BQ25756: Battery negative current reading...

Hello Alex,

Just to clarify, is the current you are trying to measure coming through the output sense resistor or is the system load drawing directly from the battery?

We don't recommend placing the system load before RBAT_SNS with the BQ25756. I recommend to place the system load to draw directly from the battery. Could you use the battery's gauge/monitor to measure the current?

Best Regards,
Ethan Galloway

Hi Ethan,

thanks for the reply...

The Load is drawing from the battery thru the RBAT_SNS resistor, we ( I am not the designer of this HW ) however expected that a 'negative' outflowing current from the battery would have been read from the BQ25756 ( no reason to not read it if not in reverse mode ) on the RBAT_SNS shunt...

I attach here a schematic pdf of out board where I represent the current flow in our particular situation by red arrows, part from the input panel, part from the battery, and while we are able to read the current thru R48, the current thru R8 is reported as 0... unless we go in REVERSE MODE ( but we have no reason to do it, as we have still power from the solar panel )...

Thanks again for your time...

Best Regards...

Alex01_mppt_schematicM.pdf

Hi Ethan,

thanks for the suggestions, I have forwarded your reply to the HW designer for him to evaluate...

Ethan Galloway said:

For charger stability, the load needs to be attached after RBAT_SNS. The charger measures the inductor current with RBAT_SNS

I am not sure the HW designer thought about this...

Ethan Galloway said:

As a workaround, could you dynamically change the charge current to meet the load current?

We could, but we have no way to measure the load current without adding a sensing element on it ( this is a costs constrained board ), that's why it was thought to use the RBAT_SNS to read the current outflowing from the battery... The designer did not expect for the current not to be read the opposite way...

We are also evaluating the BQ25750 at this point, do you think it may fit our needs...?

To make clear out application, we have a solar panel Charging the battery that is supplying a Motor and other elements which may be added by the final user... While we  may read the Motor current through the Motor driver, we have no way to establish any other current from any added load, and more, we need to keep track also of the current of the board itself ( which ha a lot of switchable components like, NFC, Radio, Slaves with other motors on RS485) as it has to eventually shut down various elements in case of battery reaching a low storage level, to save power... Previously a Gauge counter was mounted on the output line, but the final customer wants it to be cut out ( costs reason ) and make the Coulomb counting himself reading the outflow counter...that is his idea...

thanks for the support

Best Regards

Alex

Hello Alex,

I think the battery will need some sort of protection/balancing circuit from either a battery gauge or a battery monitor. Could the customer use a battery monitor to measure the current flowing in and out of the battery?

Best Regards,
Ethan Galloway

Hi Ethan,

Sorry for the delay in my reply...

Yea, we are looking into this, into adding something like a low cost current reading device at the output of the battery, it seems the only affordable option for now... We will have a discussion with the final customer at the end of this week to check what he prefers to do...

For what we have seen around, battery gauges are too expensive, in the circuit we already added a BQ34110, but it seems not to fit the application, it is very slow in sensing, and more, on the I2C bus, it is using a lot the I2C 'clock streching' feature, and we can not afford that bus being soo slow...

We are evaluating then a low side current sensing, INA185 may be a candidate for now, we will accumulate the charge by micro as we are sampling all analogs with a rate of 100Hz with a possible resolution of less than 1 mA on the current reading... 

Thanks for the suggestions...

Alex

Hello Alex,

Your welcome.

You also might be able to post an E2E question to the battery monitor or battery gauges group. They might be able to you out more on the battery protection/measurement. I can also loop them in here if you'd like.

By the way, does the customer have a method to balance the battery cells?

Best Regards,
Ethan Galloway

Hi Ethan,

sorry for the delay in my reply, but I've been busy had fixing some other stuff on the board...

So, to return to your question, no, the customer is not balancing the cells of the battery, they got simple 2 wire batteries... For the gauging instead, we are  thinking to place a low cost current sensing device with an analog return, the micro is already sampling @ 100Hz all analog inputs, and then we are going to make a current gauge by FW... not the best way, but this is what originally wanted to do the final customer...

Regards

Alex