MagTag - bad LiPo charger or bad battery?

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reticulating_splines
 
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MagTag - bad LiPo charger or bad battery?

Post by reticulating_splines »

Hey there,

I recently ordered a MagTag (which I otherwise love, such a neat lil package) and paired it with a 1350mAh Li polymer battery to use as a little unplugged weather forecast dashboard in my kitchen. Until yesterday, I had been using the board almost exclusively while plugged in to my computer, while I wrote and tested the code for the program, demo'ed about 100 different fonts before being content, etc etc. I would occasionally unplug it and let it run through a loop on battery, and everything seemed fine - I had the screen displaying the battery voltage up in the corner, and it seemed to be in the normal range of ~3.4V.

Well, yesterday, I finally "deployed" the magtag+battery and let it run. When I checked on it this morning, I noticed that the battery had died, and thanks to the e-ink screen, it did so with a final voltage around 2.7V. Seems fine.

After plugging the unit back into USB-C power and letting it supposedly charge for a while, I noticed something must be wrong with either the battery, the charging hardware on the magtag, or both. After roughly an hour, I first thought that maybe there was a combination of minor problems, because the "CHG" LED wasn't lit, and the reported voltage across the battery was still only 2.8V. I checked the voltage at & across the LED with a multimeter, and saw presumably normal values (about +5 before and about +3.5V after), so I thought maybe the LED was just a dud, and that coincidentally, the charging was also slow. However, after another few hours, I came back to the unit and noticed the voltage was still low, and much more alarmingly, that the battery seems to have expanded in thickness! Don't worry, it's been removed to "safety" (a metal coffee tin outside).

What gives? Do folks think this is indicative of an issue with the board itself, or with the battery? I don't want to buy another LiPo battery just to experience the same problem a second time. Additionally, is the LED staying dark (yet showing a ~1.5V voltage drop across it) just a red herring? I don't really know how to explain that or how it would relate. The magtag was happy to *run* off the battery until it was drained, and the solder points at the JST connector for the battery seem solid.

Thanks for any help/guesses!

EDIT: Sorry, the "BANNED" above is just...the word for the room of your home where one prepares meals...no idea why that's censored...

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mikeysklar
 
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Re: MagTag - bad LiPo charger or bad battery?

Post by mikeysklar »

The 2.7v reading is a sign that the battery pack is not doing well. The healthy range for these LiPo's is 3.2v - 4.2v. When they start getting outside that you are looking at a compromised pack.

How old was the battery pack? Do you recall the purchase date?

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reticulating_splines
 
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Re: MagTag - bad LiPo charger or bad battery?

Post by reticulating_splines »

Sigh, yep, I've come across a handful of "broken/for parts" devices in the past with the sole issue being a built-in battery that's fallen down into that range. On some of the harder-to-replace ones, like an older iPad model I came across recently, I've actually been able to "revive" them a bit with a very slow trickle charge, but definitely not going to attempt that on this suspicious-looking thing.

Annoyingly, the battery was actually brand new, just one of the millions of yellow-and-silver LiPos you can find on Amazon. Here's the brand and model, just in case anyone reading this wants to know what to perhaps avoid - although I bet the same factory makes half of the identical-looking batts out there.

https://www.amazon.com/dp/B08FD39Y5R?ps ... ct_details

My understanding is that the amount of current a LiPo battery can provide drops drastically as it nears 3V, which makes it seem bizarre to me that the MagTag would've kept pulling current from it down into the 2.8V territory. How much "zombie" draw can be pulled from the battery pins when the ESP32 is effectively "off" due to the otherwise "dead" battery? I'm surprised that being dead but having the switch set to "On" could effectively kill a battery in under 24 hours.

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mikeysklar
 
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Re: MagTag - bad LiPo charger or bad battery?

Post by mikeysklar »

Your LiPo from Amazon looks like it has reverse polarity from the Adafruit models.

amazon:
Screenshot from 2022-11-22 11-46-11.png
Screenshot from 2022-11-22 11-46-11.png (187.92 KiB) Viewed 87 times
adafruit:
328-05.jpg
328-05.jpg (431.13 KiB) Viewed 87 times

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reticulating_splines
 
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Re: MagTag - bad LiPo charger or bad battery?

Post by reticulating_splines »

Yeah, it does, but I snipped it off and recrimped the wires with fresh JST-PH terminals so I could put it on in the right orientation. I don't think it would've even turned on otherwise :p

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SMAF
 
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Re: MagTag - bad LiPo charger or bad battery?

Post by SMAF »

I bought a battery from Amazon, plugged it into Magtag and immediately smoke came out near JST connector. I disconnected the battery asap but still smells like some plastic/parts have burned badly. Later I realized that this is a reverse polarity issue. I will cross wires to fix the polarity, but I think charging will not work anymore.

We should check the JST polarity before connecting the battery.

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