powering from lithium batteries

[lerameur]

Hello,

I have an RTC running on a big 12v battery. Although I just tried using two 1.5v 357 type lithium batteries. I thought these would power my DS1337 for over a year. but after less then a day I had only 1.1v left in my batteries... Is there a way to 'hook up' the battery ? I grounded the batteries together but I use a diode on the positive to the RTC because I do not want these batteries to get a 5v charge from the LM 7805 when the main circuit gets turned on.

Ken

[Charles Linquis]

I don't completely understand your circuit, but you could put a low-leakage diode and a resistor between the +5V and the lithium battery +.
When the 5V was present, the resistor could provide all the current the RTC needs. When the 5V wasn't present, the battery would power the chip.

[aratti]

You can use two diodes one for the small lithium battery and one for the 5 volt. The small drop on the 5 volts due to the diode will not create any problem to your RCT, but it will avoid that the lithium will discharge via the 5 volts rail. The other will protect your lithium battery from the 5 volts and when the 5 volts will be off it will keep your RCT on.

Cheers

Al.

[jellis00]

What aratti is proposing with two low forward drop diodes (preferably one of the Schotkky diodes like a BAT42) is commonly called "or'ing the power sources" and is frequently used for battery backup applications similar to what you are doing.
For example, here is a crude schematic for or'ing a 5v supply with a 3v lithium battery to provide power to a RTC. However, make sure that you don't have any pull-up resistors connected to the 3v supply or it will drain the backup battery even when the 5v is present...something I learned from experience recently ;>) Your RTC normally requires pull-ups on some of its pins....so make sure they are connected to the 5V and not to the 3v....RTC will continue to keep time without the pull-ups when running on the 3v backup.
5v----------- -|>----
..........................|
...........................-------------- RTC power supply pin
..........................|
3v lithium ----|>---

[lerameur]

HI,
this is exactly what I have (except I used a normal diode), I also added a 220kohm resistor in series with the diode to keep current down. Higher then that for the resistor, the RTC would not keepa good time. The problem I am facing now Is that at the begining I had 3.09volt, its decreasing by 0.01volt everyday, Its been almost four days now and it is at 3.05volt, at this rate the battery wont last more then a few weeks... my casio wrist watch battery lasts about 4 years.... !!! and thats including the display on all that time !!

[jellis00]

Not sure why you think you need the 220kohm resistor in series with the diode...if the only current load is the RTC it won't draw any more current than it requires and most RTC chips (i.e., DS1337) only require about 150 uamps when active and only about 1.5 uamps when in standby, which is the state you should leave it in before shutting down the 5v supply. At 1.5 uamps a typical 3v lithium (ie, CR2032 which has 165 mAhrs capacity) should last 110,00 hours or 12 years. Even if you leave it in active mode rather than standby it should last about 45 days.

Make sure no resistors are connected from the 3v connection to any other locations as leakage paths.