Battery for DS1307

premkumar9 · Feb 9, 2009

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 · Feb 10, 2009

premkumar9 said:
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 · Feb 10, 2009

ericgibbs said:
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 · Feb 10, 2009

premkumar9 said:
Like this?(ref attachment). Why do you suggest a 100nF capacitor ?

hi,
Like this.

premkumar9 · Feb 10, 2009

ericgibbs said:
hi,
Like this.

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

ericgibbs · Feb 10, 2009

premkumar9 said:
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 · Feb 10, 2009

ericgibbs said:
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 · Feb 10, 2009

premkumar9 said:
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 · Feb 10, 2009

ericgibbs said:
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 · Feb 10, 2009

premkumar9 said:
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.

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 · Feb 10, 2009

ericgibbs said:
hi,
No problem.

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

What is C?

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 · Feb 10, 2009

premkumar9 said:
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 · Jul 18, 2009

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 · Jul 19, 2009

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 · Jul 19, 2009

Gayan Soyza said:
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.

Boncuk · Jul 19, 2009

Hi,

with a 3V zener the battery won't be charged to its nominal voltage.

I suggest to use an easier circuit with an LM317L wired as constant current source for 1.2mA with a feedback resistor of 1041.66~ Ω (1KΩ fixed and 100Ω trimmable. Don't use +5V at the input of the regulator since it will work stable 3V above the desired output voltage. Any voltage >7V up to the limit will do.

A Schottky diode (not even a silicon switching diode would do) won't drop the battery output voltage low enough for 3V operating voltage. I suggest to use a voltage divider of 270Ω and 1.2KΩ to reduce the supply voltage for the RTC-chip slightly below 3V (2.939V). The chip's current draw is somewhere in the nano-amp range. So a voltage divider would work fine.

Boncuk

ericgibbs · Jul 19, 2009

Boncuk said:
Hi,
with a 3V zener the battery won't be charged to its nominal voltage.
Boncuk

hi Hans.
I Know,

, its a typo it should read 3V9, I have already told Gayan.

Its used to keep a OKI RTC ic powered from a 3.6V NiCd, it switches off at about > 3.2Vish.

Regards

Boncuk · Jul 19, 2009

ericgibbs said:
hi Hans.
I Know,, its a typo it should read 3V9, I have already told Gayan.

When I read it I had already posted.

I tried to push it to 3.6V which should work with a 4.3V zener. (but increased charge current)

Its used to keep a OKI RTC ic powered from a 3.6V NiCd, it switches off at about > 3.2Vish.

I reviewed the DS1307 datasheet and it says safe and reliable operation of that chip is guaranteed at a supply voltage between 2.0 and 3.5V.

So only 100mV (theoretically) have do be dropped. With the voltage drop of a Schottky diode of ~400mV the voltage is well within limits.

Regards

Regards to you, too

Hans

audioguru · Jul 19, 2009

A 3.6V Ni-Cad battery is 4.2V when fully charged at a low charging current. Then its discharging voltage slowly drops to 3.3V when it should be recharged.

ericgibbs · Jul 19, 2009

hi Hans and Gayan,
A slightly modified circuit.

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