BQ25750: Knowing Battery State of Charge

Hello Kurt,

1.  I would not advise this. The BQ25750's ADC is intended to provide a general indicator of the voltage and current.

2. I think this might be possible? You might want to talk to BQ34Z100-G1 experts on whether this is feasible.

3. Let me attach the battery gauge experts to this thread and they'll get back to you.

Best Regards,
Ethan Galloway

Hello Ethan,

Thanks for getting back to me.  I look forward to learning more about a possibly-better battery fuel gauge for this application.

Best Regards,

Kurt

Hi Kurt,

Is your system only up to 4S? If so, why are you using a high cell count gauge?
For 4S gauge, we recommend the BQ41z50 or bq40z50. 41z is the next gen. 

In Regards to if you should use the gauge or charger's sense resistor. The only way for the gauge to coulomb count is by using the SRN and SRP pins from the IC. It is possible to use one sense resistor for both devices, but they will both need to be low-side sensing. 

I highly recommend that the gauge uses the sense resistor connected at the SRP and SRN pins.

Hello Jose,

Thanks for the posting.  I was considering using the BQ34Z100-G1 because (1) the datasheet states that it can be used with LiFePO4 batteries; and (2) it is available in distribution.  The BQ41Z50 is not yet available in distribution (although this might change by the time we draw the schematics), and the LBQ40Z50 datasheet does not state that it can be used with LiFePO4 batteries.  Using DigiKey pricing at quantities of a hundred, there is only a $0.20 difference in price between the BQ40Z50RSMR-R2 and the BQ34Z100PWR-G1.  Is there a downside to using the latter part other than the relatively small price difference?  I'm kind of stuck with the high-side current sensing with the BQ25750 battery charger controller.  However, what if I put a diff-amplifier across the high-side current sense resistor for the BQ25750 and fed this voltage directly into the SRP pin of the BQ34Z100PWR-G1 (with the SRN pin tied to the negative terminal of the battery)?  The differential amplifier output would have a higher impedance than the sense resistor, so would this work?

Hi Kurt,

I have seen many customers use the bq40z50 with LFP cells. If configured well, it will be able to perform with high accuracy. 

The bq40z50 is a better AFE device overall. This is mainly because the device offers cell balancing and it is able to read the differential ADC voltages at the cell pins. Whereas the BQ34z100 only reads the top of stack voltage, so you will need to implement cell balancing or have another IC to do this. 

BQ40z50 has had many FW changes over the years. It has many features the BQ34z100 does not have. BQ40z50 provides integrated protections. 

As for the high-side sense solution you are implementing, I cannot say if it will work or not. This will have to be tested. 

Hello Jose,

Okay, thanks for the clarity on the BQ40Z50!  I doubt that we can get the differential amplifier idea to work because all of these parts talk about looking at battery impedance.  

An additional question is about cell balancing.  Our client is planning to use a pre-packaged battery that only has two terminals brought out--therefore we can't do cell balancing (although this may be done internally).  Can we still use the BQ40Z50 to measure the state of the battery charge?

Best Regards,

Kurt

Hi Kurt,

The BQ40z50 is a Pack Side gauge, meaning it will be integrated in the battery pack.

Inside the pack, the gauge will do autonomous cell balancing. It will also track SOH and report SOC just fine. 

Hello Jose,

So, will the BQ40Z50 be able to track the stage of charge when it is external to the battery (with no access to the internal inter-cell connections)?  If not, is there TI a fuel gauge IC that will (for a four-cell LiFePO4 battery)?

Thanks!

Kurt

Hi Kurt,

Technically speaking internal cell connections will pass the same current (in series) as the external pack. All the gauge needs is to be able to track passed charge through the Sense resistor. 


It is just more common to use the BQ40z50 as PACK-side gauge. Also, many regulations are now forcing pack-side cell protection ICs nowadays. 

For TI gauge reference, please enter your filters here. 
https://www.ti.com/battery-management/fuel-gauges/products.html#338=Li-Ion%2FLi-Polymer%3BLiFePO4&1152=2-4%20Cells&1960=System&sort=1152;asc&

Hello Jose,

Thanks for your continued support!  The BQ34Z100-R2, BQ34Z100-G1, and the BQ34110 all look like good candidates for our application as they don't include any battery cell balancing but do support multi-chemistry battery fuel gauging.  I hadn't seen these earlier in my online search, so your help is much appreciated.  All three of these parts are close in price.  The BQ34Z100-R2 appears newer than the BQ34Z100-G1, so maybe that is the better of the two.  The BQ34110 is mean for rarely discharged applications.  Our application is a mix of regularly discharged and occasionally discharged (depending the user).  Are we better off with the BQ34110 or the BQ34Z100-R2?

Best Regards,

Kurt