Can I charge MLxxxx Lithium Manganese Dioxide batteries with constant current? - Page 1
Forum section: electronics / beginners

author: dazz
author_rank: Frequent Contributor
timestamp: December 06, 2023, 10:39:56 am
content: I have a bunch of ML2032 batteries, but no suitable charger. For what I could gather, it's recommended to charge these batteries with a constant voltage of 3.1V (3.3V maximum) and a máximum charging current of 2mA. The suggested circuit consists of an LDO regulator to get the 3.1-3.3V, then a current limiting resistor and a diode to prevent the battery from discharging if the supply voltage is removed. My question is, can I use 2mA constant current instead with something like this?This way I don't need the LDO regulator, which I don't have, and since it charges at constant current, it should charge much faster. Thoughts please? Thanks.

author: shapirus
author_rank: Super Contributor
timestamp: December 06, 2023, 01:22:30 pm
content: Why does the regulator have to be an LDO? As long as you have a 5V (think USB) supply, then nearly any linear reg with a respective current limiting circuitry will work.(your schematics picture isn't visible, so I'm making assumptions)

author: dazz
author_rank: Frequent Contributor
timestamp: December 06, 2023, 01:33:06 pm
content: Quote from: shapirus on December 06, 2023, 01:22:30 pmOh, you're right of course. I don't know what I was thinking.

author: dazz
author_rank: Frequent Contributor
timestamp: December 06, 2023, 11:54:07 pm
content: So any thoughts on using constant current to charge these batteries, please?

author: ArdWar
author_rank: Frequent Contributor
timestamp: December 07, 2023, 02:16:09 am
content: It's fine.In fact you're already using way too many components than most made to price products out there. They often just assume that the external supply will be always 5V or lower and simply place 2K resistor instead of your LDO or CC source.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 07, 2023, 12:05:02 pm
content: Think cheap, think small.Any R,G or Y LED will work, all you have to do is tweak R1 for 2mA.It anit clever and it aint funny , but its good enough.

author: dazz
author_rank: Frequent Contributor
timestamp: December 07, 2023, 12:10:54 pm
content: Thanks, guys!

author: tszaboo
author_rank: Super Contributor
timestamp: December 07, 2023, 01:03:53 pm
content: So now there are secondary LiMNO2 batteries, with the same name was the primary ones?Why did they use the same name?Why?Why?

author: amyk
author_rank: Super Contributor
timestamp: December 08, 2023, 02:55:39 am
content: The primary ones are CR2032.

author: IanB
author_rank: Super Contributor
timestamp: December 08, 2023, 03:12:17 am
content: Everyone above said using constant current is fine, but what about the maximum voltage of 3.1 V? Did that get lost in the noise? You still must not go above 3.1 V or you will destroy the cell. Or are we talking about putting an LED in parallel with the cell to bypass the excess current when the voltage reaches 3.1 V or so?Basically, two components: 5 V supply, series current limiting resistor, parallel LED for voltage limiting. Is that the idea?(The LED lights up when the cell is fully charged.)

author: dazz
author_rank: Frequent Contributor
timestamp: December 08, 2023, 03:39:11 am
content: Quote from: IanB on December 08, 2023, 03:12:17 amYes, the TL431 is there to cap the voltage at some 3.1V, although I'm testing the circuit at 3.17V because I read somewhere that these batteries can be charged up to 3.3V. Are we sure 3.1V is the absolute max then?I also added some extra circuitry for the LED that indicates the charge is complete. Also, I removed the diode that keeps the battery from discharging through the 431 resistors because I won't leave the batteries in the charger while unplugged. The 1N5817 seems to make the last portion of the charge very slow because it drops 150mV even at very low currents of less than 0.5mA. The whole idea of the constant current thing is to make the charger a bit less painfully slow (it takes over a day to charge a 60mAh cell)EDIT: Here's the datasheet, although I got my 2032's from Aliexpress, so...https://www.abcde.de/data/ml2032_datasheet.pdf

author: IanB
author_rank: Super Contributor
timestamp: December 08, 2023, 04:36:56 am
content: The Maxell datasheet you linked shows a maximum voltage of 3.3 V (not to exceed). However, lower voltages could extend the cycle life.Also, the same Maxell datasheet advises that those cells should not be discharged. They are intended for float applications, like battery backup for memory, where they maintain a 80-100% state of charge at all times. If you discharge them much below 80% you will seriously shorten their cycle life. This means the practical capacity is about 10 mAh, not 65 mAh. This also means they won't actually take a day to charge, since you will only be replacing 10 mAh, not 60 mAh.But essentially, if you want these cells to be a rechargeable replacement for CR2032 cells, you are backing the wrong horse.

author: dazz
author_rank: Frequent Contributor
timestamp: December 08, 2023, 05:00:09 am
content: Quote from: IanB on December 08, 2023, 04:36:56 amOh, I missed that part. Oh well, I paid less than $1 per battery, so even if they only last 10 or 20 cycles it's a good deal. We'll see.

author: tszaboo
author_rank: Super Contributor
timestamp: December 08, 2023, 09:04:15 am
content: Quote from: amyk on December 08, 2023, 02:55:39 amThese aren't:https://eu.mouser.com/ProductDetail/Murata-Electronics/CR2032R-HO6?qs=GBLSl2AkirteBGL7Xnaujw%3D%3Dhttps://www.saft.com/products-solutions/products/lm-m

author: amyk
author_rank: Super Contributor
timestamp: December 09, 2023, 05:37:59 am
content: Quote from: tszaboo on December 08, 2023, 09:04:15 amCR2032 is not rechargeable. ML2032 is.

author: tszaboo
author_rank: Super Contributor
timestamp: December 09, 2023, 01:28:14 pm
content: Quote from: amyk on December 09, 2023, 05:37:59 amThat's my point. The CR batteries are also Lithium Manganese Dioxide, that aren't rechargeable.

author: dazz
author_rank: Frequent Contributor
timestamp: December 10, 2023, 02:53:55 pm
content: Another quick question, please. I'm using this circuit to discharge one of these ML2032's at a constant current to measure it's capacity.Is this a valid circuit? Or is part of the current being provided by the 5V power supply instead of the battery? The battery is voltage source V8 in the circuit. The thing is it's been 40 hours of 1mA constant current discharge and the curve is showing more capacity than the one in the datasheet, so I'm starting to suspect my circuit is not doing what I want it to do.

author: dazz
author_rank: Frequent Contributor
timestamp: December 14, 2023, 04:28:27 am
content: Quote from: dazz on December 08, 2023, 03:39:11 amOh, I'm dumb. All that stuff just to light up an LED once the charge is complete was unnecessary when I can simply use the current through the TL431 to do that, right? Like this:It simplifies the circuit a lot, and avoids relying on the BE junction drop to set the output voltage at which the LED lights up, which is temp dependent. I don't think I even need a current limiting resistor for the LED, because the constant current source should cap the current at 2mA, right? I'll give it a try.BTW, I measured an impressive 93mAh of capacity for one of these ML2032's at 1mA, and it's not my test circuit,because I tried a different one with an LM317 and no external power supply, and the results are very similar.

author: dazz
author_rank: Frequent Contributor
timestamp: December 14, 2023, 03:05:32 pm
content: OK, I tried that last circuit, but it's not working as it should. Here's a pic with the voltages I get. For some reason now I get 3.6V at the output instead of 3.1V, I dont' understand why. Isn't the 431 supposed to start conducting when the voltage at REF is 2.5V, and with that resistor divider, shouldn't that happen when the output is at 3.1V instead of 3.6V?EDIT: I lowered the resistor values by a factor of 20 in the divider, now the output voltage is OK, but the LED just barely lights up, and it stays that way even when I try to charge a fully discharged battery (the LED should be off then)