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I have seen this rechargeable battery back up circuit on web.The resistor is used to limit the current.The diode in series is mainly to prevent the back feeding the power supply during battery operating mode.But I am not sure what is the application of another diode which is in parallel to the resistor.What would be the impact if we I remove the diode.

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The resistor limits the charge current, but when the battery's in use, you don't want to limit discharge current the same way - you want the battery to be a low impedance source for the circuit it's powering. The diode bypasses the charge-limiting resistor so it can discharge a lot faster (and more efficiently) than it charges.

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  • \$\begingroup\$ Thank you Nick. Since I am float charging my battery(security systems),I wanted to eliminate both the resistor and the diode.Even if the battery develops shorts,my AC-DC Converter has current limitations.Would it work in that case? \$\endgroup\$ – ANONYMOUS Nov 4 '15 at 12:03
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    \$\begingroup\$ @sanju As long as your supply's current limit is less than the battery's charge rate limit, and you retain the diode between the plugpack and the rest of the circuit, that should work fine. \$\endgroup\$ – Nick Johnson Nov 4 '15 at 12:07
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    \$\begingroup\$ Make sure the float voltage is appropriate for the battery. If you have a wide range of ambient temperatures, you may need to temperature compensate the float voltage. Also, batteries should have fuses in series, just like other power sources. A 2.1 Ah lead battery might not need a special fuse, but the fuses for large batteries need to be capable of interrupting very large DC currents. (see MIDI fuses, for example). \$\endgroup\$ – mkeith Nov 4 '15 at 18:58

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