UL-approved coin-cell charging schematic (ML-1220)

[ricko_uk]

Hi,
the attached schematic shows the battery charging circuit for a coin cell battery ML1220 that has been previously accepted in a client's product that received full UL certification.

The rechargeable coin cell full part number is ML-1220/F1AN coin cell battery (datasheet attached).

When a new product has recently been submitted with the same schematic, the test house said it does not meet the UL approval criteria because UL approval requires that "Conditions of UL approval: The maximum current must be restricted to 300mA when protective components have been subjected to short- or open-circuiting."

Obviously u can put a fuse inline but I am pretty sure that is not the solution that would normally be used. Perhaps placing two 6R resistors in series instead of a single 12R (for R2)? That would require double fault which I believe does not need to be "covered"?

I looked everywhere for an UL-approved charging circuit for ML1220 (and similar) coin-cell batteries.

Does anybody have any such circuit or information about a solution that is UL-approved.

Thank you

[ricko_uk]

Hi all,

could anybody offer any suggestions?

Thank you

[Peabody]

Below is the ZS-042 DS3231 module.  Its charging circuit is a regular diode and a 200R resistor.  But I very much doubt the design is certified by anyone.  And most people disable it because these modules usually have a CR2032 coin cell, which is not rechargeable, not the LIR2032 for which the charging circuit is intended.

[ArdWar]

How is that first design even managed to get UL approved in the first place? The charge resistor is way too small even in the best case scenario.

Let's assume typical test scenario of 3.3 Vcc, 2.5 Vbat(low), 0.3 Vf diode, and 1 mA charge current (typical 1220).
You need (3.3 - 0.3 - 2.5) / 0.001 = 500 ohm charge resistor.

[ricko_uk]

Thank you both.

The client relaxed the requirements so we are now using a supercap.

[floobydust]

I think it was salvageable, resistor values were way too low for R2, R5 etc.  There are NiMH as well.
FYI MCP7940N is piece of junk RTC look at the silicon errata. It's cheap for a reason.

[Peabody]

That's really awful.  And it looks like they've only discovered new ones every few years.  Who knows how many more are still waiting to be recognized.

I think this one is unusable.  But is there another cheap RTC that's ok?  I've always used the DS3231SN, but it's not cheap.

[floobydust]

Sort by price and it's the cheapest RTC available. Microchip CEO Ganesh Moorthy on the golf course thinking he's beat everyone 
I mention it as an example of Microchip rolling out shoddy product, and then not fixing the silicon after even 10 years. ACROSS THE BOARD MCP794xx, MCP795xx all are copy'n'paste same bugs i.e. "Date incrementing at noon" (plus rollovers) and then firmware hoops to workaround for the entire family. It's absurd they tested so poorly to begin with and continue selling them. But USD $0.59/100-lot and add $10 worth of hassle and $100 worth of headache some would say a bargain.

I don't use cheap RTC's anymore - either they drift too much or I've gotten burned with bugs in them.
Years ago it was PCF8583 that didn't reliably set the alarm flag, product stayed asleep and I had to helicopter out to sites $1,000/hr. Total nightmare.

It appears a maker favorite is the DS1307, but over 26 years old from the days of the 8051. But it's drifty and good luck fixing that with no tuning caps there. USD $3.13/100-lot.

I'm having success with the DS3231N but it's high cost is crazy -  USD $8.70/100-lot. The MEMS version is lower cost DS3231M USD $7.33/100-lot.

Looking at an example of a newer part MAX31329 USD $2.70/100-lot  looks interesting but LGA-10 package, no trim, aging, drift spec etc. a dumbed down datasheet. It has internal xtal, battery trickle charge diodes/switches/resistors.
But who has time to mess around and try it? Nothing for how it performs over temperature.

[Marco]

If there was some open source firmware for MSP430 for temperature compensated RTC it would seem the entire market would be redundant.

[thm_w]

I think it was salvageable, resistor values were way too low for R2, R5 etc.  There are NiMH as well.
FYI MCP7940N is piece of junk RTC look at the silicon errata. It's cheap for a reason.

Started reading through, oh thats not so bad you just have to turn off the clock while you are writing current time...
Next bug "if you turn off the clock while writing and then turn it on again the day value can be wrong" lol

[Peabody]

Another interesting one is the NXP PCF2127.  It has a built-in crystal, temperature compensation, aging adjustment, and 512 bytes of static ram.  But still not cheap - over $4.

The advantage of the DS3231 to me as a hobbyist is that a lot of the RTC modules from the Far East use it.  Of course they no longer contain genuine chips, if they ever did.