Deep cycle lead acid batteries are used in the PV systems to store energy from the panels during the day for use when there is no sunshine. Thus, the buffer charge stored in the batteries bridges the time gap between power generation during the daytime and demand after sunset hours. Since unpredictability is in-built in the solar PV systems due to varying sunshine intensity during the day, systems must be designed so that batteries are not adversely impacted. General problems encountered with the batteries in the solar PV systems are:
- Difficulty to maintain them in fully charged condition,
- Difficulty in recovery from deep discharged conditions during non sunny days,
- Putting additional loads by the user,
- Low acceptance of charge by the batteries at low charging currents.
People normally recommend low maintenance flooded tubular LA batteries which require less frequent water addition. For instance, EXIDE offers their TORR range of Tubular batteries with plates casted at high pressure (100 bar) to ensure void free structure and consistent grain orientation. The batteries have low self discharge rates of less than 3% per month at 30˚C and water topping is required every 8 – 10 month. EXIDE claims that these batteries can sustain partial state of charge for up to six months and have long lives of 5 – 10 year.
For people preferring sealed batteries, EXIDE has EP series of Powersafe batteries. These are VRLA AGM batteries with 3 – 5 year life depending upon how they are used (or abused!).
Although batteries come in various designs, preventing the following 4 conditions enables them to perform optimally for a long time. These are well known issues in battery circles which the PV system designers should also be familiar with.
- Electrolyte Loss
- Very deep Discharge
In flooded or non-sealed batteries, conditions of high temperatures, high charging rates, and over-charging can lead to loss of electrolyte which is sulfuric acid. It can result in a situation where parts of the plates are above the electrolyte level which will lead to reduced battery performance. In sealed batteries, high charging currents and over-charging will cause an increase in temperature and pressure which can eventually result in the release of gas (and possibly loss of electrolyte) from valves. Even permanent damage can occur. Thus, the sealed (or maintenance free) batteries are particularly vulnerable to temperature and over-charging effects.
Excessive overcharging increases the water consumption in flooded or vented batteries and consequently the batteries require more frequent water addition. The danger is greater in the valve regulated (or sealed) lead-acid batteries which may overheat or dry out resulting in a loss of capacity. Overcharge and charging rates can be controlled by use of proper charge controllers. A controller senses the battery voltage and reduces or stops the charging current when the voltage gets high enough.
During lead acid battery discharge, lead sulfate crystals are deposited on the plates as part of the normal electrochemical reaction. During charging, the chemical reaction is reversed and the lead sulfate crystals are converted back to lead on the negative electrode and lead oxide on the positive electrode. But if the battery is left in partial state of charge (not fully charged) for sometime, or is in use but not reaching a fully charged state, the lead sulfate crystals can harden and will not convert back to lead or lead oxide during charging. When this happens, the battery capacity is reduced.
This effect will occur more quickly at higher temperatures. Thus, a PV system should be designed in a way that partially state of charge does not sustain for long and the battery gets back to full state of charge as quickly as possible.
The lead acid battery chemistry depends upon the electrolyte which is a sulfuric acid solution in water. Needless to say, the solution should be uniform and homogeneous throughout the battery cells or compartments. However, since sulfuric acid is denser than water, a situation can arise where the acid concentration is higher at the bottom of the battery than at the top – this is stratification. It will reduce the battery performance.
In flooded batteries, stratification can develop if a battery is not being fully charged; when a battery is fully charged some gassing takes place which mixes the electrolyte layers. Stratification is not a permanent effect and a single full charge should sufficiently mix the electrolyte, but if it persists for some length of time it can result in sulphation at the bottom where the acid concentration is high.
Very Deep Discharge
Although deep cycle batteries can normally withstand up to 80% discharge, it is recommended to allow only 50% as the maximum discharge and leave 30% for emergencies. The less deeply these batteries are cycled, the longer they will last. Therefore, for maximum battery life it is best to shallow cycle the deep cycle batteries! For instance, for its TORR range of solar Tubular batteries EXIDE gives the following cycle lives:
1500 cycles at 80% DoD,
3000 cycles at 50% DoD, and
5000 cycles at 20% DoD
As seen from above discussion, allowing a battery to stay in a state of not being fully (or nearly fully) charged can lead to the sulphation problem. What is still worse is discharging beyond 80%; it creates possibilities of causing irreversible changes to the battery’s chemistry and can possible cause severe permanent damage.
You may like to read Battery Charging Basics