Lead acid battery

A lead acid battery with the case removed. The inside of a lead acid battery is composed of lead plates immersed in an electrolyte solution.

Lead acid batteries rely on reversible chemical reactions between lead and acid to provide energy when it is needed and to store enegy when it is being produced. They have been around for over 150 years and have proven their durability, low-cost, recyclability, and performance under variable conditions to the point that nearly every automobile on the planet relies on a lead acid battery to start and run. Lead acid batteries have been the preferred form of energy storage for off-grid PV systems since they first began being built for the same reasons, thus most off-grid PV components are built for use with lead acid batteries in 12V, 24V or 48V configurations. Lead acid batteries have the added advantage of coming in a variety of voltages (2V, 6V, 12V) and amp-hour ratings (5Ah to 5000+Ah). But due to their high density of lead, this batteries are extremely heavy. A 12V, 225Ah flooded lead acid battery weighs around 60kg, which is approaching the upper-limit of what is easily movable without equipment.

Just like how a Solar PV module is comprised of various different PV cells connected together in series that each produce a certain voltage, lead acid batteries are composed of series connected cells with each one producing around being having a nominal voltage of roughly 2V. This means that a 12V battery will be comprised of 6 cells.

There are many different types of lead acid batteries, but they can initially be divided into two categories: starter batteries and deep-cyle batteries.

• Starter batteries are used in cars and are intended to provide large amounts of power for short periods of time with a shallow depth of discharge. They work well for this purpose, but they are unable to continuously supply power beyond a shallow depth of discharge without severely shortening their cycle life. These batteries will fail prematurely in a PV system and are not worth investing in.
• Deep-cycle batteries have a more robust design that enables them to continuously supply high amounts of power to a deeper depth of discharge. These batteries are heavier and cost more than starter batteries, but are the appropriate battery for use with a PV system.

This page will focus on deep-cycle batteries, which can be further divided into various categories.

Flooded lead acid (FLA)

The original lead acid battery design. They are the simplest, most durable and cheapest of the lead acid batteries. They are slightly more durable as they tend to be more forgiving of deep discharges than maintenance-free VRLA batteries.

Characteristics

• They require monthly maintenance as the batteries will lose water as they charge and discharge. Durable, long cycle life and cheap only if properly maintained. If maintenance is not performed regularly, the batteries will rapidly fail and replacement will cost significantly if maintenance-free batteries had been used initially.
• Release significant amounts of hydrogen gas.
• Have a liquid electrolyte solution inside that requires that they remain upright.
• Can undergo an equalization charge which can help to prolong their cycle life.

Considerations for use

• If there is any doubt about the end-user's ability to maintain the battery bank, then maintenance-free batteries should be considered.
• End users must have appropriate safety equipment and training for servicing batteries or a technician must do the work.
• Require users to have regular access to distilled water was any other form of water has impurities and will damage the battery.
• Batteries should be stored in a secure space with proper ventilation due to the combined hazard of the batteries spilling acid and their tendency to realease significant amounts of hydrogen gas.
• May not be the best battery type with either extreme high or low temperatures.

Valve-regulated lead acid (VRLA)

In the 1970's maintenance-free lead acid batteries began to enter the market. These batteries are designed to address several of the primary concerns that arise with FLA batteries. They are less durable, have a shorter cycle life and cost significantly more than flooded lead acid batteries. They are less foriving of deep discharges. There are two primary sub-categories of VRLA batteries: AGM and Gel.

Characteristics

• They do not require maintenance.
• They are sealed (although not completely) so there is little to no offgassing.
• They do not have a liquid electrolyte solution inside and are additionally sealed to prevent leaks, therefore they do not have to be positioned upright at all times.

Considerations for use

• Work well for end users that may not perform maintenance.
• Work well for locations in which there is not a seperate space that can be dedicated to energy storage.
• Require a higher budget.

Absorption glass mat (AGM)

The electrolyte solution is contained within mats of fine glass fibers. These batteries cost on average 1.5-2 times as much as FLA and have a shorter cycle life when compared to a properly maintained FLA battery.

Specific considerations for use

• Perform better than FLA and gel batteries in cold environments. FLA batteries can freeze and be damaged during charging/discharging.

Gel

The electrolote solution is made into a gel paste. Gel batteries are the most costly lead acid option. They prefer slow charging and discharging, which is not ideal for renewable energy systems.

Specific considerations for use

• Perform better than AGM and FLA batteries in hot environments.

Notes

https://www.trojanbattery.com/pdf/Signature_Trojan_ProductLineSheet.pdf https://www.trojanbattery.com/pdf/AGM_Trojan_ProductLineSheet.pdf https://www.trojanbattery.com/pdf/GEL_Trojan_ProductLineSheet.pdf