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Lithium battery voltage tolerance

I have been troubleshooting a circuit involving a Maxim DS1306 real-time clock (RTC) with battery back-up. I connected Vcc of the RTC to the same 3.3 V supply as required by the micro that talks to it. The battery is a Panasonic BR2330 (allegedly a 3 V part): -

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R2325_BR2330.pdf

The RTCs appeared to be failing in droves, in that I could program them once, but after a single power cycle they refused to respond to the micro any more. Replacing the chip always worked, but only for another power cycle.

After barking up a lot of wrong trees, I found postings on other forums indicating that the DS1306 switches from normal mode to battery-backup when Vcc drops below Vbat, but it doesn't switch back to normal until Vcc rises above VBat + 0.2 V.

The Maxim data sheet: -

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says that a device in battery mode cannot be written (page

9, last sentence of paragraph under "POWER CONTROL"). On-line user comments indicate that it cannot be read in this mode either. At this point I realized that if the second statement is true, a DS1306 in battery mode cannot be distinguished from a dead chip.

I measured the battery voltage and found the 3 V battery supplied 3.22 V, and my 3.3 V supply is unlikely to rise

0.2 V above this--ever. I pulled 7 more boards off the shelf to compare battery voltages:

#1: 2.9995 V #5: 2.9887 V #2: 3.1975 V #6: 2.9919 V #3: 3.0579 V #7: 2.9919 V #4: 2.9894 V

The expected load on the battery of the DS1306 is 550 nA @

3 V (pg 16).

If you look over the BR2330 data sheet you find no indication it can rise above 3 V, unless you hike up the ambient temp well above 20 °C (which I don't).

I was wondering what range of voltages other members of this forum have observed from this battery. ============================================================ Gary Lynch | To send mail, change my | domain name from "no$pam" gary.lynch@no$pam.org | to "ieee". ============================================================

Reply to
Gary Lynch
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Are you charging the battery? The Maxim data sheet said something about charge but I didn't read it carefully.

Batteries with a (really) full charge often go significantly above their nominal voltage. (I'm not a wizard on Lithium chemistry.)

What's the voltage on the battery on a working setup? Does it change if you let it sit there for a while?

What happens if you let the unit with 3.2V sit around for a while? Is that the voltage when the board is powered off?

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Reply to
Hal Murray

Gary Lynch wrote in news: snipped-for-privacy@Your-Name-Here.News:

Did you manage to confirm later that the RTCs were not dead, only in a state that couldn't be written to or read from?

Either way, something I've seen used with backup batteries might work for you, a series pair of 1n4148 diode, and a 1K resistance. The resistor would prevent any chance the battery can fry the chip in the absence of main power (I know, pretty stupid if that occured given the purpose of the IC but in some devices that can pull a pin low the chance exists of damage that way, and the 1K will allow enough current for the backup purpose). The diode will drop the voltage for you, likely enough to get the headroom you need for 3.3V to rise beyond the 0.2V you need. So long as the backup battery is only held out of circuit, and does not need a charge current, this should work ok.

Reply to
Lostgallifreyan

Lostgallifreyan wrote in news:Xns9C9138F22E408zoodlewurdle@216.196.109.145:

A thought on that diode... maybe use Schottky. I was thinking earlier of something I'd seen on a 3.6V circuit. In this case it's a battery (3.2V coin cell?) that is often considered dead once it falls below 2.6V so a smaller voltage drop is better.

Reply to
Lostgallifreyan

I was thinking the same thing. Test, the "dead" units and see if they come back to life when reinserted, and put a diode in line with the battery to drop the V a little bit. I use the DS1307 and the lithium coin cell we use drops to ~3V after about a day of use and then hangs there for the rest of the life of the cell. A Schottky diode in line with the cell might just do the trick.

Reply to
WangoTango

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