BQ24172: NiMH chargining termination?

Hi Jackson,

I do not understand the 100mA trickle current that you are seeing.  When charging NIMH at <0.5C, the charger's safety timer is used to terminate the NiMH battery before the battery voltage reaches VBATREG as set per the external resistors.  Therefore you have to set VBATREG just above the voltage that you expect the NiMH batteries to charge to before the timer expires.  In other words, the constant voltage portion of the charge cycle is really battery OVP.  Are your batteries reaching that voltage before the timer expires?  With TTC capacitor installed, termination is not disabled.

There is no other method to terminate charge for NiMH using the BQ24172.

Regards,

Jeff

Jeff:

Thanks for your reply. 

We have modified our FB pin voltage divider to terminate charge at about 1.45V/cell, and we have enabled the TTC capacitor (0.1uF).  With these settings we are getting successful charges of or NiMH packs (90% capacity in about 4 hours -- we will probably adjust TTC cap to a shorter time period).

A related question, on re-starting the charge cycle, to try to maintain a fully-charged battery:

If we discharge our NiMH battery pack to below V_RECHG (on BQ24172, this should be 2.05V, if we read the data sheet correctly) we still do not see the charge cycle begin again.  Instead, we are seeing about 0.5Hz on STAT pin, and we cannot figure out what fault condition is present.

We would expect, given our resistor divider on FB pin (1M/313K) to have charging restart at about 8.6V, but we are not seeing a re-start even at 7.5V.

As a matter of fact, it is really not clear what has been triggering a re-start unless we have a fully discharged battery pack (about 6V).

Toggling ISET to below 40mV and back above 120mV does not seem to reset the fault condition and initiate charging.

--Jackson

Jeff:

We plan to terminate on battery voltage (about 8.8V).  We would also enable TTC for about 4 hours, as a backup.  It takes about 3.5 hours to terminate our 2200mAh battery @ 8.8V, with 1A starting current.

Additionally we will have the thermistor from the NiMH pack as a safety backup.

I think that the fault I was seeing may be the result of a damaged BQ24172.  I could not detect a battery to start charging at all, but I did swap out the PCB with another one and it begiins charging correctly.  I'll keep an eye on this, but I think it was a false alarm.  Thanks for the additional testing and confirmation.

Are there risks to the the BQ part if we try to start charging while there is still a load on the battery?  If there is a power outage, there may be active loads, until the MCU detects loss of mains power and shuts off power to all external loads.

Thanks,

Jackson