powering from lithium batteries

[falingtrea]

Standard diodes proably have too high of a reverse leakage current. What I have see as a recommendation is either a schottky diode and a series resistor or two schottky diode in series. At issue is reverse current charging of the primary battery. Litium coin cells do not handle charging very well.

[jellis00]

I have an application wired per my crude schematic above with Schottky BAT42 diodes on each leg which has been running on 5v for many weeks and the lithium CR2032 backup battery is still showing 3.11vdc. So I don't see the need of the resistor in series with the schottky diode or two shottky diodes...doing this just drops the supply voltage to the DS1337 some more by the addition of the voltage drop across the resistor or the additional diode.

The DS1337 requires pull-up resistors between the I2C pins, the ALARM pins and the 5v source AHEAD of the schottky diode and NOT TO THE 3v SOURCE or the downstream connection to the Vcc of the DS1337. Otherwise a leakage path will exist from the DS1337 pins where the Pull-ups are connected to the MCU, and the MCU (which will sink current even when its 5V Vcc is shut off) will drain the backup battery. This recently happened to me and drained my backup battery in a few days while the 5v supply was shut off. I had to cut traces and re-wire the pull-ups to the correct location to eliminate the leakage path.

So I recomment to lemeur is to replace your standard diodes with low drop Schottky diodes (like the BAT42) and to double check how you have your pull-up resistors wired. That may be why your backup battery is draining so quickly.

[Balachandar]

In my propeller clock that I designed some years ago, I used Philips 8563 RTC that is known for its very low power consumption. I used neither diodes nor resistors and connected pin 8 of the RTC directly to + terminal of 2032 Lithium battery. The 4.7K pull-up resistors of the RTC are connected to 5V. Since 2032 battery is not meant to be charged, there is no need to connect it to 5V rail. The only device that the Li battery is powering up is the RTC. So, how long the battery lasts is purely dependent on the power consumption of the RTC and the shelf life of the Li battery. In my circuit, even after 4 years, the RTC is working fine.

I am sure, DS1337 is very similar to Philips 8563 and you can try connecting the battery to the RTC without the diodes and resistors.

Bala

[falingtrea]

I have an application wired per my crude schematic above with Schottky BAT42 diodes on each leg which has been running on 5v for many weeks and the lithium CR2032 backup battery is still showing 3.11vdc. So I don't see the need of the resistor in series with the schottky diode or two shottky diodes...doing this just drops the supply voltage to the DS1337 some more by the addition of the voltage drop across the resistor or the additional diode.

The two diode or diode/resistor was a UL requirement on my design. Lithium coin cells cannot be back charged above a certain current level.