Please note that automobile batteries should not be used for Solar Photovoltaic(SPV) applications. Automobile batteries are meant for delivering high current for a short time. The battery is then quickly recharged. On the other hand, PV batteries operate in a different way and they should deliver over a long period and to be recharged slowly. So never use Automobile batteries for Solar Photovoltaic applications. In this context, it is essential that we know the various battery specifications for SPV applications.

Battery capacity:

It  is a measure of a battery’s ability to store or deliver electrical energy, commonly expressed in units of ampere-hours(Ah).

An Ampere-hour is equal to the transfer of one ampere over one hour. For example, a battery which delivers 5 amps for 20hrs is said to have delivered 100 ampere-hours.

The capacity of  a battery depends on several constructional factors like the quantity of active material, the number and physical dimensions of the plates, and the electrolyte specific-gravity.

We should also know that battery capacity also depends on the operational factors like the discharge rate, depth of discharge, cut-off voltage, temperature and cycle history of the battery.

Sometimes a battery’s capacity is also indicated as energy storage capacity. It is expressed in Kilowatt-hours(KWh), which can be approximated by multiplying the rated capacity in ampere-hours by the nominal battery voltage and dividing the product by 1000.

For Example :

A nominal 12V, 100Ah battery has an energy storage capacity of (12 * 100)/100 = 1.2 Kilowatt-hours.

Battery Cycle :

It refers to the process of charging and discharging of a battery.Battery discharge is the process that occurs when a battery delivers current, quantified by the discharge current rate. Charging is the process when a battery receives or accepts current, quantified by the charge current or rate.

A discharge followed by a recharge is considered as one cycle.

The discharge can be very small or shallow, or it can be very severe or deep. All batteries can be cycled, but the question is how deeply and how many times before a permanent loss of capacity occurs.

One common use of battery is called “float service”. In a float service applications(eg. UPS), the battery storage is not regularly used, and is available on a standby basis for emergencies only(eg. loss of main utility power). Batteries are trickle charged at a small rate. When they are needed, they are discharged relatively quickly, but then gradually and fully recharged from maximum utility power over many days. Then they float gain for weeks or months. Battery manufacturers often claim 20 years of usable life for batteries in float service. This however is NOT the way, the battery in PV applications will be used!

You should know that the batteries used in photovoltaic applications will definitely be subjected to cycling on a daily basis, and perhaps also deeply cycled. So useful service life in PV “cycling service” will be less(5-10years).

PV systems need deep cycling batteries!

Charge/Discharge rates:

The rate of charge or discharge of a battery is expressed as a ratio of the nominal battery capacity to the charge or discharge time period in hours. For example, a 100 Ah battery is discharging at the rate of 2 Amps. The time to completely discharge a fully charged battery at this rate would be the capacity divided by the current or 100Ah/2 Amps = 50 hrs. So you can say that the battery is discharging at the “50 hour rate” or at “C/50”.

Rate in Amperes = C/T    (C= Capacity in Amp/hours, T= Time in hours)

This notation is helpful, because it allows you to express relatives rates of battery charge and discharge without referring to the exact size of a battery.For example, most manufacturers recommend charging their batteries no faster than the C/5 rate to limit gassing and overcharge. This means 20 amps for a 100Ah battery, and 100 Amps for a 500 Ah battery. Moderate charge rates are around C/20 or C/30, while trickle charging at C/100 will hardly produce any gassing at all in most batteries.

Batteries used in typical PV systems experience very low rates of charge and discharge compared to industrial applications. For example, the maximum charge rates from the PV array to battery are commonly about C/40, and typically discharge rates supplied to the load may be as low as C/100 to C/200. Contrast to this, a typically industrial forklift battery might be discharged   in one 8-hour shift.

Cold temperature reduces the Capacity:

battery capacity goes down as temperature goes down. This is because the chemical reactions go slower and less active materials can be accessed and converted when cold.

Fast Discharging Reduces Capacity:

As batteries are discharges and reaction products like water are produced. During fast discharges,  the reaction products get in the way of fresh electrolyte , so capacity is limited. In contrast, if the battery is discharges slowly. the fresh electrolyte can more efficiently penetrate into the plates and more capacity is made available for discharging.

Depth of discharge(DOD):

The depth of discharge(DOD) of a battery is defined as the percentage of capacity  that has been been withdrawn from a battery compared to the totally fully charged capacity. By definition, the depth of discharge and state of charge(SOC) of a battery add to 100 percent. in PV systems it is usually mentioned either as the allowable or maximum DOD or the average daily DOD.

Caution:

Maintaining a state of charge(SOC) at 80% is allowing a 20% depth of discharge(DOD). Don’t get confused between the two!

Maximum DOD:

It is the maximum percentage of full-rated capacity that can be withdrawn from a battery. It is a maximum discharge limit for a battery, generally dictated by the cut off voltage and discharge rate. In stand-alone PV systems, the low voltage load disconnect(LVD) set point of the battery charge controller dictates the maximum DOD limit at a given discharge rate.

Average daily DOD:

It is the percentage of the full-rated capacity that is withdrawn from a battery with the average load profile. If the load varies seasonally, for example in a SPV lighting system, the average daily DOD will be greaterin the winter months due to the longer operation period of nightly loads.

Courtesy : flickr.com