Lithium phone battery charger design....info needed

[rgbphil]

Hi all,

I was wondering if you could help out with a non-LED question.

I'm designing a small device for a client and they want to use a Motorola Razr battery type BR50 internally. Current draw is quite minimal, but the device is semi-sealed so replacable batteries aren't an option.

I've found a suitable chip, the Microchip MCP73863 to charge the battery off a USB connection but am having trouble trying to figure out the hot/cold thermistor values needed to set the trip points for this chip.

I've taken apart a BR50 battery, found the thermistor SMD, nominally 10kohm which is the only thing connected to the 4 pin connector on the device apart from the battery connections and what is obviously a one-wire data or other battery ID pin. There are a few other miscellaneous parts which I presume are under/over voltage protection mosfets and maybe a eeprom. The 10k chip is not connected to anything else except for the connection pin and the neg-ve battery lead, which led me to assume it's the thermal sensor.

When heating up I can see the thermistor change value, but only slightly. I took apart the battery, removed the chip and tested it seperately but even when stuck in the freezer or touched with a soldering iron it only changes value by about 200-300ohms. For the MCP7386x chips you need to have a hot/cold ratio of about 3-1 to set the trip points. I would've imagined that freezer+soldering iron would exercise the thermistor across it's range. I've found datasheets for similiar SMD thermistors, and they have a much greater range of change....so I'm puzzled.

Any links to data on the thermistors used in these battery packs or more detailed charging data for mobile phone batteries? Googling about I can only find thousands of sites trying to sell me batteries, nothing on design data.

Thanks
Phil

 

[VidPro]

mabey it aint the thermal sencor at all, but the ground lead for the INFO data?
but at 10k its rather high for that :-(
i donno its all above my skill level. so i am not going to be much help.
but when they put thermal protection on packs, it is usually directally tied to the protection curcuit of the pack, and its either a thermal breaker, or a thermal fuse. i dont see a lot of thermal monitoring capacity, but then i couldnt identify MOST of them tiny parts.

 

[VidPro]

oh and what good is a SMD (tiny parts) thermal monitor, unless the PCB (printed curcuit board) is touching the battery itself? which wouldnt work at all because they arent going to jam parts into the bagged cell.

if you got a bagged cell, you just need the correct reactionary thermister, and put it on the side of the bag.

 

[Morelite]

The thermistor is not used as a thermal sensor, it is a in rush current limiter. It can be a positive temp. coefficient or negitive. Depending on if it is NTC or PTC it will change the amount of current going through it. A NTC thermistor will allow more current as it heats up (from the load and not the cell temp), a PTC will limit current as its temp. raises.

 

[VidPro]

here are examples of (easy human capable) breakers and thermal disconnects for li-poly.
http://www.batteryspace.com/index.asp?PageAction=VIEWCATS&Category=868

 

[VidPro]

here is a DATA source for the NTC things from one company.
the (I) thing there can bring up spec sheets and app notes.
http://www.vishay.com/thermistors/ntc/
(slow site)

and for the PCT
http://www.vishay.com/thermistors/ptc/

 

[rgbphil]

Thanks for the replys...

The 10k device in question was on a cct board indeed placed jam packed close to the battery and right in the middle where thermal monitoring would be expected.

None of the other SMD 'resistors' changed value with heat/cold at all, so I discounted them. The one in question did.....however not as much as expected.

I've seen data (I think posted by Newbie) with the polyswitches/thermal cutouts etc, however these would be in series with the power from the battery pack. There are other devices on the cct board connected this way.

This device was seperate and only connected to the neg-ve battery lead and one pin on battery connector (I beeped out all connections). If you check the datasheet for the MCP73863:
http://ww1.microchip.com/downloads/en/DeviceDoc/21893c.pdf
and other charger chips eg from linear etc, they all mention a thermistor used as a thermal sensor, typically connected to neg-ve like this......so I'm deducing that's what it is....might be wrong, but it fits all the criteria except for the lack major movement (there is a little) between freezer temps and soldering iron temps.

For example for an NTC SMD thermistor with an R25=10k (ie resistance at 25 degrees celcius), the vishay datasheets show R0=29k and R55=3.3k (ie roughly 3-1 ratio between lithium operating parameters (ie 3-1 from room to too cold and 3-1 from room temp to too hot)....exactly how the charger chips are meant to work.

I've organised someone with a Razr phone to help out sometime in the next week. We'll measure the thermistor input at room temp on an exhausted battery......then fast charge it with a charger known to get it toasty. Hopefully it will show behaviour as expected when warmed up this way. Maybe there is something strange like needing a minimum amount of current to read a thermistor properly and my DMM isn't supplying that when reading resistance????

Phil

PS: Although small, these chips can be soldered by hand without resorting to tricks like putting a board on a frypan. You just need a fine tip, good magnification/light and design your PCB tracks to come straight out rather than under or to the side. Then 'push' a nub of solder and solder to the slight amount of metal on the sides. Solder will creep a little under the chip and along the sides.

 

[VidPro]

Phil Says PS: Although small, these chips can be soldered by hand

only by sheer geniouses , the rest of us need more solder wick, lots more PATIENCE, and a little artistic realisation that its aint gonna be pretty

of the few thermisters i played with, you can squish them between your fingers, and watch the resistance change on even cheap meters, and that is only 68*F to 98*F.

BTW , i prefer pica thermal fuses to the breakers, ONLY because if the thing was heating up to the thermal breaking points available like 55*C and such, i would not want it to re-engage, i want it to STOP completely and not re-start. but nothing i used them in would be going to other users.

 

[rgbphil]

Actually I was somewhat worried myself about how dodgy the result would be, but was pleasently suprised. I one of those magnifiers with lenses for both eyes, a head lamp on top (of my head....duh!) and a bright desk lamp. Looks like a borg when decked out, but you don't need a degree in neurosurgery or an electron microscope.

You're right about the patience though, it takes about 20mins to solder in a 16pin QFN package. If you get a bit of blobbing, then your solder wick comes in handy there as well. It's remarkable the utility of using the surface tension of the solder.....I suppose that's how the units are soldered by machine when using re-flow anyway. The key I think is not to actually touch the chip, but heatup the track and solder and let them do the work. The other trick was to use a PCB landing design for the QFN with longer pads and extend tracks to the pads outwards without bends or joins close to the pins.

Thanks for the heads up on watching the resistance change with just fingertips....that's what I was expecting. Don't know why this one is so un-reactive.....might just be bad luck, I'm hoping that it isn't some sort of special low R/C ratio thermistor or else the charger chip selected will need a re-think.

Surely someone out there has been making chargers for cell phone batteries and has data on the thermal monitoring? I've seen an instructable using the same chip, but they didn't bother with the thermistor sensor....which is asking for trouble.


Phil