BQ24618: Premature Charge Termination

Tim Harvey84

Part Number: BQ24618

I designed a new board with a BQ24618, basically using the circuit in the datasheet, for 2 series LiIon cells making a 7.4V nominal 8.4V max battery.

It seems to be charging as expected, then all of a sudden it suspends/ends charging. If the batteries were very depleted, this happens long before 8.4V. If power is cycled it continues where it was and keeps going. It might take several tries to get it close to done.

I have charge current at 2.9A, termination at 0.1A though I believe that gets clipped to 0.25A.

Looking at what could stop the charging:

- CE is not going low. It is connected directly to VREF.

- The adapter is not going into overvoltage, undervoltage or getting disconnected.

- The chip is not getting hot.

- TS is 1/2 VREF. We are not using a thermistor on the batteries.

- I have a 0.1uF cap for TTC, which should give us 9.3h. This is happening in less than 30min, then 20min, then 5 min, ... as it charges up.

- VREF and REGN look okay, but if they glitch I can't see it.

Any thoughts as to what is going on?

I do see noise on the CHARGE/STAT1 signal that I do not see on the DONE/STAT2 signal. Just enough to make the LED glow a bit.

Thanks, Tim.

over 2 years ago

0 Michael Emanuel35 over 2 years ago

TI__Mastermind 41745 points

Hello,

Can you send a schematic?

Regards,

Mike Emanuel

0 Tim Harvey84 over 2 years ago in reply to Michael Emanuel35

Prodigy 180 points

Sure,

BQchargerSchematic.pdf

0 Tim Harvey84 over 2 years ago in reply to Tim Harvey84

Prodigy 180 points

I changed Iset2 to 0.3A, and ACset to 4A. Discharged the batteries some, and started charging. It ran for half hour or hour. I didn't note the start time. The current was as expected. When it dropped to about 1.2A with VBAT = 8.4V, it stopped charging with STAT1 and STAT2 off. Cycle power and it continued where it was for about 15 minutes and stopped at about 0.76A. Cycled power and it continues for another 15min, and stops at about 0.5A. Cycle power and it continues. 15min later it hits DONE.

Another Engineer said she shorted out the cap for TTC and it charged forever. I have not confirmed this. That is my next thing to try. If that continues on CHARGE, it does not look like a VREF or REGN error does it?

It acts like a 100nF cap is not the 9.333h I am expecting.

I am closer than last year when the thing did absolutely nothing on a previous board. But something is still not right.

0 Michael Emanuel35 over 2 years ago in reply to Tim Harvey84

TI__Mastermind 41745 points

Hello,

When the charge stops prematurely can you measure VCC, REGN, ACSET, ISET1, ISET2, SRN, VFB, VREF, STAT1, STAT2, CE, PG, and ACP?

Please capture waveforms of input voltage, PH voltage, SRN voltage, and charge current.

Please capture waveforms of STAT1, STAT2, PG, and VREF.

Regards,

Mike Emanuel

0 Tim Harvey84 over 2 years ago in reply to Michael Emanuel35

Prodigy 180 points

You want them captured right at the event? Or just anytime before and after?

Shorting CTTC to VREF passed the first try, and the second time with a short discharge between. I will try it with much deeper discharges, but it takes time to do big tests with a 5.2mAh battery!

0 Tim Harvey84 over 2 years ago in reply to Tim Harvey84

Prodigy 180 points

I mean a 5.2Ah batteries.

0 Michael Emanuel35 over 2 years ago in reply to Tim Harvey84

TI__Mastermind 41745 points

Hi,

I need to see the values of these pins right after charge is suspended. DC values while charging would also be helpful. Have you confirmed the TTC capacitor? Shorting TTC to VREF disables the safety timer so I believe the TTC capacitor may have a role here. What is the parasitic capacitance between TTC and GND with the TTC capacitor attached when running?

Also STAT1 and STAT2 OFF indicate, "Charge suspend, timer fault, overvoltage, sleep mode, battery absent". Are you sure none of these faults are present? Safety timer seems like a suspect.

Regards,

Mike Emanuel

0 Tim Harvey84 over 2 years ago in reply to Michael Emanuel35

Prodigy 180 points

The thing is working when I disable the safety timer. I'll just go with that. Everything else seems fine and is as expected.

Thanks.

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BQ24618: Premature Charge Termination