Batteries

Technically, a battery is any device that can store energy.  But what we typically refer to when we think “battery” is an electrochemical device that converts chemical energy into electricity through a galvanic cell.  A galvanic cell is a device consisting of two electrodes of different metals that we call anode and cathode, and an electrolyte chemical (usually acid) solution.  When we connect two or more of these galvanic cells in a series connection we call this a battery.  For example, the left side of the graphic below shows a single cell of a multi-cell battery connected in series inside of a casing.

PV System Batteries

In a stand-alone off-grid system, batteries store energy created by the Solar PV panels to use later at night where utility grid power or other energy source power is not available.  In an off-grid PV system, batteries are typically the second most expensive component of the system.   Therefore, the type and care of off-grid PV System batteries is critically important to the long-term cost-effective operation of the system.

A battery’s capacity for holding energy is rated in amp-hours, 1 amp of current delivered for 1 hour = 1 amp-hour.

Also, battery capacity is typically listed in amp hours at a given voltage.  For example, a battery rated at 100 amp hours (20 hour reference) will deliver 5 amp hours for 20 hours before being discharged.  Note that manufacturer’s typically rate storage batteries using a 20-hour rate.

Battery Cycles

Another way to rate batteries is by their charge “cycles”, where a “cycle” is one complete discharge and recharge of the battery.   These ratings apply to batteries as either “shallow” cycle or “deep” cycle.   Shallow-cycle batteries, such as those used in automobiles, are designed to deliver several hundred amperes of current for just a few seconds, i.e., a high charge, in order to turn the starter to start the car.  Once started, the car’s alternator takes over and the battery is quickly recharged.   Shallow charge batteries, therefore, are used in applications where a large charging current is needed for a very short time.

Conversely,” deep-cycle” batteries are designed for long-term energy storage where only a few amperes of current are need for hundreds of hours between charges.   This is typically what is required for an off-grid PV system and, therefore, deep-cycle batteries are best suited for stand-alone PV power systems.

Keep in mind that these two types of batteries are designed for different applications and should never be interchanged.

Also keep in mind that battery cycle of a battery depends the depth of discharge for a given battery, where this can have a major affect on the lifetime of the battery.  Depth of discharge (DOD), also called “state of charge” of a battery is a measure of how much a battery is discharged (DOD) or how much energy remains in the battery (State of Charge).  It is important to remember that batteries should never be discharged beyond their rated DOD.  Discharging a battery below its DOD rating can cause damage to a battery which will affect its life time.  This can also lead to over-charging a battery to bring it back up to full charge, which can cause boiling of the electrolyte and further damaging the battery.

The following is a list of basic rules to follow to help you extend the life of your batteries in your Off-Grid PV System.

1. Always install and/or replace batteries in sets as batteries like to be together in the same group.  If you have 8 batteries in your system, avoid replacing just one or two batteries.  Try to install them all at the same time, and replace them all at the same time.  (However, it is also a good practice to rotate two batteries at a time in your set with fresh batteries as long as you rotate through the entire battery array at even time intervals).
2. Always check your battery connections; avoid battery post corrosion.
3. Try to keep the battery enclosure temperature somewhere between 50-90°F.  This will help extend the life of the battery as well as its efficiency.
4. Once they are installed, try to avoid moving or disturbing your batteries as much as possible.
5. Use a hydrometer to test the electrolyte level of each and every battery cell at least 2-3 times each year.   Recording these levels readings can help with changing trends to determine when to replace your batteries.