I am designing a system that contains an RTC to for displaying the date and time on a user interface, as well as log events with time stamps. In this particular case, the system gets powered-on at intervals ranging from once a day to once every two weeks. When used, it runs on a DC power supply, which is derived from the mains AC power supply. The system can remain powered-on/used for any time duration between 30 minutes to 4 hours at a time. When powered-off, the mains power (and thus the DC power supply to the circuitry) gets disconnected. This is when the RTC needs to make use of a backup battery (or similar backup) solution in order to keep the RTC running. The RTC that I intend to use is the Microchip MCP79510.
Of course the most common solution for such an application would be to simply use a standard coin cell battery (such as the CR1220 or CR2032). However, the system needs to be designed in such a manner that the normal user does not have any physical access to the internals of the system, where the backup battery is located. Only authorized personnel should have such access. So in order to prevent visits to the system by such personnel just for the sake of replacing a battery every time it has been depleted (even though this could only be once every 5-10 years, and there could be a number of these systems implemented in the field), I am looking at an alternative solution to provide this backup power.
My first thought is to just use a rechargeable coin cell battery, with a small charging circuit. This should theoretically never require a battery replacement, unless the battery itself is malfunctioning. I have done some reading regarding charging these rechargeable coin cell batteries. I am well aware that non-rechargeable coin cell batteries are... non-rechargeable...
With that said, I have read that trickle charging should never be used for these types of batteries, since the constant current charging mechanism could lead to a charging voltage higher than the battery is rated for. Therefore, my understanding is that I cannot use a "charging" circuit that always keeps the battery at its fully charged voltage, as used with other types of battery applications. Am I understanding this correctly, or am I mistaken?
My first preference for this type of battery is the something like the Panasonic VL1220. For this particular battery, there is a document that outlines a few charging circuits, of which the below circuit seems to be the "standard" one to use:
Doesn't this circuit constantly charge the battery, as is typically recommended against?
During further reading, I came across the Microchip MCP73831 charge management controller. While it seems like an attractive alternative to the discrete circuit solution, it also mentions a constant current charging functionality, which is (according to my understanding) not suitable for charging these type of batteries. Could this device be configured properly for charging rechargeable coin cell batteries, such as the case in my application?
An alternative solution to the battery setup is to use a supercapacitor. Although it is a much simpler solution, this would heavily rely on the system being powered-on before the supercapacitor is depleted, in order to recharge the supercapacitor. The operating voltages for these types of capacitors put a heavy restriction on the total backup time, especially if there exists the possibility of the system not being powered-up early enough to recharge the supercapacitor before it has already been depleted. Furthermore, the physical size of very high capacitance supercapacitors also becomes an issue. Therefore, the supercapacitor solution is probably not ideal.
UPDATE: From the comments thus far and Neil_UK's answer, it seems that the MCP73831 is not necessary, and that the above circuit would be sufficient for the application.
UPDATE 2:
In the case of using a rechargeable coin cell battery, is the expected lifetime of the battery itself in the same order of the non-rechargeable coin cell batteries? If this is the case, then perhaps it is inevitable that the battery will have to be replaced every 5-10 years, irrespective of whether it is the rechargeable or non-rechargeable type (which means I might just as well go with the non-rechargeable battery type).
Answer
Panasonic warn against constantly supplying a current, the sort of thing you can do with nickel chemistry batteries, if the current source can supply a voltage higher than 3.6v. With a 5v supply, there would be a great temptation to simply put a resistor in series to limit the current to the battery. That would work for NiMH, but would damage the Vanadium/Lithium battery.
The illustrated circuit does not supply 5v to the battery. R1 and R2 reduce the voltage, and D1 reduces it further. Warning, it does rely on the input voltage being a good 5v. If you supply (say) 9v to that charging circuit, it will damage the battery. Unless you have a good 5v, you should instead use one of the regulator circuits suggested by Panasonic, for instance #3 or #4.
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