LTC4015 unable to charge a fully discharged battery (BMS discharge power path is closed)

Hi Ravid,

There is no way to bypass it using the LTC4015, but you could add a path to pull it up externally.

That said, part of the battery detection feature is sourcing current into the battery, and this is designed to pull pack-protected batteries back into normal operation. This may not be working for you as a result of the load that is on the battery which could be sinking this current and leaving the battery in pack protect.

Could you try disconnecting the load as a test and see if the LTC4015 is able to pull the battery out of pack protect? If that works, you could consider putting a UVLO on your load to prevent this from happening.

Regards,

Zack

Hi Zack,

Unknown said:

Could you try disconnecting the load as a test and see if the LTC4015 is able to pull the battery out of pack protect? If that works, you could consider putting a UVLO on your load to prevent this from happening.

It will not succeed because there is an hysteresis between turning off the discharge power path and turning it back on - when the battery minimum cell voltage gets to 2.6V it will disconnect the discharge power path, and only when the minimum cell voltage raise above 2.8V for a few seconds it will turn back on.

I thought about connecting a 10V LDO from panel input to BATSNS with a series diode (to prevent current from battery output to LDO output when battery turning back on). That way when the panel will be connected and the battery is fully discharged, the BATSNS will sense 10V and the LTC4015 will start charging. I tested it and it worked!

I have another question regarding LTC4015. I don't understand how the UVCLFB function is working. The DS states the following: "Maximum charge current is tapered off when this pin is below 1.2V, at 1.15V the charge current is zero.". But when an 18V PS is connected and the battery is charging I measure about 570mV on UVCLFB pin. According the DS it is not suppose to work am I right?

BTW the MPPT pin is set high (MPPT function is ON).

Thanks,

If your battery is ok with a permanent charge path to 10V, that could be a possible solution. I would recommend some series resistance for current limiting.

You can ignore that UVCL information when MPPT is on - it is controlling the UVCL on its own in that case. If MPPT was not on, then it would follow the behavior described based on your voltage divider to the UVCLFB pin.

Regards,

Zack

Unknown said:

I would recommend some series resistance for current limiting.

Well noted!

Thanks for the great help!