DS3231 RTC - Remove Battery and use 4.5v (3xAAA) as backup?

Other Hardware Sensors
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I have a DS3231 RTC in my 5v Arduino Pro mini project. It's a low power project and I'm also fighting for space. Project is powered from 3xAAA.

I have already removed the charging resistor so I could replace the rechargeable battery with standard CR2032.

I also removed the power indicator and the 4 i2c pull up resistors (top set of four)

All the above was based on this blog Using a $1 DS3231 Real-time Clock Module with Arduino | Underwater Arduino Data Loggers and works great.

Now I find I need to save more space so I wondered if I could ditch the backup battery completely and just wire the backup circuit into my main battery circuit.

So the question is can I just de-solder the battery holder and connect the +/- terminals left on the board to my main battery circuit.

My project shuts of power to the I2C connection when it sleeps so I'm thinking that the DS3231 will just switch to low power mode and switch to the battery connections (now connected to my 3xAAA) to keep the clock ticking!

I am assuming that the difference in voltage won't matter (3v expected from CR2032 and 4.5v from my AAA's) as the DS3231 is 3v or 5v.

Any comments for and against this Noob idea gratefully received.

Thanks

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coastline:
I have a DS3231 RTC in my 5v Arduino Pro mini project. It's a low power project and I'm also fighting for space. Project is powered from 3xAAA.

I have already removed the charging resistor so I could replace the rechargeable battery with standard CR2032.

I suspect you might be better off leaving things the way they are. The way you are going suggests that you are likely to find that the only thing you have that is reliable is the clock

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And when you change the AAA batteries, the RTC time will need to be re-set if you do away with the CR2032.

Also, since the backup circuit is 3.3V, it might not be happy with 4.5V.

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Two silicon diodes in series would drop about 1.3V, from 4.5 to 3.2, 1 silicon and 1 Schottky would drop about 1V.

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outsider:
Two silicon diodes in series would drop about 1.3V, from 4.5 to 3.2, 1 silicon and 1 Schottky would drop about 1V.

It's may works but an electrical engineer wouldn't do it.

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outsider:
Two silicon diodes in series would drop about 1.3V, from 4.5 to 3.2, 1 silicon and 1 Schottky would drop about 1V.

Yes, but does he want to set the clock/calendar every time he changes the AAA batteries?

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Nick_Pyner:
I suspect you might be better off leaving things the way they are. The way you are going suggests that you are likely to find that the only thing you have that is reliable is the clock

Always looking to learn so any elaboration would be welcome!

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OldSteve:
And when you change the AAA batteries, the RTC time will need to be re-set if you do away with the CR2032.

Also, since the backup circuit is 3.3V, it might not be happy with 4.5V.

Currently estimate a change of batteries once a year so not too concerned about re-setting the date and time.

I was hoping that the DS3231 would be tolerant to the 4.5v (dropping to about 3.8v with drain) as the Spec on the device claims 3v or 5v. I can't see any circuitry on the board that would drop 5v to 3v so I assumed that the device was as a whole was happy with either.

I quite expect my noob assumptions to be dashed by the experts - as I said I'm here to learn. My whole project (which should have been way beyond my ability) is only possible because of forums like this one.

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outsider:
Two silicon diodes in series would drop about 1.3V, from 4.5 to 3.2, 1 silicon and 1 Schottky would drop about 1V.

Thanks I,'ll take a look at that.

I'm hoping the backup circuit is the same as the main circuit and is tolerant to 5v or 3v.

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-Standby:
It's may works but an electrical engineer wouldn't do it.

It would be great if you could elaborate on your reasons and perhaps suggest an alternative.

Thanks

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Two silicon diodes in series would drop about 1.3V, from 4.5 to 3.2, 1 silicon and 1 Schottky would drop about 1V.

For which current ?
If I = 1µA, V will not be equal to 1 volt, rather to 0,1 or 0,2 V
How many mA is needed to power a DS3231 ?

When the current will be known you should check in the diode's datasheet the actual drop voltage .
Caution should be taken with Shottky diode. Behavior of a Schottky diode is not as simple as that of a "classic" diode .

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coastline:
I was hoping that the DS3231 would be tolerant to the 4.5v (dropping to about 3.8v with drain) as the Spec on the device claims 3v or 5v. I can't see any circuitry on the board that would drop 5v to 3v so I assumed that the device was as a whole was happy with either.

Have you considered looking at the chip's datasheet? I said "it might not be happy with 4.5V". I didn't say I knew for sure.

After perusing the datasheet, I now do know for sure - 4.5V is fine on the 'VBAT' pin of the chip. It's "Recommended Operating Conditions" state "2.3V to 5.5V".
So if re-setting the time/date isn't a concern, you're good-to-go, connecting the 'VBAT' pin to your AAA batteries.

DS3231 datasheet

And although in the end, 4.5V is OK, this wasn't necessarily a good assumption:-

I can't see any circuitry on the board that would drop 5v to 3v so I assumed ...

No circuitry was needed on the board to drop the voltage to 3V since only a 3V battery was connected to the 'VBAT' pin. No circuitry is necessary to drop 3V to 3V. :slight_smile:

Edit: You might not even need anything connected to the 'VBAT' pin, since you don't need secondary battery backup. Either way, when the main battery supply is disconnected, the power goes off.

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Hi OldSteve,

Thanks for responding.

Have you considered looking at the chip's datasheet?

Duh, of course I didn't - I'm a noob!

I will admit to being intimidated by datasheets that I have looked at in the past. I'm often left concerned because I didn't really understand all of the information presented. However in hindsight I guess power requirements should be (and in this case are) pretty easy to read.

I guess it would be better to ask for confirmation of what I thought the datasheet meant than not to look at all. You see, I learnt something new today! Thanks.

No circuitry was needed on the board to drop the voltage to 3V since only a 3V battery was connected to the 'VBAT' pin. No circuitry is necessary to drop 3V to 3V.

What I meant here was that I could see no regulator on the vcc line to drop it to 3v (from the 5v supply). So my assumption was that the circuit as a whole was 3v tolerant not just the battery input. Again I was trying to look at the board itself to figure out what was going on when the datasheet was the place to go.

Edit: You might not even need anything connected to the 'VBAT' pin, since you don't need secondary battery backup. Either way, when the main battery supply is disconnected, the power goes off

My reasoning behind wanting the VBAT connection is reduced power consumption. My project is in deep sleep mode for the majority of time. I shut off power to everything unnecessary (even going to the trouble of de-soldering power led's from the DS3231 and the Pro Mini and also the pro mini power regulator). I assumed that the DS3231 would shut down non essential power consumption on a loss of VCC to preserve VBAT battery life. The datasheet appears to confirm this.

So my understanding (which could be completely wrong) of the datasheet appears to indicate that the current usage between standby (no i2c activity) powered by VCC = 171.5µA (avg) and when powered by VBAT = 3.53µA (avg).

So as you say it looks like I can provide power to both VCC and VBAT at 4.5v (dropping to 3.8v as AAA's drain). When my project sleeps I shut off power to VCC and the DS3231 switches to low power mode and takes it's supply from the VBAT line also at 4.5v (down to 3.8v). Pretty cool!

Thanks OldSteve I appreciate you taking the time to point me in the right direction for this information. In hindsight it was the obvious place to look and a typical noob thing not to do, so thanks for the hand holding and not scolding me for being an idiot. It's not that I'm lazy I have done a huge amount of reading to get to the point I am at but sometimes you can't see the wood for the trees.

They really should develop a beer transfer system so proper thanks can be conveyed. ;D

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OldSteve:
Have you considered looking at the chip's datasheet?

coastline:
Duh, of course I didn't - I'm a noob!

I will admit to being intimidated by datasheets that I have looked at in the past. I'm often left concerned because I didn't really understand all of the information presented. However in hindsight I guess power requirements should be (and in this case are) pretty easy to read.

I guess it would be better to ask for confirmation of what I thought the datasheet meant than not to look at all. You see, I learnt something new today! Thanks.

The more you look at datasheets, the more you'll understand them. The datasheet should be your first port-of-call when you use a device you haven't used before, (and when you use a device that you have used before, for that matter).

My reasoning behind wanting the VBAT connection is reduced power consumption. My project is in deep sleep mode for the majority of time. I shut off power to everything unnecessary (even going to the trouble of de-soldering power led's from the DS3231 and the Pro Mini and also the pro mini power regulator). I assumed that the DS3231 would shut down non essential power consumption on a loss of VCC to preserve VBAT battery life. The datasheet appears to confirm this.

Yep. So I guess you plan to power the RTC's Vcc from a Pro Mini pin. Sounds like a plan to me.

So my understanding (which could be completely wrong) of the datasheet appears to indicate that the current usage between standby (no i2c activity) powered by VCC = 171.5µA (avg) and when powered by VBAT = 3.53µA (avg).

Sounds right to me.

They really should develop a beer transfer system so proper thanks can be conveyed. ;D

I don't actually drink the stuff. All I drink is coffee-flavoured milk. (Strange but true. :smiley: )

Anyway, it sounds like it's all coming together. Good luck with the rest of the project.

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So I guess you plan to power the RTC's Vcc from a Pro Mini pin. Sounds like a plan to me.

That's right. I'm already powering both the DS3231 and a 1602 LCD from a mini pin. Works well and well below DATASHEET specs for the mini pins :wink:

coffee-flavoured milk

:astonished:

Thanks again.