Need help identifying a simple circuit

[JohnnyR]

Hello all.
I don't know much about circuits and parts but here's my situation;

I'm doing a review on an electronic device and part of it is a USB charger containing a 10430 Li-Ion battery (3.6V AAA size). The previous model didn't have any overcharge protection built in so you can imagine that was quite a dangerous situation. I know the usual strict rules on Li-Ion charging and how dangerous it can be to overcharge them so I've had contact with the manufacturer and told them they needed to go back to the drawing board and make it save.
Now I've received the new model but looking at the inside I can hardly imagine that this is a state of the art Li-Ion charger
Anyway, all it has to do is stop charging when it reaches 4.3 volts (at most) so I was wondering if anyone can tell me if this circuit can do that. Aside from the 2 resistor and 1 capacitor I don't know what the U1 part is. There's no printing on it and it's got 5 contact points but as far as I can see only 3 of them are used (upper 2 and middle lower).
Any help would really be appreciated.

**broken link removed**

 

[Nigel Goodwin]

U1 is probably an IC, it's quite possible it's doing everything that's required, but without knowing what IC it is, it's impossible to say.

 

[Sceadwian]

There are basically two rules for safe lithium cell charging. Don't charge over 1C and don't charge over 4.2volts per cell. If those two conditions are met as long as there isn't a physical cell failure the charge isn't going to hurt anything. If you want truly safe there should be a thermal sensor attached to the lithium cell and that temperature should be monitored.
Even if the charger charges the cell at more than 1C all it's going to do is shorten cell life, the important part is the over voltage protection. The higher the voltage goes over the 4.2v max and the higher the ambient temperature is the chances of a thermal run away and pack bursting starts to occur, even then it takes a pretty decent amount of time and available current during over voltage to cause a lithium pack to burst.
That IC looks to me like a voltage regulator, if it's a more advanced IC it could be a simple charger IC that limits voltage and current. I personally wouldn't use such a simple charge circuit but it should be safe.

 

[JohnnyR]

Thanks for the replies.
I know all the basic Li-Ion charging rules. But does the board layout make it possible for the IC, without knowing its specs, to stop giving a charge current to the battery? I can't figure out how the IC would be able to measure the battery voltage in this layout. Also, it looks like the middle contact line is running underneath the IC straight from + to - but that would mean it's short circuiting everything... ????

 

[Diver300]

I think that the wire going between the top two pins of the IC goes to the bottom left one.

That makes the IC in series with the +ve feed to the battery. I guess the resistors set the output current.

The circuit does seem sparce for a Li-Ion charger. There seems to be no way of monitoring temperature.

 

[Sceadwian]

That trace doesn't look like it connects vertically, it looks like it goes to the large square pad to the lower left of the IC Which means that the +/- IN voltages only go to the IC, and the battery leads are connected to the two lower left pins. C1 is a bypass cap for the IC. R1 and R2 are in parallel, not sure what they're for but must be some kind of feedback to the IC. Seeing as how the IN + voltage only goes to the IC it can interupt the current to the battery/circuit. Seems like a very basic charging circuit, I'm sure there are dedicated lithium charge ICs in that kind of form factor. If you're so concerned ask the maker to identify the IC.

 

[Pommie]

I'm going to have a guess at a MAX1551.

Mike.

 

[JohnnyR]

Wow, what a find!
Indeed that seems to be it. It's even got thermal protection

Thanks Pommie!

 

[Sceadwian]

I don't see an external temperature measuring line on the chip so it's probably inside of it, which means the chip would have to be mounted very close to the battery to use the thermal cutoff.

 

[audioguru]

I am shocked that Maxim doesn't say that the battery is not fully charged when its charge indicator turns off. The battery is still charging with 50mA of current and might be only 70% fully charged.

When the charging voltage is 4.2V and the charging current reduces to 0.03C then the battery is fully charged.

 

[Diver300]

The Max1551 has thermal protection, but it only protects itself from getting too hot, not the battery.

Li-Ion batteries shouldn't be charged above 40 deg C or so, but the self heating of any battery charger is too much for the sensor to be on the chip. That is why most charging circuits have a thermistor on the battery.

 

[JohnnyR]

I am shocked that Maxim doesn't say that the battery is not fully charged when its charge indicator turns off. The battery is still charging with 50mA of current and might be only 70% fully charged.

Looking at the DC CHARGE CURRENT vs. BATTERY VOLTAGE diagram you can see the charge current is dropping above 3.8V But even at 4.0V the charge current is still 200mA.

 

[Pommie]

The Max1551 has thermal protection, but it only protects itself from getting too hot, not the battery.

If you look at the circuit board above, the battery is located on the other side from the chip and therefore the battery will heat the chip.

Mike.

 

[Sceadwian]

The battery leads being on the other side of the board give no indication of the physical location of the battery, only it's leads, even a small air gap with no thermally conductive conformal coating would effectively 'disconnect' the battery from the charging chip, and even if it has a very good heat linkage to the board going through FR4 is at best a bad idea. If it is to be considered 'closely coupled' with the cell that charging chip needs to be directly attached to the cell with a good thermally conductive epoxy, or shrink wrapped onto the cell itself.

 

[JohnnyR]

On the other side of the board is a plastic battery holder. So forget about thermal conductivity

 

[audioguru]

The IC protects itself from getting too hot but it doesn't stop the battery from catching on fire.

 

[Pommie]

The battery is millimeters away from the chip and will shut down long before the battery catches fire.

Mike.

 

[Ubergeek63]

Wow, what a find!
Indeed that seems to be it. It's even got thermal protection

Thanks Pommie!

That is no surprise. Lithiums are so picky they are as likely to become blow torches as they are to charge.

Dan

 

[Sceadwian]

Lithium packs aren't really that picky, you really have to over charge them for a long period of time with a large over voltage potential to get them to go up. Though not ideal, the thermal cutoff of the IC will at least keep gross overchanging from occuring, then again I'm not sure if that's possible from a 5 volt power supply.