BQ25731: Setup options for use with SLA or LiFePO4 batteries

Justin Grange

Part Number: BQ25731
Other Parts Discussed in Thread: BQ24650, BQ24610,

Hi,

We built a board with the BQ25731 on it and have it working well with a stack of Li-Ion batteries. The EVM is specifically set up this way. We wanted to modify the design to use a 12V sealed lead acid battery (cell voltage 2.1V) or a stack of series LiFePO4 batteries (cell voltage 3.2V) and we were not clear on how best to implement the changes required.

The website specifically states that this is possible and we have seen documentation for other chargers that were modified to support LiFePO4 such as in SLUA565 for BQ24650 and SLA such as in SLUA992 for BQ24610.

These are fairly straightforward chargers with simple STAT pins.

How do we modify the hardware and setup the firmware to support the different cell voltages?

We are currently assuming:
Modify the voltage divider feeding CELL_BATPRESZ from VDDA down to GND.
That sets the charge voltage. We are assuming we would set it to 12.6V for a lead acid battery and a 4S LiFePO4 pack and 16.8V for a 5S pack.
Then we need to setup the charge voltage and charge current in firmware to dial it in and modify the termination voltage as well.

Is there an example of this we could reference?

Thanks for your expertise on how to do this safely.

over 2 years ago

0 Pengzhao Zhu over 2 years ago

TI__Genius 11385 points

Hi Justin,

Unfortunately, we don't have any specific example to do that but your high level thought process is correct - you will need to change the charge voltage + adjust charge profile. As you should have a MCU in your system, you can modify the charge profile (adjust current and voltage as needed) to suit your battery chemistry.

In summary:

1. You will need to adjust the charge profile. However, this can easily be done by changing the charge voltage and charge current of the IC.
2. I would also recommend a high-accuracy ADC to monitor the battery voltage in real-time to help charge profile adjustment.

Thanks,

Peng

0 user4125449 over 2 years ago in reply to Pengzhao Zhu

Intellectual 330 points

Hi Peng,

you're sure the charge profile can be changed by setting charge voltage and charge current? Nothing more to it?

Thanks

Matthias

0 Pengzhao Zhu over 2 years ago in reply to user4125449

TI__Genius 11385 points

Hi Matthias,

Charging a battery is all about changing the charge voltage and charge current. MCU has full control of the end application system once it has the flexibility to adjust charge voltage and charge current. You have full control.

Thanks,

Peng

0 user4125449 over 2 years ago in reply to Pengzhao Zhu

Intellectual 330 points

Hi Peng,

ok, I got it, but I was expecting the BQ25731 was doing this job for me. I don't want to control charge and voltage from the MCU, we want to offload this. Do I get you right, the BQ25731 can't do that for Li-Fe-Po4 batteries?

best regards

Matthias

0 Tiger Zhou over 2 years ago in reply to user4125449

TI__Mastermind 32945 points

Hi Matthias,

You need a standalone charger, which sets the charge voltage and current by resistors. BQ25731 is a digital charger. It is designed to interface with a host controller or MCU. I think you got the idea.

Regards,

Tiger

0 user4125449 over 2 years ago in reply to Tiger Zhou

Intellectual 330 points

Thank you, got it.

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BQ25731: Setup options for use with SLA or LiFePO4 batteries