Solar battery selection, management and maintenance for photovoltaic (PV) systems should not be an afterthought (but rather a critical thinking process) because battery life and performance will ultimately affect the overall performance and operating cost of the system.
When this important first step is thoughtfully realized, other procedures like battery management and maintenance are more likely to be effective and successful. Unfortunately, many intending users of PV systems erroneously assume that any rechargeable battery should provide energy storage and supply. Granted that rechargeable batteries are long-lived products which, in general, may be used many times over if they are charged and discharged properly (G. Pistoia et al, 2001, p 15), however, some rechargeable lead-acid batteries left in low state-of-charge (SoC) for long periods lose some of their capacity due to a permanent chemical change in the plates called ‘sulphation’ (Mark Hankins, 2010 p 53).
This article attempts to explain the various segments of batteries commonly found in the markets, battery technologies, major types of lead-acid batteries, state-of-charge and heat management in lead-acid batteries and maintenance practices required to improve the performance of a chosen battery type.
The task of solar battery selection requires that one should be equipped with the various market segments of batteries. The major market segments of batteries include primary batteries, starting lighting and ignition (SLI) batteries, industrial rechargeable batteries and portable rechargeable batteries (G. Pistoia et al, 2001, p35). Primary batteries are often non-rechargeable and are designed to be used once and discarded. They can be made from zinc-carbon cells or alkaline zinc-manganese dioxide cells (ScienceDirect). Other segments of batteries are usually rechargeable and they are manufactured using various technologies including Lead-Acid, Nickel-Cadmium, Lithium-Ion, etc.
Amongst these technologies, Lead-Acid batteries are the most readily available solution to the problem of storing solar electric energy (Mark Hankins, 2010 p 41). So, let’s know more about these batteries.
Lead-Acid batteries are generally classified into two groups, Deep Cycle batteries and SLI-type batteries. The latter is easily self-discharged when left standing uncharged for a given period (Mark Hankins, 2010 p 53). They are designed to be moderately discharged on a regular basis. Thus, they are also called shallow cycle batteries. They are not suitable choices for energy storage in photovoltaic systems (Mark Hankins, 2010 p 55). Instead, they are frequently used in automotive starting, lighting and ignition (SLI) systems.
Depending on the chemistry and manufacturer of the SLI batteries, a common problem that makes them unattractive for photovoltaic applications is that some of them, when left in a low SoC for over a month, may not accept their rated charge capacity or they may not accept charge at all (Mark Hankins, 2010 p 53). It is on this deficiency and coupled with their very short life-span in off-grid photovoltaic systems that it is strongly advisable to avoid automotive-type (SLI) batteries (Mark Hankins, 2010 p 58).
On the other hand, deep cycle batteries are the preferred choice for many solar electricity applications. When fully charged, they tend to retain their charge even when left uncharged for long periods. They are further divided into flooded lead-acid and valve-regulated lead-acid batteries. Each of these sub-groups is further classified into two subcategories as shown in the table below (D.A.J Rand et al, 2014 p 109).
|Parameters||Lead-Acid Deep Cycle Batteries|
|Technology||Flooded (FLA) Form||Valve-Regulated (VRLA) Form|
|Applications||Automotive, some PV Systems||PV Systems|
|Maintenance||Periodic additions of distilled water||Avoids the need for water maintenance.|
|Spill-proof||Acid is mobilized, spillable||Acid is immobilized or non-spillable|
|Duty||Float duty (Shallow cycle)||Deep-discharge duty (Deep Cycle)|
|Overcharging||They are less susceptible to damage by overcharging.||It causes greater temperature increases in VRLA batteries than in flooded batteries|
|Heat generation||They give off explosive hydrogen gas when they are being charged. Do not smoke or carry open flames in battery storage rooms||Closer attention has to be paid to the generation and management of heat in VRLA batteries than in flooded counterparts.|
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Just as one needs to monitor the amount of fuel left in a car’s petrol tank, one needs to keep track of how much energy remains in the battery. This is accomplished by monitoring the state-of-charge (SoC) of the battery. The SoC is a measure of the energy remaining in the battery. It tells you whether a battery is fully charged, half-charged or completely discharged (Mark Hankins, 2010 p 48). This is often indicated on the charge-controller or solar inverter as a percentage ranging from 0 to 100. The state-of-charge of a lead-acid battery usually decreases as its voltage also decreases (Mark Hankins, 2010 p 49). For lead-acid deep-cycle batteries there is an inverse correlation between the DoD of the battery and the SoC (Wikipedia). Therefore, an optimal compromise must be established between the two disproportionate parameters to guarantee longer cycle life. One recommendation is that even deep cycle batteries should not regularly be discharged below 60% DoD (or 40% SoC) (Mark Hankins, 2010 p 52). If the need demands that a battery should be discharged beyond this threshold, then it should not exceed 70% DoD (or 30% SoC).
After a deep discharge, it is advisable to always let the battery regain full charge by allowing it to reach a maximum state-of-charge before its next use (Mark Hankins, 2010 p 53).
Keep state-of-charge records of your battery to enable you detect when a battery is degrading or when a cell has gone bad. A typical template that may be adopted for recording state-of-charge is shown below.
Batteries generate heat during charge-discharge cycling and this must be dissipated to the environment to prevent the battery temperature from rising continuously. Proper heat management will ensure that the battery temperature does not exceed a safe level and will maintain all the cells within as small a range of temperature as possible. Allowing temperature gradient can lead to premature failure (D.A.J Rand et al, 2014 p 10).
The combined processes of radiation and free convection of air are inadequate to remove all of the heat that is conducted through the battery container (D.A.J Rand et al, 2014 p 12)
Do not mix battery types in your battery bank. All your batteries should be of the same type and manufacturer, and about the same age (Mark Hankins, 2010 p 41). Old or poorly performing batteries decrease the performance of those to which they are connected. Various battery chemistries, in general, usually have different operating voltages, energy ratings and cycle lives (if they are rechargeable) (G. Pistoia et al, 2001, p 15; Mark Hankins, 2010 p 42). Even within the same battery chemistry family, there will be variations to suit specific applications (G. Pistoia et al, 2001, p 16)
Batteries wear out, no matter the type and make. Plan for the replacement of every battery in your battery bank at the same time. It is often tempting to replace some and leave others to save cost. But this may sometimes turn out to be a regrettable decision.
A battery is made up of cells and sometimes only one or two cells may be bad. It is usually more economical to purchase a new battery rather than repair a single bad cell
Keep all battery surfaces clean especially the top where the terminals are often located. This reduces high rate of self-discharge caused by electrical conduction through acid mist accumulating on top of the battery (Mark Hankins, 2010 p 59).
D.A.J Rand et al, 2014, Valve-Regulated Lead-Acid Batteries, Elsevier B.V.
Mark Hankins, 2010, Stand-Alone Solar Electric Systems, Earthscan Ltd
G. Pistoia et al, 2001, Used Battery Collection and Recycling, Elsevier Science B.V.