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For a wireless phone 4Ah is huge. But 9 volts? That seems odd, all modern cell phones use a single lithium cell. More details are definitely in order.
Alternatively, it might even work from 8V so an LM7808 might work.
Hi guys
Dear Robert-Jan
The phone is Panasonic model NO.KX-TC1743B (cordless phone)
it use external adapter the input is 220v 50 Hz
The out put is 9v 350mA so it charges the batteries in hand set but if there is
A failure in power source the phone will not work (receive or send calls)
The base unit will not communicate with hand set
so all I want to do is to make circuit To charge External Battery at the same time to make it work maximum 4 hours
Best regards
Danny
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Hi Robert-Jan,
A battery is only partially charged when its voltage reaches a certain level. The battery current and its voltage determines when it is fully charged.
Many products that use a rechargeable battery have the battery being trickle-charged all the time. Then the power supply powers the product and trickle-charges the battery at the same time without switching.
The voltage of your zener diode varies significantly because it does not have enough current in your circuit. Add a resistor from the base of the BC547 to the negative of the battery.