Solar Power Batteries Guide

For environmentally conscious homeowners looking to reduce their carbon footprint or those planning to live off-grid, going solar is an appealing option. With dropping prices on photovoltaic solar panels and advances in storage batteries, solar energy systems are now accessible as DIY projects. Lithium iron phosphate (LiFePo4) batteries in particular are well-suited for small-scale residential solar storage.

Unlike traditional lead-acid batteries, LiFePo4 batteries are lighter, more efficient, and have a longer lifecycle. Their safe lithium iron phosphate chemistry prevents overheating and makes them non-combustible. For DIY solar projects, LiFePo4 batteries are easier to maintain than lead-acid alternatives. Their simplified charging parameters and tolerance for partial states of charge means you don’t have to babysit the batteries as much. LiFePo4 batteries also retain over 90% of their capacity after thousands of cycles, giving them a longer usable lifespan.

Installing LiFePo4 solar batteries is straightforward for DIYers. Once your rooftop solar panels are hooked up to a charge controller, you simply wire the batteries in sequence to create a battery bank scaled to your energy needs. Off-grid LiFePo4 battery banks can store surplus solar energy during the day to power lights, appliances, and devices at night when the sun goes down. Going solar with LiFePo4 batteries allows environmentally conscious homeowners to take control of their own energy production and reduce reliance on the centralized grid. The falling prices on solar panels combined with efficient new storage options like LiFePo4 batteries makes going off-grid easier than ever.

Sizing Your Battery Bank

The total capacity of your batteries should be enough to store the electricity generated by your solar panels for use when the sun is not shining. This is typically measured in watt-hours (Wh) or kilowatt-hours (kWh). Batteries also have a maximum C rating for charge and discharge. 1C = 100% of rated capacity per hour.

Keep in mind that lead-acid based batteries can only be discharged to 50% of their rated capacity without damage, while lithium LiFePo4 batteries used for solar systems can usually be safely discharged to 10-20% of full capacity when cycled daily. Therefore, you will need up to twice the amount of battery capacity for lead-acid types.

Batteries can be connected in series and/or parallel to achieve the desired voltage and power configuration for your system.

Choosing the Type of Batteries

There are different types of batteries available for solar power systems, each with its own advantages and considerations:

1. Lead-Acid Batteries
   Lead-acid batteries are like the starting batteries you have in your gasoline or diesel car or truck. They are designed for very high current loads for short periods of time to start the vehicle motor. Flooded (wet) lead-acid batteries contain liquid sulfuric acid which reacts electro-chemically with lead plates to create electricity. These batteries are designed to be rugged for use in cars, but typically last only a few years.

   They are very heavy, somewhat fragile, and create flammable gases like hydrogen when charging or discharging. This off-gassing can cause corrosion of nearby metals including battery terminals and copper wire. You may have seen green corrosion on the battery terminals of your car.

   Due to their chemistry, lead-acid batteries can only be discharged to 50% capacity without damaging the battery, so they are effectively only useful for 50% of their listed power rating. When used in stationary applications, car batteries should not be used because sediment builds up on the bottom of the cells during cycling and will eventually short out the cells. Special stationery or deep cycle AGM marine batteries are better suited for this use.

2. LiFePo4 Lithium-ion Batteries
   Almost all new solar systems use lithium-ion batteries, specifically LiFePo4 lithium-iron-phosphate batteries. This chemistry is the most stable and lightest weight type of lithium-ion battery. This type of lithium-ion battery does not have the same combustion, flammability, and overheating issues as other types of lithium-ion batteries and the high volumes sold have made them more affordable. They typically have 3,000 to 5,000+ cycle warranties to degrade to 80% capacity.

   There are several styles or configurations of lithium-iron-phosphate batteries for DIY use:

• Raw cells
  Raw LiFePo4 cells can be purchased from distributors or manufacturers and DIY builder can create their own battery systems. LiFePo4 cells are typically 3.2 vdc nominal voltage with various storage capacities, including the common 100 Ah and 280 Ah sizes. For comparison 100 Ah Eve cells cost about $43 each plus shipping from China (4 required for 12 VDC) = $172.

  Just like standalone batteries, raw cells can be combined in serial and parallel combinations to create the desired capacity. This allows batteries of any voltage and ampere hour rating to be constructed.

  Raw cells must be properly mounted to prevent vibration that may loosen connections. Some types of cells should also be banded together to prevent swelling.

  Raw cells must be connected to a Battery Management System (BMS) to control the charge and discharge to the cells to prevent over charging and discharging which will damage the cells. The BMS also monitors each cell’s charge to ensure the battery energy is well balanced.

  The BMS will also monitor and manage each cell to provide equal charging. Good BMS’s will also have over and under temperature cut-off sensors and over current protection.

  BMS’s have a maximum charge and discharge rate, and a maximum voltage which limits how many batteries can be placed in series. Some BMS’s will include Wi-Fi battery monitoring systems for cell status and SOC (state of charge).

  Today, raw cells may not be less expensive than other packaged products like Server Rack batteries, especially when you consider the warranty plus all the components needed like circuit breakers, BMS, fuses, case, and temperature sensors.

• Standalone batteries
  The market for standalone LiFePo4 lithium-iron-phosphate batteries has exploded in the past couple of years. Chinese manufacturers like Ampere Time and Chins have led the way to lower cost higher quality LiFePo4 batteries in 12, 24, and 48 VDC configurations, and storage ratings of 100, 200, and 300 ampere-hours. Most can be connected in serial and parallel configurations to provide more power. These batteries include BMS managers and often include low temperature protection.

  They are great for installations in RV’s, cabins, outbuildings, and smaller solar systems. In larger systems, they do not offer the same built in communication and management capabilities as Server Rack batteries. They are sold by Amazon, Walmart, and other large outlets and online. They are often available from US distributors with quick shipping and US based support.

  Example costs of these Chinese made batteries in June 2023:

• Ampere Time (Amazon) 12 VDC 100 ampere-hour for $349.00 including shipping. At 1280 watt-hours that is $0.27 / watt-hour.
• Chins* (Amazon) 12 VDC 100 ampere-hour batteries for $289 including shipping. At 1280 watt-hours that is $0.22.5 / watt-hour. By comparison, the Chins 24 VDC 100 AH version is $639 or $0.25 / watt-hour.
  * $30 Discount applied.
  

  Example costs of “Made (or assembled) in USA” batteries in June 2023:

  Battle Born still assembles their batteries in the USA and makes high quality batteries. However, they are also the highest cost at $925 for 12 VDC 100 ampere-hour or $0.72 / watt-hour, often 2-3x as expensive as the competition. This is a battery and BMS only product.

  There are also batteries available from most manufacturers with additional features including low and high temperature protection, built-in heater to protect against freezing temperatures, and remote monitoring via Bluetooth. They are often labeled “Plus” or “Smart” batteries. These features usually increase the battery costs

• Server rack batteries

  Server rack batteries have recently become more popular for solar systems. As the name implies, these batteries were originally designed to provide back up power for computer servers and for cell phone towers. They are built in metal enclosures and designed to fit into a standard 19” wide electronics rack. They are modular and can be stacked to provide more voltage or current. Most include BMS, circuit breaker, monitoring panel, and a communications link for connecting to All-in-One or smart inverter systems like Victron.

  Most are sold by US based companies, have local shipping and support. They also have US-based warranties.

  Amazingly, these systems have decreased in price and in June 2023 you can buy a 100 ampere-hour 48 VDC (5120 watt-hours) EG4 basic server rack battery with BMS, temperature sensing, and communications for $1399. That is $0.27 / watt-hour. Contrast this with 4 x 12 VDC 100 ampere-hour batteries from Battle Born for $3700 or $0.72 / watt-hour without communications features.