the battery very early. For making your rechargeable batteries last longer requires to not letting them completely discharge.
A very useful circuit shown here will do this job very efficiently by alarming when your battery has come to a certain voltage level, for example a 12V lead acid battery shows 11.6 when it is 90% empty. So adjust the circuit with the 10K variable resistor to alert before the battery reach that level or set the circuit to alarm when the battery reach to 11.8V or 11.9V.
The circuit is using two transistors and a 555 timer IC. As the battery runs down and come to the preset level it will provide the beep beep signals and flash an LED. The beeping signals and LED flash rate can be adjusted with the 100K variable resistor. The circuit will not require any external power supply and it will run with the same battery. The circuit will only activates on the preset level and otherwise it will remain in standby due to which it will drain only micro ampere from the battery.
It can also be used for other type of batteries with variety of voltages. For example 4.5V, 6V, 9V etc. batteries type are NiCd, NiMH etc. For using with batteries below 9V use minimum value zener diodes for example 4.3V or 3.3V.
For making the batteries to last longer requires care of them, one of the main factor that weakens the rechargeable batteries is deep discharging them. Every time a battery gets completely discharge its efficiency decreases to some extent and continuously deep discharging damages the
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