Save your spare batteries from self discharge due to improper charge- discharge cycles. If you forget to charge the sealed lead acid battery for long period, it will not accept full charge during charging and its performance deteriorates. A solar panel can solve the problem and since it charges only during day time, over charging can be prevented.
You can connect 12 volt batteries parallel to the set up.
A 12 Volt 150 mA solar panel provides sufficient current and voltage to charge the battery. Since the battery is under slow charge, it can be left unattended. To keep the battery in top condition, it is better to discharge to 50% once in a week using a 12 volt car bulb. 12 volt 150 mA Solar panel cost only Rs.300. If you have standby NiMH batteries, use a 6 volt 100 mA Solar panel for charging. Fix the solar panel in a place where direct sunlight is available.
About Solar Cell
These are photoelectric transducers generating electric current by accepting light energy. The photocell has many p-n junctions connected in series. One of these junctions is very narrow to allow the passage of light energy. When sunlight falls on the p-n junction, electron-hole pairs are formed. These charge carriers generate current proportional to the incident light energy. The sunlight incident on the semiconductor junction of solar cell collides with the valance electrons. This causes formation of charge carriers which cross the p-n junction in an opposing manner and creates a voltageacross the p-n junction. Approximately 0.6 volt will be generated from each solar cell if the sunlight is strong.
About Lead Acid Battery
Battery cycle
One complete discharge and recharge forms a battery cycle. This cycle is usually a discharge from 100 % to 20 % charge followed by recharging from 20 % to 100 %. Life of the battery depends on the number of deep cycles per day. For example, if a battery is discharged 50 % per day and recharged to 100 %, its life increase to twice than a battery discharging and recharging occasionally. Therefore it is better to discharge and recharge emergency lamp battery and inverter battery daily to increase its life. It is also better to start vehicles daily even if it is not running. This prevents self discharge and increases the efficiency for cycling.
Self discharge
If the lead acid battery is not discharging through the load, self discharge takes place @ 4 % per week at 27 degree. For example, a 125 Ah battery shows self discharge @ 5 Amps current per week if it is not discharging through the Inverter.
Memory Effect
‘Memory effect’ causes the battery to ‘remember’ how much charge was released on previous discharge. So that the same amount of energy is released with every charge/discharge cycle. The battery then keeps the ‘memory’ of the primary platform and considers it as the end of discharge for the next cycle even though the battery can be discharged further. If a ‘memory effect’ develops, during the next discharge cycle, battery only remembers this. Memory effect is high in Lead Acid battery and low in NiCd battery. Memory effect is negligible in NiMH battery.
About NiMH battery
The NiMH battery is similar to NiCd cells in structure but it uses a hydrogen absorbing alloy as negative electrode instead of cadmium. The positive electrode is a nickel oxy hydroxide (NiOOH).Alkaline batteries have approximately 3000 mAh capacity with a 1000 mA load. Digital camera with LCD and flash light can draw over 1000 mA quickly and depleting the alkaline batteries after a few shots. NiMH batteries on the other hand can easily handle the high current levels and maintain their full capacity. AA type NiMH cells have a nominal capacity (C) ranging from 1000 mAh to 2700 mAh at1.2 volts with a discharge rate of 0.2C / hour.
Typical NiMH battery is rated 1.2 volts/cell. This is the nominal voltage of the cell that is discharging at the rate of C/10 at 25 degree centigrade to an end voltage of 1 volt. The battery pack has three 1.2 volt cells making a total 3.6 volts. When a fully charged battery is used, the discharge voltage starts from 1.5 volts followed by a sharp drop to 1.3 volts. The voltage in the cell remains 1.3 volts to 1.2 volts until another voltage drop occurs during a deep discharge. The nominal voltage of the battery can be calculated by dividing the battery power by its capacity.
Fully charged battery loss it capacity when not in use for long time. The self discharge rate of NiMH battery is 5-10% on the first day. Self discharging property makes NiMH battery unsuitable for many light duty uses such as digital clocks, remote control devices etc where battery would normally be expected to last many months. Temperature at which the battery is stored is very important. Higher temperature above 55 degree increases self discharge rate.
Filed Under: Electronic Projects