Battery Size and RTC

mctrivia

looking on digikey for non rechargable 3V lithium bateries requiring a holder i find the max storage of each size to be:

10mm - 30mAh
12.5mm - 48mAh
16 mm - 130mAh
20 mm - 225mAh cheapest
23 mm - 560mAh
24.5 mm - 1000mAh

What I am wondering is how big of a battery do I need to run DS32C35-33IND# for 10 years or so. Datasheet lists 4 different currents
Active Battery Current: 70uA
Time Keeping Battery Current: 3.0uA
Temperature Conversion Current: 575uA
Data-Retention Current: 100nA

I am pretty sure I can ignore the first one as I will not be requesting information from the ic when on battery power.
Do I need to add the bottom 3 to get 578.1uA or 50675mAh none of these batteries would last even a year if that is so.

I also have SparkFun's RTC and it comes with a battery rated for 10 years but is not as accurate as the above IC and requires an external crystal.

dMajo

mctrivia,

"active battery current" is the current used to communicate with the device powered by Vbat (no Vcc). The device is able to perform i2c communication except for the fram
"timekeeping battery current" is the current used to keep the rtc running in absence of power (Vcc)
"temperature conversion current" is the current used to perform temperature calibration.

I assume that when no Vcc also the prop will be off (so no need for communication)

10· 1 year= 365*24= 8760h*3uA = 27mA or 8760h*100nA=1mA without rtc running (just keeping the time at when the power was loss)
8760h/60=146h*575uA=84mA (the temp calibration happens once a minute)

so total power 27mA+84mA=111mA ==> if space is not your problem go with the cheapest one 225mA

PS: be shure you have understood also Note 3
····· Note 5 says that the temp cal is already averaged in the timekeeping current so probably the second line is not necessary but I usually do as above

Post Edited (dMajo) : 2/10/2009 5:23:54 PM GMT

mctrivia

thanks for the correction. if I am not mistaken that 111mAh is for 1 year. still means the 1000mAh could do me 9 years.

dMajo

Yes, you're right: I have updated

Carl Hayes

Any of these sizes should suffice for you. However, it's hard to find button-cell holders for the 10mm size -- thus for my own stuff I chose the 12 mm ones. Then I discovered that many button-cell holders don't hold the CR1216 very tightly (it's quite thin), so I changed to CR1225 which is thicker and works well.

I recommend (also from recent experience) putting a 0.1 microfarad capacitor across the battery, because the mechanical contacts in the button-cell holder sometimes will drop out when there is vibration or shock. The 0.1 cap will run the RTC for about 50 to 100 milliseconds, which is plenty to get past a shock-induced loss of battery contact.

On eBay several vendors sell CR1225s for about 13 cents each, quantity 40, and under a half buck, quantity ten -- brand new in the manufacturer's packaging. I have found them perfectly satisfactory. Don't pay $3 each at your local supermarket!

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· -- Carl, nn5i@arrl.net

mctrivia

ok so with note 5 i can ignore temperature so I need 270mAh for 10 years so i can save 4.5mm and $3 by gong with the 225mA battery at the sacrfice of only getting 8 years. Now to see how much room i can squeeze out of this board. May need to make stand offs and put the battery over top of the rtc.

note 3: what is VIH?

dMajo

Is the voltage for the high level "1". As the logic sees the 1 at Vcc*0.7 (3.3*0.7 so >2.3) they aware you that if this voltage is below the Vbat-0.6 (so 3.0/3.6-0.6=2.4/3.0) you will still be able to communicate but because of the internal scl/sda structure you will drain much more current from the battery. I assume much more than IBATLKG (maybe could it be the difference between the typ and max?). Just be sure to have high levels above 2.8V (prop Voh>2.85 @ Vdd=3.3V).

mctrivia

if the prop has no power though would not scl and sda be 0V?

dMajo

Yes, but this will be logic 0. The problem happens if you drive them high (Vih) with let's say 2.4v: this is logic 1 and below vbat-0.6v. Maybe the increased leakage is because of some forward biased diodes somewhere in the internal design of the chip, I don't know. But this will not happen at logic low so just be low or high enough. There is also a nonsense if this happens at low levels because they are supposed to be, when on battery backup, in absence of main power: otherwise why the backup if it will not withstand the power outage

mctrivia

i am battery backing the prop also though at much shorter duration then the clock. but i am monitoring if on line or battery so i will make sure that states are locked low when in battery mode and wait until back on system power before checking the time.
thanks a lot for the help.