By PS Deodhar
Off grid photovoltaic (PV) solar power equipment requires a battery to store the energy accumulated during the sunny hours for use at night. In most applications, lead-acid batteries are preferred as these are maintenance free and do not emit any harmful gas during the charging process. Usually, PV solar power units use 12 V or 2 V sealed maintenance free valve regulated lead acid (SMF VRLA) batteries specified in ampere hour (Ah) capacity. Ideally, a 12 V, 40 Ah lead acid battery will give a discharge current (called ‘C’) of 40 A for one hour or 10 A for four hours. However, such a battery, which is otherwise ideal for solar applications, does not perform well at the 1C discharge rate.
To obtain a reasonably high discharge capacity, manufacturers commonly rate these batteries at 0.1 C or 10 hour discharge; that is, 4 A for a 40 Ah battery. Remember that even the best lead acid battery, at this slow discharge rate, will never give 100 per cent of its rated output capacity.
The life of a lead acid battery depends on, and is directly related to, how deeply the battery is discharged. For instance, a 12 V, 40 Ah battery can ideally store 12×40=480 Wh of energy, but to avoid damaging the battery or drastically reducing its life, you should not extract all its stored energy.
When the battery is discharged, its terminal voltage starts dropping. You should stop discharging the battery when the terminal voltage reaches 10.5V. The battery will certainly be damaged irreversibly if it is discharged beyond that level.
Some batteries come with automatic protection against this. In such cases, discharging beyond a limit is prevented unless the battery is recharged. This feature is especially useful for solar use at home since home users cannot detect whether they are discharging the battery too deeply.
When fresh charging fails due to successive cloudy days, you will invariably end up discharging your battery deeply and, in the process, damaging it or drastically reducing its life. Therefore, such deep discharge protection must be made mandatory.
The charge-discharge cycle life of a sealed lead acid battery is also directly related to its repetitive discharge rate. In solar applications, the battery is charged once every sunny day. So for a single day that has one charge-discharge cycle, the battery will provide good service for:
- 200 to 220 days (one year) with 100 per cent depth of discharge (full discharge)
- 400 to 450 days (two years) with 50 per cent depth of discharge (partial discharge)
- 1000 days (four years) with 30 per cent depth of discharge (shallow discharge)
- 1500 days (six years) with 15 per cent depth of discharge (rated discharge)
There are SMF batteries available for solar use, which come with integrated protection to assure four to five years of minimum battery life. The protection feature automatically prevents the battery from being discharged beyond 1.7 V per cell, and also does not allow it to be discharged at rates higher than 0.15 C of its rating. Such batteries do not require any maintenance.
The author is ex-chairman, Electronics Commission, Government of India, and a former advisor to late Prime Minister, Rajiv Gandhi.