Non rechargable battery circuit

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Hi there,

I am designing a circuit using an ATtiny13V. The circuit will be in aplastic box that will be installed and located in a non accesible place.

There is a very long life non recargable battery inside used as back up power supplier. The whole circuit will be supplied by the mains power supply via a DC circuit. An ATmega88 always check the main supply connections from 10 points. It is very critical for the system not to stop even for a second transmiting the data.

There is a need for the system to know as soon as possible where the problem was each time the mains fails in order to transmit the data to a data collector.

The problem is that when the power supply fails a circuit controlled by the controlled, without using a relay, to put inside the non rechargable battery even if this event will be happened once in a year. Also the battery must not be parallel with any kind of load when not used, to save its energy. Therefore the leackage current from the battey goes in this circuit must be absolute 0 (I mean 0,00000000000000000000A). The battery must be discharged by itself as the manufacturer informs (less than 1% at 25 degrees each year)

I tried a mosfet circuit but the leackage current is anacceptable. Does anybody has any idea?

Thank you very much.

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer

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In a similar situation we have a nominal 5V Vcc AVR system running from a regulator, etc. There is a diode to Vcc from the 5V so the AVR actually runs at about 4.6V. There is a 3.6V lithium battery as the backup battery, also with a diode to Vcc. So when main power fails the AVR is running off the battery at about 3.3V.

this works quite well, as the keep-alive tasks are not high-power. You said "transmit" which can be a power sucker. Note that the Shottky diode drop will go up quite a bit as power draw increases (say, to over 1/2 volt during a 100mA RF transmission) and then the system may become starved running at 3V or below.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Note that Schottky diodes have greater reverse leakage currents than normal diodes.

But maybe that will help against self-discharge if there is a few uA going to the battery.. But when the mains fail, the few uA will go into the other direction..

- Jani

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Why can't you use a relay - reliability issue or something?

A compact relay that is powered by the mains input that will switch across to your backup battery when the coil is unpowered will provide total isolation to the battery in normal mains mode. You can size a capacitor as needed to keep the circuit alive as the relay switches from mains to battery and back again. Use large capacity quality ceramics (X5R etc) since they are less leaky and less temperature sensitive than tantalum/electrolytics.

Schottky diodes are very leaky compared to standard diodes - they become even more leaky at high ambient temperatures.

cheers,
george.

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Thank you all

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer