Battery for DS1307 - Electronic Circuits Projects Diagrams Free

premkumar9 · 10th February 2009, 06:33 AM

Hi,
DS1307 is a real time clock. I use it in my project. The battery for backing up during power failure is specified as 2 to 3.5 V. I used CR2025 3V lithium battery which is working fine.
Can I use 3.6V 60mAH Ni Cd battery instead as in the circuit with a zener ? What is the resistance value recommended?

ericgibbs · 10th February 2009, 08:11 AM

Quote:

Originally Posted by premkumar9

Hi,
DS1307 is a real time clock. I use it in my project. The battery for backing up during power failure is specified as 2 to 3.5 V. I used CR2025 3V lithium battery which is working fine.
Can I use 3.6V 60mAH Ni Cd battery instead as in the circuit with a zener ? What is the resistance value recommended?

hi,
You could just connect a low voltage diode in series with the battery positive terminal.
A BAT85 or schottky low voltage diode should be OK.
Also I would connect a 100nF cap from the DS1307 battery pin to 0V.

premkumar9 · 10th February 2009, 09:57 AM

Quote:

Originally Posted by ericgibbs

hi,
You could just connect a low voltage diode in series with the battery positive terminal.
A BAT85 or schottky low voltage diode should be OK.
Also I would connect a 100nF cap from the DS1307 battery pin to 0V.

Like this?(ref attachment). Why do you suggest a 100nF capacitor ?

ericgibbs · 10th February 2009, 10:11 AM

Quote:

Originally Posted by premkumar9

Like this?(ref attachment). Why do you suggest a 100nF capacitor ?

hi,
Like this.

premkumar9 · 10th February 2009, 10:17 AM

Quote:

Originally Posted by ericgibbs

hi,
Like this.

Can you just show the charging of this battery from Vcc (5V) also?

ericgibbs · 10th February 2009, 10:27 AM

Quote:

Originally Posted by premkumar9

Can you just show the charging of this battery from Vcc (5V) also?

hi,
The charge circuit is additional to the simple voltage dropper I posted.

Are you asking for a charger circuit.?

premkumar9 · 10th February 2009, 10:35 AM

Quote:

Originally Posted by ericgibbs

hi,
The charge circuit is additional to the simple voltage dropper I posted.

Are you asking for a charger circuit.?

Here, earlier I used a 3V (non rechargeable) battery. Now i am thinking of using a rechargeable battery marked as 3.6V. I want the charging from 5V supply of the IC. But the IC data sheet says the recommended battery voltage as 2 to 3.5V.
So I want to
1. Limit the voltage.
2.Charge it from 5V
Earlier I connected the 3V battery directly to the IC pin with out any extra cap or R .

ericgibbs · 10th February 2009, 10:50 AM

Quote:

Originally Posted by premkumar9

Here, earlier I used a 3V (non rechargeable) battery. Now i am thinking of using a rechargeable battery marked as 3.6V. I want the charging from 5V supply of the IC. But the IC data sheet says the recommended battery voltage as 2 to 3.5V.
So I want to
1. Limit the voltage.
2.Charge it from 5V
Earlier I connected the 3V battery directly to the IC pin with out any extra cap or R .

hi,
You are confusing two different points.

1..If you use a 3.6Vbty you need to drop the voltage by using the simple diode dropper in my post. This will meet the Vbty 2V to 3.5V requirement stated in the DS1307 data.

2.. The battery 3.6V NiCd charger is a different/additional circuit.

premkumar9 · 10th February 2009, 11:14 AM

Quote:

Originally Posted by ericgibbs

hi,
You are confusing two different points.

1..If you use a 3.6Vbty you need to drop the voltage by using the simple diode dropper in my post. This will meet the Vbty 2V to 3.5V requirement stated in the DS1307 data.

2.. The battery 3.6V NiCd charger is a different/additional circuit.

There is a communication gap. In my OP i asked for resistance value to charge the bat from 5V (pls refer fig) , not for dropping the voltage.

Both the above (charging also) are my requirements.

ericgibbs · 10th February 2009, 11:55 AM

Quote:

Originally Posted by premkumar9

There is a communication gap. In my OP i asked for resistance value to charge the bat from 5V (pls refer fig) , not for dropping the voltage.

Both the above (charging also) are my requirements.

hi,
No problem.

Have a look at this circuit, keep the charge current to C/0.05 for simple float charge.

BTW: Looking at your OP, there is no reference to Charging or a 5V supply.

Quote:

DS1307 is a real time clock. I use it in my project. The battery for backing up during power failure is specified as 2 to 3.5 V. I used CR2025 3V lithium battery which is working fine.
Can I use 3.6V 60mAH Ni Cd battery instead as in the circuit with a zener ? What is the resistance value recommended?

premkumar9 · 10th February 2009, 02:45 PM

Quote:

Originally Posted by ericgibbs

hi,
No problem.

Have a look at this circuit, keep the charge current to C/0.05 for simple float charge.

What is C?

Quote:

BTW: Looking at your OP, there is no reference to Charging or a 5V supply.

I showed a resistance connected to Vcc(5V). Then my doubt was about the value of R(indicated charging current)

ericgibbs · 10th February 2009, 03:09 PM

Quote:

Originally Posted by premkumar9

What is C?

I showed a resistance connected to Vcc(5V). Then my doubt was about the value of R(indicated charging current)

hi,
C for the battery you have [60mAH] is 60mA.

So C/0.05 = 0.06/0.05 = 1.2mA.

Gayan Soyza · 19th July 2009, 07:48 AM

Hi eric in your last schematic how did you calculate charging rate C/0.05 ?? The zenner votage is below the battery voltage V(z)3.0 < V(b)3.6V !!!!

MikeMl · 19th July 2009, 08:38 AM

Yes, I have the same question. The posted circuit is a constant-voltage charging circuit; not a constant-current method required by Nicads.

Nicads are probably the worst battery technology to use to back-up a real time clock. The useful life of the Nicad even under constant-current trickle charge will be less than the shelf life of the Cr2025 it replaces.

A better approach is to diode isolate the Cr2025 so that while the 5V is available, the operating current for the RTC comes from the 5V, and the battery supplies current to the RTC only in the absence of the 5V.

If you insist on using the Nicad, just calculate a resistor that will limit the charging current into the Nicad to less than the 0.05C rate. Under long term charging, the Nicad's terminal voltage will likely climb higher than the stated 3.6V, say 3.8V, so if the capacity (C) of the Nicad is 0.06Ah, the 0.05C rate would be 3mA, so R=(5-3.8)/0.003 = 400Ω

ericgibbs · 19th July 2009, 09:17 AM

Quote:

Originally Posted by Gayan Soyza

Hi eric in your last schematic how did you calculate charging rate C/0.05 ?? The zenner votage is below the battery voltage V(z)3.0 < V(b)3.6V !!!!

hi Gayan,
Its a typo it should read 3v9, there isnt a 3v0 zener to my knowledge,?

The circuit shown is taken from the OKI, RTC clock applications book.
I used it for a number of years with OKI ic's and NiCd without any problems.

IMO, ideally is requires a lowish value resistor in series with the battery.

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10th February 2009, 06:33 AM	#1
premkumar9	Battery for DS1307 Hi, DS1307 is a real time clock. I use it in my project. The battery for backing up during power failure is specified as 2 to 3.5 V. I used CR2025 3V lithium battery which is working fine. Can I use 3.6V 60mAH Ni Cd battery instead as in the circuit with a zener ? What is the resistance value recommended? Attached Thumbnails

19th July 2009, 07:48 AM	#13
Gayan Soyza	Hi eric in your last schematic how did you calculate charging rate C/0.05 ?? The zenner votage is below the battery voltage V(z)3.0 < V(b)3.6V !!!! __________________ Gayan My Website http://gsmicro.blogspot.com/

19th July 2009, 08:38 AM	#14
MikeMl	Yes, I have the same question. The posted circuit is a constant-voltage charging circuit; not a constant-current method required by Nicads. Nicads are probably the worst battery technology to use to back-up a real time clock. The useful life of the Nicad even under constant-current trickle charge will be less than the shelf life of the Cr2025 it replaces. A better approach is to diode isolate the Cr2025 so that while the 5V is available, the operating current for the RTC comes from the 5V, and the battery supplies current to the RTC only in the absence of the 5V. If you insist on using the Nicad, just calculate a resistor that will limit the charging current into the Nicad to less than the 0.05C rate. Under long term charging, the Nicad's terminal voltage will likely climb higher than the stated 3.6V, say 3.8V, so if the capacity (C) of the Nicad is 0.06Ah, the 0.05C rate would be 3mA, so R=(5-3.8)/0.003 = 400Ω __________________ Mike ML. Last edited by MikeMl; 19th July 2009 at 08:56 AM.