Chip that fix battery charging and power supply

Thread Starter

StefanTh

Joined Nov 23, 2019
4
Hi

We have an RTC chip that is normally feed with the main supply on the PCB but we also want it to be feed with battery backup when main power disappear.

Is there a chip that can charge a standard (like NiMh) rechargeable button cell battery and also switch between the main supply and the battery?

When the main supply is available the chip is feeding the RTC chip with the main supply and recharging the button cell.
When main supply disappear the chip switch, seamless to the battery as a power supply to the RTC chip.

I have a faint memory of seeing such chip somewhere but can not recall it's name or number, I guess there are a lot of them.

PS
The main supply will still feed the normal power connector of the RTC but this chip will be connected to the battery pin of the RTC
DS


Regards
 

jpanhalt

Joined Jan 18, 2008
8,303
Which is fine as long as you fit a rechargeable battery...
The chip comes with a black-box warning in big font to make sure only a battery of the correct type is installed by the user. Nothing is foolproof, though. Since the TS wrote,
Is there a chip that can charge a standard (like NiMh) rechargeable...
I assumed it was a given.
 

Thread Starter

StefanTh

Joined Nov 23, 2019
4
Well, we have MCP79410, it is equal to the DS3231 in the sense that it have a specific battery input pin.

We are looking into three different solutions ,
1) We go with a 5.5V supercap that is charged via +5V and is feeding the BAT input of the RTC
2) We go with button cell, non-rechargable, that is feeding the BAT pin and is monitored and alarm when the voltage goes to low
3) We go with button cell, rechargeable, that is feeding the BAT

1) and 2) is pretty straightforward, but not 3), which initiated the thread.
2) and 3) will need at least one extra chip
1) With a 7.5F, will hold the time around 298 days

If the solution is not, "wow this chip, with 3 external components, solves everything", we most likely go with 1) or 2), leaning towards 1).

The easy solution we looking for is a chip that have at least
one 3.3V - 5V input,
one pin for a rechargeable (button cell) battery and a
power output pin (goes to the BAT pin of the RTC).

The power out pin is feed with the input 3.3 - 5V if it is present.
The power out pin is feed with the rechargeable battery if 3.3V - 5V is not present
The battery is charged if 3.3V - 5V if it is present.


> https://forum.allaboutcircuits.com/threads/real-time-clock-rtc-breakout-boards-warning.118173/
The way the button cell is charged which the thread is warning about can not be considered because
the solution will be placed in a commercial product.
 

jpanhalt

Joined Jan 18, 2008
8,303
Well, we have MCP79410, it is equal to the DS3231 in the sense that it have a specific battery input pin.

1) and 2) is pretty straightforward, but not 3), which initiated the thread.
2) and 3) will need at least one extra chip
??? Solution 3 (shown in post #3 ) requires a resistor and diode, no extra chip.
 

ericgibbs

Joined Jan 29, 2010
9,044
hi Stef,
The DS3231 module I posted has a CR2020 battery, which is lightly charged while the main 5V supply is present and it will maintain the RTC for many hours when the 5V is absent.
I have three systems which have been in constant use for about 3 years, without any problems.

All the module require are the power and MCU control wires. [ which can be up to 2 mtrs in length if screened.]

E

BTW: I consider the statements in the posted link regarding a CR2020 battery warning is not substantiated in practical experience
 
Last edited:

Thread Starter

StefanTh

Joined Nov 23, 2019
4
I think i finally found the chip I saw long time ago

http://www.ti.com/lit/ds/symlink/bq25050.pdf

This one will use the main power to charge the battery and drive the load (RTC).
When the main power disappear, it will switch to use the battery to drive the load
(Using the External Power Path Control Feature )

By this, you get a well controlled way of charging the battery and auto switch between
the sources.

This one is from Ti but I guess there are others out there.

Regards
 

jpanhalt

Joined Jan 18, 2008
8,303
The system your RTC cam e with works fine. Why complicate it?

If that is for "learning," the Microchip MCP73831/2 is a nice chip and has an output that can be used for state of charging (see: Adafruit's eval board). You can also get a board from Microchip,I think. Those are very small chips.
 

Thread Starter

StefanTh

Joined Nov 23, 2019
4
MCP73831 will work as well, if there is no problems to add the BAT input to the cell directly, which most likely will not be a problem, the BAT pin will use 700 nA and tolerant up to 6.5V

But this threads original question is solved.
 
Top