room plus require ventilation and airflow to allow any trace gases to escape during charging and keep the batteries cool in higher temperatures.
On the other hand lithium batteries have a much higher energy density and thus require much less space, however the biggest advantage is that lithium battery modules do not vent any gases during charging and so do not require the same amount of ventilation. This means they can also be stored in most non-habitable rooms or garages without any concerns. Also lithium batteries have individual monitoring systems which monitor temperature so confined spaces and over-heating are generally not a problem.