UL-approved coin-cell charging solution? - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: June 28, 2024, 03:14:14 pm
content: Hi,after moving to supercap (see my reply weeks ago below) and spinning 10 expensive PCBs, Intertek said it would need to be a complete re-certification at the cost of several thousands of pounds. So the client want to go back tot he rechargeable battery.Issue (1) mentioned below is easily fixed by changing the charging resistor to 510R.But issue (2) still needs to be solved. i.e. limiting the charging current to less than 300mA in case of the charging resistor failure to short circuit (i.e. 0R).Does anybody have a UL-approved circuit that meets the second requirement too?Thank youHi,the attached schematic shows the battery charging circuit for a coin cell battery ML1220 that has previously received full UL certification by Intertek. The rechargeable coin cell full part number is ML-1220/F1AN coin cell battery (datasheet attached).It turns out that the new engineer at Intertek is much more knowledgeable and thorough than the previous one because when we tried to use the same circuit it would not pass for two reasons:1) charging resistor value too high and2) component failure of the charging resistor to short circuit then the charging current would far exceed the maximum allowedTheir wording was the UL approval criteria 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

author: ricko_uk
author_rank: Super Contributor
timestamp: July 01, 2024, 03:05:36 am
content: Hi all,could anybody offer any suggestions?Thank you

author: Peabody
author_rank: Super Contributor
timestamp: July 01, 2024, 03:43:51 am
content: 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.

author: ArdWar
author_rank: Frequent Contributor
timestamp: July 01, 2024, 03:57:34 am
content: 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.

author: ricko_uk
author_rank: Super Contributor
timestamp: July 02, 2024, 10:07:48 pm
content: Thank you both.The client relaxed the requirements so we are now using a supercap.

author: floobydust
author_rank: Super Contributor
timestamp: July 02, 2024, 11:57:38 pm
content: 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 thesilicon errata.It's cheap for a reason.

author: Peabody
author_rank: Super Contributor
timestamp: July 03, 2024, 02:11:01 pm
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: July 03, 2024, 08:34:29 pm
content: Sort by price and it's the cheapest RTC available. Microchip CEO Ganesh Moorthy on the golf course thinking he's beat everyoneI 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 partMAX31329USD $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.

author: Marco
author_rank: Super Contributor
timestamp: July 03, 2024, 09:48:19 pm
content: If there was some open source firmware for MSP430 for temperature compensated RTC it would seem the entire market would be redundant.

author: thm_w
author_rank: Super Contributor
timestamp: July 03, 2024, 10:58:27 pm
content: Quote from: floobydust on July 02, 2024, 11:57:38 pmStarted 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

author: Peabody
author_rank: Super Contributor
timestamp: July 03, 2024, 10:59:12 pm
content: 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.

author: ricko_uk
author_rank: Super Contributor
timestamp: November 19, 2024, 02:24:58 pm
content: I updated the OP... but in case you are just reading this reply, here is the same info:After moving to supercap and spinning 10 expensive PCBs, Intertek said it would need to be a complete re-certification at the cost of several thousands of pounds. So the client want to go back to the rechargeable battery.Does anybody have a UL-approved charging circuit?There are two issues:(1) the charging current needs to be limited and that is easily fixed by changing the charging resistor to 510R.(2) limiting the charging current to less than 300mA in case of the charging resistor failure to short circuit (i.e. 0R). This needs to be solved.Where can I find a UL-approved charging circuit?Thank you

author: tszaboo
author_rank: Super Contributor
timestamp: November 19, 2024, 02:29:17 pm
content: Quote from: ricko_uk on November 19, 2024, 02:24:58 pmYou should read the UL file of the battery. You can register and read them for free at UL product IQ.That's good for starters.A few thousand dollars is not much for these leeches. They wanted 10K plus to do a damn IP test on my product. You know, chuck it in water drink a coffee and then see if water went into it. 40 minute in total.

author: nctnico
author_rank: Super Contributor
timestamp: November 19, 2024, 04:51:06 pm
content: Quote from: ricko_uk on June 28, 2024, 03:14:14 pmA (PTC) fuse should do. With the resistor failed, the board needs a repair anyway.

author: SeanB
author_rank: Super Contributor
timestamp: November 19, 2024, 06:16:47 pm
content: Simplest solution is to split the charge resistor into 2 parts, and thus you can use 2 270R resistors in series, placing them a little away from each other, or even one on each side of the battery itself, which will meat the UL requirement along with also preventing a fire if the battery positive is shorted to ground. Make both resistors higher in power than 0.125W, and both as fusible, and your issue is solved, as they will forever operate even shorted across the rail, or if the cell is shorted, and will fail open if say the battery is removed and replaced with say mains supply.

author: ricko_uk
author_rank: Super Contributor
timestamp: November 19, 2024, 08:33:31 pm
content: Thank you allYes, putting two resistors is the simplest option but the engineer was being difficult about it. And the client already raised a complaint about it, so hopefully they will accept it as a solution.Adding a fuse apparently is not an option because they are not guaranteed to fail open. Apparently there are some conditions whereby they can fail to short instead of open (which to me does not make much sense it being a fuse...).That is why I was just wondering if there were other requirements or official UL schematics.Thank you all as always