Could I please have some feedback on this circuit.
Should the correct re-charge circuit for the faulty Chinese Tiny RTC modules look something like this?
I would use a chip dedicated for the purpose of recharging LiPo batteries, like this: https://www.sparkfun.com/products/10217 To use either circuit the battery has to be disconnected from the DS1307.
The DS1307 will keep time for several years on a standard 2032 cell, so there is really no need to worry about using a rechargeable battery.
As for feedback on your circuit: it isn't completely clear what it is supposed to do, but it won't work properly to recharge a LiPo battery, as it lacks an accurate reference voltage. What is the PNP transistor supposed to do (it is backwards with respect to power supply polarity)? What does the switch do?
The BC558 feeds into the Vbat pin of the DS1307 and is only turned on when the 5V supply is turned off.
The BC548 is the opposite and feeds current into the 3.6V battery positive, at about 4V through the voltage divider, when the 5V supply is switched on. And at the same time the BC558 turns off and prevents 4V feeding into the Vbat pin.
It is meant to just tickle charge the battery and roughly 10uA rather than at 40mA where you would need to voltage ref to turn off the charge current.
The DS1307 data sheet warns against having any circuitry between the backup battery and the Vbat input, including a diode.
jremington:
The DS1307 data sheet warns against having any circuitry between the backup battery and the Vbat input, including a diode.
I found the attached datasheet for LIR2032 coin cells and noted this:
Charge method
CC/CV
(Constant Current-Constant Voltage)
current:0.5C
Voltage:4.2V
End current: 0.02C
So how about this circuit?
All that needs doing is to adjust the pot to get 4.2V at the positive battery terminal (without the battery in place).
And there are no components between Vbat and the battery positive.
CoinBatteryLIR2032_.pdf (43.4 KB)
The absolute maximum voltage at the Vbat terminal of the DS1307 is 3.5 V. The battery will charge to higher than that.
jremington:
The absolute maximum voltage at the Vbat terminal of the DS1307 is 3.5 V. The battery will charge to higher than that.
Damn....this is a difficult problem to solve.
What about one of these reed switch relay devices: http://www.jaycar.com.au/productView.asp?ID=SY4030&CATID=79&form=CAT2&SUBCATID=979#1
Surely this would be OK with the DS1307 - no diodes or transistors between the battery and the Vbat pin.
The coil could be controlled by a BC558.
The designers of the DS1307 went to great effort to create a circuit which will keep reasonably accurate time and run off of a standard coin cell battery for up to 10 years without connection to any other power source.
It is therefore no surprise that you can't easily adapt this circuit to a rechargeable battery, for which it was not designed. There simply isn't any point in trying!
JohnLincoln:
I'm intrigued as to why you are spending time, money and effort on a faulty device. Wouldn't you be better off investing in a working one?
I tried the fix where you remove R4, R6 and D1 (the faulty charging circuit) and it seems to work without it. Since then I have not succeeded in causing the DS1307 to loose its time, once I set it, by disconnecting the arduino USB cable and the power to lines to the Tiny RTC module etc.
There is not a great deal that can't be done in electronics these days and there must be a way to trickle charge the LIR2032 with puting a diode or resistor between the Vbat and the batery positive and without apply greater than 3.5V to Vbat.
This reed switch chip thing I found at Jaycar seems to fit the bill. The only issue I can think of is that it will switch on and off slower than a transistor which would mean that 4.2V will be applied to Vbat very briefly.
Perhaps the better option is to simply provide a separate charging circuit for the LIR2032 so that you remove the LIR2032 from the RTC.