BQ27426: Can this fuel gauge be configure for 2 cells?

Jackson A

Part Number: BQ27426
Other Parts Discussed in Thread: BQ40Z50-R2, BQ4050, BQ40Z50, BQ27520-G4

Hi,

Can the BQ27426 be configured for 2 cells? Looking for a fuel gauge that can support 2 cell LiFePO4 chemistry. Liking the BQ27426 due to its simplicity.

Many Thanks.

Jackson

over 3 years ago

0 Rushi_Dalal over 3 years ago

TI__Intellectual 2025 points

Jackson,

BQ27426 is designed for one cell applications, but we have seen a few customers uses a resistor divider from a 2S stack. This method has additional inaccuracies to due to resistor tolerances.

I would suggest looking at BQ40z50-R2, this Impedance Track(TM) gauge has a LFP_Relax bit in FW to handle the flat OCV profile of LFP cells. This fuel gauge requires a bit more configuration than BQ27426 (also based on Impedance Track(TM) algorithm).

On the other hand, we also have CEDV(TM) gauges such as BQ4050. We have seen customers used CEDV gauges for LFP as well. For CEDV to work effectively, it does requires periodic deep discharges down to 7% occasionally to learn the capacity as the battery ages. It is the better algorithm for the applications where impedance track is not ideal (highly pulsed loads and no rest periods).

Thanks,

Rushi

0 Jackson A over 3 years ago in reply to Rushi_Dalal

Prodigy 50 points

Thanks Rushi.

Your suggestion seem good but probably overkill for what I need. Plus the BQ40Z50 doesn't seem to have much stock around.

I still prefer the BQ27426 if I can get away with a simple voltage divider on the BAT pin. Do you have any documents/ref designs on a 2S setup?

Thanks,

Jackson

0 Rushi_Dalal over 3 years ago in reply to Jackson A

TI__Intellectual 2025 points

Jackson,

How many samples of BQ40z50 would you need for your evaluation? I have some internal stock I may be able to ship to you.

Unfortunately, I don't have a reference design regarding a 2S setup with BQ27426. Due to the flat OCV profile of LFP cells, adding a resistor divider would only add to gauging error in my opinion.

The CEDV bq4050 may be the way to go if your load profile allows such discharges as mentioned earlier.

Thanks,

Rushi

0 Jackson A over 3 years ago in reply to Rushi_Dalal

Prodigy 50 points

Hi Rushi,

I've moved away from the BQ27426 due to its small package adding to the cost of the PCB. So I'm on the search for a simple battery gauge that will be mounted in an industrial product.

The battery pack will have its own protection. I'm looking at the bq27441 and bq27542. Is there a big reason to choose one over the other? Do they both support LFP cells?

Thanks,

Jackson

0 Rushi_Dalal over 3 years ago in reply to Jackson A

TI__Intellectual 2025 points

Hi Jackson, do you have a package requirement? If you need QFN then BQ27411/BQ27441 and BQ27542 would be the fit. BQ27542 would be more geared to handle LFP due to LFPRelax bit in the FW. BQ274xx does not have that.

Thanks,

Rushi

0 Jackson A over 3 years ago in reply to Rushi_Dalal

Prodigy 50 points

Thanks Rushi.

If I go with the BQ27542 can I configure it the same for a dual cell LFP battery with a voltage divider on the BAT pin?

Regards,

Jackson

0 Jackson A over 3 years ago in reply to Rushi_Dalal

Prodigy 50 points

Hi Rushi,

Is there any stock of the BQ27542? It seems difficult to find.

Jackson

0 Rushi_Dalal over 3 years ago in reply to Jackson A

TI__Intellectual 2025 points

Jackson, our TI.com e-store seems to be out of stock (at the moment) due to very high demand. I would suggest searching Arrow or Mouser to quickly get supply as it seems they have some quantity in stock. We replenish our e-store as well, but I'm trying to suggest the quickest option for you at this time.

Thanks,

Rushi

0 Rushi_Dalal over 3 years ago in reply to Jackson A

TI__Intellectual 2025 points

Jackson, this is a tricky case. The problem is for LFP the nominal voltage is about 3.2V, and the our minimum supply needed for the chip is 2.7V for no operating restrictions. So you would need a way to ensure the battery doesn't discharge too low.

Thanks,

Rushi

0 Jackson A over 3 years ago in reply to Rushi_Dalal

Prodigy 50 points

Rushi. Are you able to provide 5 samples if we run into any sourcing issue?

0 Jackson A over 3 years ago in reply to Rushi_Dalal

Prodigy 50 points

If I set the voltage divider to within the 2.7-5.5V operating range I think I should be ok. Thanks for all your help. Much appreciated.

0 Rushi_Dalal over 3 years ago in reply to Jackson A

TI__Intellectual 2025 points

Yes if you cannot procure though distributors then please reach back to me and I will check if we have anything internally. Please continue to monitor the TI store stock as well.

Thanks,

Rushi

0 Jackson A over 3 years ago in reply to Rushi_Dalal

Prodigy 50 points

Hi Rushi.

I'm stuck with the thermistor. I've noticed it is recommended for <2V. If this thermistor is located inside the battery pack, is it ok to step the voltage down with a divider. Or is it recommended to place the thermistor on board? I'm not against either just would expect better temp sensing when the thermistor is inside the pack.

Thanks,

Jackson

0 Rushi_Dalal over 3 years ago in reply to Jackson A

TI__Intellectual 2025 points

Jackson, as this is a pack side gauge, I believe it's better optimized to place thermistor inside the pack and as close to the TS pin as possible as mentioned in the datasheet.

Thanks,

Rushi

0 Jackson A over 3 years ago in reply to Rushi_Dalal

Prodigy 50 points

Rushi. It does say it can be used for pack side and main board with non-removable battery. I plan to use it on the main board. Does this mean the thermistor must be on the main board away from inside the pack?

Jackson

0 Rushi_Dalal over 3 years ago in reply to Jackson A

TI__Intellectual 2025 points

Jackson, yes a pack side gauge can still be used on the main board (or system side) as long as the battery is non-removable. Typically the thermistor is still part of the pack. The pack would still have connections for Pack+, Pack-, and TS (thermistor).

Thanks,

Rushi

0 Rushi_Dalal over 3 years ago in reply to Jackson A

TI__Intellectual 2025 points

Please take a look at the typical application diagram of BQ27520-G4, it may help you to visualize: https://www.ti.com/lit/ds/symlink/bq27520-g4.pdf , bottom of page 1

0 Jackson A over 3 years ago in reply to Rushi_Dalal

Prodigy 50 points

Thanks Rushi. Appreciate your help.

Jackson

0 Jackson A over 3 years ago in reply to Rushi_Dalal

Prodigy 50 points

Hi Rushi. We're running into sourcing issues. Am I able to get in touch with you via email to discuss options?

0 Rushi_Dalal over 3 years ago

TI__Intellectual 2025 points

Hi Jackson,

Yes please send me a connect request via E2E and we can proceed from there.

Regards,

Rushi

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BQ27426: Can this fuel gauge be configure for 2 cells?