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12V battery charger circuit design help


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Hi Sir,
        I am quite new to electronics. I have a modem (from Teracom) powered by 12v(DC) 1.5Amps adapter. I want to run this modem using a 12v battery which needs to be automatically charged using 230V 50Hz AC power. I came across many schematics when searching on the internet. Following are the two links I think we can start with:

http://www.circuit-projects.com/battery/charger-circuit-for-6v-or-12v-car-battery.html

http://electronicsuite.com/power-supply-2/12v-car-battery-charger-suite/#axzz1QP3aTqRg

I have a 12V 4.5Ah battery.

Can you please help me start designing the circuit?

Thanks.

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Thanks Hero999 for your quick response. Although I posted a few links but I am not sure whether these circuits are really reliable. I mean as I am new to electronics I don't know whether these really are the circuits to start with. If they work perfectly then I am going to start my project.

Thanks.

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The first circuit seems to be the best one.

Ideally the battery should be charged at a constant current until the voltage reaches 14.5V, then the current is stopped until the voltage falls to 13.8V at which point the charger can be switched to float mode which just regulates the voltage at 13.8V indefinitely so the batteries can safely be left connected without either overcharging or discharging.

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Hi,
    I am almost done with designing the PCB. But one more question I want to ask is: Should I directly connect the battery to the modem or is there a need of any isolation or some kind of protection circuit required; like current limiting.
Any suggestion will be appreciated.

Thanks.

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