All solar systems operate on the same basic principles. First they convert sunlight into DC power through the photovoltaic PV effect. This DC power can either be converted by a solar inverter into AC power or stored in a battery and run home appliances. Depending on the solar system type, any excess solar energy can then be fed into the electricity grid for credits (on-grid solar) or stored in various battery storage systems (off-grid solar).
When it comes to using off-grid solar power systems, you should evaluate your solar battery options. Battery systems utilized for solar energy storage are available in two main types, lithium-ion and lead-acid (AGM and Gel).
The Best Battery Storage For Off-Grid Solar Systems
Majority of advanced solar energy storage systems use rechargeable lithium-ion batteries and are available in several shapes and sizes. Today, lithium-ion batteries are widely considered the best battery type for general energy storage applications due to their increased energy density and efficiency. Most lithium batteries utilized for home energy storage incorporate lithium iron phosphate LiFePO4 cells due to their enhanced useful life, lowered cost and safety.
Battery Capacity And Depth Of Discharge DoD
Battery capacity is usually measured in either Amp Hours Ah or kilowatt-hours kWh. The amount of energy utilized, referred to as the depth-of-discharge or DoD, is taken as a percentage of the total batteries capacity. Generally, lithium LFP batteries discharge up to 90% total capacity of 10% State of Charge (SOC). The conventional lead-acid gel & AGM batteries are usually not discharged more than 30-40% daily except in emergency backup situations.
The maximum ‘daily depth of discharge’ DoD allowed for each type is:
- Lithium-ion = 80 to 90%
- Lead-acid AGM = 15 to 30%
- Lead-acid Gel = 20 to 40%
- Lead-Carbon = 20 to 50%
It’s best to always refer to your manufacturers specifications. Your solar battery’s warranty may be void if the deep discharge occurs lower than what is defined under the warranties conditions.
Battery efficiency, also called round trip efficiency, refers to the charging or discharging efficiency or losses during use. Usually, losses from lead-acid batteries are much higher at 15-20%, while most lithium-ion batteries are significantly lower at 2-8%.
Another issue with lead-carbon or acid battery systems is the much longer charge time than lithium systems. A lead-acid charge cycle can take up to 2 to 4 hours, as per the discharge depth. During intermittent weather conditions, the charge efficiency can be relatively low, and you may not achieve a full charge, which can decrease battery life. In contrast, lithium batteries can charge quickly, usually in 1 – 2 hours, and efficiently absorb energy at much higher rates.
Also, unlike lead-acid or carbon batteries, lithium-ion batteries can tolerate a partial state of charge POS for extended time periods without degradation. Inadequate charging of lead-acid batteries regularly, mainly occurring during the winter, can significantly reduce most lead-acid AGM and Gel batteries’ lives.
Battery Management System BMS
Many lithium batteries come with an internal battery management system BMS, monitoring cell temperature, and voltages. This can shut down the battery if the temperature surpasses an upper or lower limit, typically above 45°C and below 5°C.
Advantages Of Lithium Solar Batteries
Lithium batteries have a compact size and weight due to their high energy density. Generally, a lithium LFP battery is about 25% of an equivalent lead-acid battery’s size and weight, which is helped by an increasingly higher depth of discharge DoD available in a lithium battery.
Lithium battery systems are usually made up of smaller, easy-to-handle sizes, providing increased flexibility when sizing a system. This also enables extra capacity to be added if required at a later stage.
- Extraordinary efficiency – up to 98%
- Increased energy density – compact and lightweight
- The usable capacity of up to 100%
- Partial state of charge PSOC, not an issue
When under a load, a lithium’s voltage drop is relatively low and won’t usually fluctuate more than 1-2 volts from nominal even under an increased load. This is because of its increasingly lower internal resistance compared to lead-acid batteries. This results in low losses during charging and discharging processes, ultimately resulting in increasingly higher round-trip efficiency.
Dandelion Renewables can help if you’re considering an off-grid solar power system or if you need help regarding different battery storage options. We provide residential and commercial energy storage services across Alberta, Saskatchewan, and BC. Check out our expert solar power services in detail or contact us for personalized service.