Fona battery charger issue

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whoops42
 
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Fona battery charger issue

Post by whoops42 »

It's all fun and games until someone lights the house on fire.

My Fona w/ SIM800 has been quite a fun thing to play around with, Unfortunately, all good things must come to an end.

While swapping the SIM card around between a phone and the Fona module I let some smoke out.

<///whooops////> :/

I'm not entirely sure what burned out but I suspect it is some component of the battery charge system.

3 things may have happened:
either I plugged it in on my ribbon connector wrong and ( with the way it is wired shifted all inputs one to the right) swapped bat and ground (most likely)
I touched something and created a short(second most likely)
without the sim card in and the unit powered on something fried for some reason (super doubt it)

here's what I know:
The Fona works and will send messages/ communicates fine.
AT+CBC returns battery info but indicates unit is not charging (0 ,some percent %, some voltage)
The charge and done LEDs are both lit like when the battery is not connected.

from this I surmise that the unit is not completely fried, the unit is getting power from battery, the battery charge IC may or may not be functional.
and either the charger IC is gone or there is something like a diode that has blown and no power can reach the battery.

While I could not find the specific chip online, i probed around with a multi-meter and did some digging and found that, if what I think is the charge IC is actually the charge IC, then it follows a pretty standard layout. (p1 charge, p2 gnd, p3 battery, p4 vcc, pin 5 program)

It also looks like the charge and done LEDs are reverse polarity so that when (maybe?) current goes one way the charge is lit and (maybe) when current goes the other way done is lit. (makes some sense given that from the datasheet for a similar IC something like 2 micro amps can flow back into the IC when it's in shutdown state. not sure if that's enough for an led but....)\

ANYWAY, all that rambling aside, my questions are
0, Is it worth pursuing fixing this or should I just get either a new fona, or a new discrete little 3.7 usb battery charger board to wire in?

1, if I did get another charger
like this fancy lil one right here (https://www.adafruit.com/product/1904?g ... gLgf_D_BwE)
would there be any reason I couldn't program for 500 ma and connect that to the battery and the battery to the board as normal?


1.5 IF i did attempt a fix, would anyone who is more familiar with this circuit be willing to volunteer a way to test that charge IC?

1.75 , is that actually the way those LEDs work?

2, is the little black SMD due east of the battery JST and due southwest of the SMA antenna in fact a diode? (this I can probably find out myself but cant hurt to ask here and now)

3 what capacitance should I expect for the little SMT cap between the above diode and the key button?

3.14 what is the make and model of the charge IC or is it just a standard design that can can be comparable to several other 5-6 volt lithium 3.7 v current programmable chargers in same package

I'll keep chuggin away on this and try to draw out the charge circuit in detail and see if I can figure this out on my own/ see what all else could be the problem, but figured I might as well hedge my bets and use my "ask the experts/community" lifeline.
so anyway thanks very much and if you have any advice besides "don't do dumb things and expect not dumb results", i'd love to here it
very best regards
-whooops42

ps in the attached photo of the fona i circled the little smd I think is a diode, the 5 pin smd I think is the Charge IC, the little SMT cap that I'm wondering about testing, and those two LEDs that are simultaneously lit.
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adafruit fona pic w/ some components circled
adafruit fona pic w/ some components circled
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adafruit_support_mike
 
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Re: Fona battery charger issue

Post by adafruit_support_mike »

Of all the parts you've circled, the LiPo charger (red circle) is the most likely to be damaged. It's an MCP73831, if you want to try to replace the part itself:

https://octopart.com/search?q=mcp73831& ... -5&start=0

The diode marked in green is an MMSZ5231BT1G 5.1V Zener diode, there to protect the SIM800 module if more than 5V gets connected to the LiPo jack. That could be damaged by reverse voltage, since Zener diodes are just regular diodes when current flows through them the other way. It would look pretty much like a short circuit to a battery connected backwards:

https://octopart.com/search?q=mmsz5231b ... 31&start=0

The capacitors are 10pF and 33pF power decoupling, and are very unlikely to have been damaged.

The LEDs are connected to the MCP73831's status pin, and if both of them are lit, it strongly suggests the MCP73831 is dead. The orange CHARGE LED is connected to the LiPo's positive terminal, then through a 470-ohm resistor to the MCP73831's status pin. It should only light if the status pin is low. The green DONE LED is connected to GND, and goes to the MCP73831's status pin through a 1k resistor. It should only light if the status pin is high. If they're both lit, it means the status pin isn't high or low, and the two LEDs are basically in series with a 1.5k resistor between them.

You can clip out the MCP73831 one lead at a time, then remove the pins one by one with a soldering iron, and leave that space vacant if you want to connect an external charger like the Micro-LiPo. That would work, and the best place to make the connection would be between the BAT and GND pins at the edge of the board.

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whoops42
 
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Re: Fona battery charger issue

Post by whoops42 »

Clipped out the old chip, desoldered the leads, ordered me a few mcp73831t-2ati/ot 4.2 charger ICs, carefully wedged my iron in and soldered a new one in and it seems ok. Went through a charge cycle,( yellow to green) and topped the battery to about 4.19v . Seems ok, thanks!

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adafruit_support_mike
 
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Re: Fona battery charger issue

Post by adafruit_support_mike »

Glad to hear it's working for you. Happy hacking!

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