Batteries in off-grid renewable energy projects are key components since all of the electrical energy will be stored in a battery bank. Having an adequately sized and well maintained battery bank will greatly improve the overall performance of your renewable energy system.

 How are batteries rated?

Batteries come in different voltages; the most commonly used are 6V and 12V. They are rated by Cold Cranking Amps (CCA), Reserve Capacity and Amp-Hour (AH). For renewable energy applications the Amp-hour (AH) is the important rating. It is a measure of how many amps can flow out of the battery in one hour. Batteries are commonly rated using the “20 hour rate”. For example, a 100AH battery rated at 20 hours means that it can deliver 5A every hour for 20 hours before it is fully discharged (100AH / 20H = 5A). In a nutshell: the higher the AH rating on a battery, the more energy it can store.

How deep a battery is discharged is called depth of discharge (DOD); so in our previous example, a fully discharged battery is 100% DOD. Discharging a battery and then charging it to some DOD value, is referred to as a cycle.

 Battery Clarification

Batteries are classified in three main groups: starting, marine and deep-cycle. Starting batteries, such as car batteries, are designed to provide high starting currents for a short period of time (to start an engine for example). If used for deep discharge, they last on average about 150 cycles, but under shallow discharge (5% DOD) they can last for thousands of cycles.

Deep cycle batteries are designed for deep discharge (80% DOD) for many cycles. For this reason they are highly recommended in renewable energy applications. Marine batteries are a hybrid between true deep cycle and starting batteries.

 Types of Batteries

The two most common types of batteries in renewable energy projects are Lead Acid and Absorbed Glass Mat Sealed Lead Acid (AGM).

Lead Acid: These are the most common. They have the longest life, least cost per amp-hour and least sensitive to overcharging (note that overcharging any battery will decrease its life). The down side is that they require regular maintenance: adding water, cleaning the top terminals and equalizing charges. These batteries need to be placed in well-ventilated areas.  6 Volt Golf cart batteries are commonly used lead acid batteries for renewable projects.

AGM: The biggest advantage of this type is that they are maintenance free and are completely sealed, thus allowing them to be placed indoors. The down side is that AGM are more sensitive to overcharging and are more expensive.

 How long will my battery last?

There are many factors that determine battery life such as depth of discharge, temperature, maintenance, number of cycles, etc. which makes estimating battery life difficult.

Below we give an approximation of battery live under deep cycle for the three different battery types. Figure 1 shows the approximate life of deep cycle batteries under different depths of discharge for different periods of time.

• Starting (car batteries) = 2 to 8 months
• Marine = 1 – 5 years
• Deep Cycle = 4 – 8 years

Deep discharge cycles shorten battery life. For this reason, we recommend not to discharge the battery bank by more than 50% DOD if used on a daily basis.

 Battery State of Charge

This term is used to indicate how discharged a battery is. This value is determined by measuring the battery voltage or specific gravity. Voltage is measured by a voltmeter and specific gravity by a hydrometer (this can’t be used on AGM’s since they are sealed). The table below gives an approximation of a battery state of charge given its voltage 2 hours to 3 hours after a charging cycle.

Percentage of Charge	12 VDC	24 VDC
100%	12.70	25.40
90%	12.58	25.16
80%	12.46	24.92
70%	12.36	24.72
60%	12.28	24.56
50%	12.20	24.40
40%	12.12	24.24
30%	12.04	24.08
20%	11.98	23.96
10%	11.94	23.88
Taken form the Trojan Battery Company L-16 battery in static condition

 Connecting Batteries 

Multiple batteries can be connected together in series or parallel to increase voltage and/or system AH.

Batteries in parallel maintain the same voltage, but the AH is added (see figure 2). Here,

1. The positive terminal from battery A is connected to the positive terminal of battery B.
2. The negative terminal from battery A is connected to the negative terminal of B

Batteries in series maintain the same AH but the voltage is added (see figure 3). Here,

1. The positive terminal from battery A is connected to the negative terminal of battery B.

Note: the arrows indicate the free battery terminal for connecting the inverter and charge controller.

You can also combine batteries in series and parallel. For example, for a 12V 400AH battery bank using 6V batteries at 200AH each, you would need to connect the batteries in this way (figure 4):

These are two groups of 2 batteries in series (for 12V), that are then connected in parallel to produce 400AH. This is equivalent to four 12V batteries at 100AH each in parallel.

 Battery Maintenance

Having a well maintained battery bank will greatly improve the overall performance of your renewable energy system. When maintaining batteries make sure they are isolated (not connected to any energy source or inverter). Some basic steps to follow to prolong battery life are:

• Regularly check the battery bank state of charge
• Clean the battery terminals if there is any sign of corrosion
• At least perform an equalization charge twice a year (for non-sealed lead acid batteries).
• If using Lead Acid (non-sealed) batteries check that the electrolyte level is marked at full level and add distilled water if necessary.

During an equalization cycle, the batteries are charged at a high voltage for a period of time (normally two or more hours depending on the size of the bank) to remove sulfates accumulated on the battery plates. If batteries are frequently heavily charged and discharged, then it is recommended to perform an equalization cycle every 30 days. Consult with your battery manufacturer about equalization cycles before starting this process and always use appropriate safety precautions when handling batteries.