2S Li Ion battery getting disconnected from charging IC during charging

[Sai teja]

Hello,
         I am using MCP73213 IC for charging 2S 990 maH Li Ion battery.  On initiating charging, my battery is getting electrically disconnected from charging IC. I found out this by measuring voltages at terminals. Voltage at battery terminals of charging IC is 8.2 V and voltage across battery is 7.8 V.
I am really confused why is this happening. Where the same charging circuit is functioning well with 2200 maH Li Ion battery. What is the reason behind this weird behavior? Please some one help me out in figuring out the issue.

Datasheet of MCP73213 IC is linked below.
http://ww1.microchip.com/downloads/en/DeviceDoc/20002190C.pdf

[KL27x]

Voltage at battery terminals of charging IC is 8.2 V and voltage across battery is 7.8 V.

Is it possible that there is a max current cutout circuit on the battery, and you are measuring voltage at the true battery terminals directly? And the charger IC is connected on the other side of this circuit? In this case, it would theoretically be possible you are measuring two difference voltages and there's an actual disconnect between the charger and the battery. But I think it would be obvious to you if this were the case.

As long as these points you are measuring from at in fact in continuity, then what you are measuring is just the voltage drop in the wires/traces between the charging IC and the battery terminals. If the voltage at the IC is 4.1 per cell, it is doing its job. Stop worrying about the IC. It is holding up its end of the bargain within 0.1V; it can't make your battery charge any faster than that. Fatten up your wire/traces.
 

[jbb]

One of the cells may have more voltage than the other.  If the pack has a good protection circuit, it will measure both cells and trip if either cell is too high.  Can you measure the voltage across each cell?

[KL27x]

If you want to increase charging speed but you need long wires/traces between the charger IC and the battery, you may want to look for an IC which has a "voltage sense pin." This will read the voltage at the battery, so that the IC can actually increase voltage above 4.2V to compensate for the voltage drop across the charging conductors. A charge IC/circuit can't do this blindly without possibly damaging the battery; it needs a low current voltage sense connection.

"Voltage sense" is what I call it in my head, anyway. I am pretty sure this is what it is called in the industry when it comes to voltage regulation circuits. But I have a way of misremembering things to fit my own thought process.

At ANY rate, there is no way you are going to hook up a dead li ion battery to a charger and for the terminals to instantly show 4.1+ V per cell. That would mean it is at least halfway charged, instantly. So with a voltage sense circuit, the scenario you just described will still happen. It may be your battery is in the constant current stage, limited by a maximum amount of current. This is perhaps why you are recording 4.1 V per cell at the IC output, rather than something that would round up to 4.2V. If you had current sense line, this wouldn't necessarily change. 7.8V recorded AT the battery terminal may simply be the voltage which results in the max charge current. It's in the constant voltage phase, where the battery is like ~ 2/3rd charged, where a voltage sense will decrease your charge time.

[soubitos]

My IC of choice for a 2S charger would be TP5100.. although it requires more components and PCB space, i think it comes with more features and is capable for higher currents.....