Getting started with bq27541-G1

I wish you luck with this as it is a similar task to the one I have been attempting with the bq27531 and the quality of the documentation (especially the TRMs) is woeful!

I definitely advise buying the Evaluation modules as it is absolutely necessary to use them to generate the Golden Image - there simply isn't enough documentation to allow you to do this solely on your target board.

I found that bqCONFIG provided most of the things I needed - though getting a version that works is a bit of a challenge. The versions on the family support pages wouldn't connect to the I2C EVM and I doubt TI have fixed this yet. I eventually tracked down a 1.5.5.0 version posted in the forums. I do advise downloading it and running it to prove to yourself that you have the software necessary.

bqConfig has a step-by-step process for generating the Golden Image, including a step to enter the chemistry ID.

The next challenge is to get that Golden File into chips on your board. If you can bring out the I2C bus from your board so it can be driven from the MSP430 EVM module, it is relatively easy to program the values to the chip. Hold in mind that this will be part of your production process so you will need to connect it to the MSP430 for every board you make; don't rely on tacking a couple of wires onto the bus!

It is possible to write your own software to program the Golden Image into the chip but TI simply won't publish the boot-flash documentation. There are some misleading flow-charts describing the operation but these are not a good starting point - if an error is made the chip is ruined. Other forum members have put sniffers on the I2C bus with the EVM to backward-engineer the process and you'll find a few helpful posts on that in the forums.

Another stumbling point is the SEAL/UNSEAL operation. This is very poorly documented in the TRMs and seems to catch a lot of people out if the postings on the forum are anything to judge by.

The UNSEALING is achieved by writing the key as a command in the control() register.... it's rather like having the key as a special ADDRESS you write to instead of writing special DATA to a well-known address. It is vital that the keys not have values used by any other command in the control operation - this is made extra tricky because TI don't tell us all the commands that are used in the control register! You must also make sure that the keys are different - if you make them the same it is impossible to write them both! In summary - don't change them until you've become an expert!

On one of your questions, regarding the thermister... I do not understand why one might think that the temperature of the drive-chip is more safety-critical than the temperature of a Li battery given the failure modes of these cells. The on-chip temperature measurement is really only applicable to batteries that INCORPORATE the chip (so they are effectively measuring the cell temperature) - but you have said that your chip is on the host board. I would very strongly suggest reviewing your design in this regard!

The very best of luck!

Joe

Ah, I'm sorry, on the thermister question... I wonder if you meant that the thermister incorporated in the cell is being connected to the charge controller, so that controller is responsible for stopping the charge current if the cell gets too hot?

This is a fair thing to do if you don't trust the bq27541 to be as good as the charger chip at this safety critical task. In this case using the internal thermal device will give you some small measure of improvement in charger control as you might reasonably judge that normally the cells will be at ambient temperature, which will be registered by the chip. You will degrade some of the features of the chip in extending battery life but it will be difficult to suggest which design choice is better, trusting the bq27541 v. degraded battery life.

Hi Joe,

Thanks for your reply... although I feel even more worried about our decision choosing the bq27541-G1 now...

First of all, you are right; I might not have made the connection scheme of the thermistor clear enough. So there is a thermistor directly glued to the cell. Monitoring the cell temperature is done by the Charger IC which will reduce charge / discharge current during thermal overload conditions. As far as I know, the bq27541-G1 does not have any safety features integrated that could limit the current. This is why we definitely want the charger IC to monitor the cell temperature directly. So you are probably right and we have to sacrifice some Impedance Track accuracy and use the Gas Gauge's internal thermistor.

By the way: what to do with the thermistor input on the bq27541 when using the internal thermistor only? Can it be left floating? I could not find an answer in the datasheet.

You are right, testing the device features using an EVM should be first step… but there is no EVM with the bq27541-G1. The only EVM available has the bq27541 populated and my distributor tells me it is an End-of-Life product. That’s why we refrain from buying this outdated EVM. So we put lots of effort into designing a target board that also provides us the possibility for easy connection of the EV2300 and an external load circuit to perform the learning cycle. That’s why we should be able to program the Golden Image into the chip with the EV2300 and hopefully won’t need to use our host system to do so.

Thanks for pointing out the dangers of the Sealing commands! Will keep that in mind until it becomes imminent.

The bqCONFIG version you mentioned, did it give you the ability to perform the automated learning cycle?

Thank you!

Best regards,

Daniel

The bq27541 does seem to have functionality to control the charging based on temperature - with over- and under-temperature limits.

"The bq27541-G1 uses temperature to monitor the battery-pack environment, which is used for fuel gauging and cell protection functionality."

I wish the documentation was clearer as to what this does exactly, and it will depend on how you are controlling the cell too, but the internal schematic suggests that the FETs in negative line could be turning off the charge current if the temperature is outside of specified range and I would expect this of a cell controller... It would be so nice if the TRM said such things explicitly! It would be so nice to be able to tell customers that their devices won't catch fire because the TI chip will turn the current off if it gets hot...

I know the EVM isn't quite the same as the chip you're going to use but with such limited documentation you might need to try things to see what they do!

The bqCONFIG I have does perform the learning cycle but doesn't perform auto chemistry selection - which is ok for you as you're cell is already listed!

I think you are referring to the Typical Application circuit on the first page of the datasheet (showing a complete pack), where external FETs and an additional protector IC are shown? Because I cannot find any hints that the bq27541 has internal FETs to cut current flow.

Joe Skaife said:

The bqCONFIG I have does perform the learning cycle but doesn't perform auto chemistry selection - which is ok for you as you're cell is already listed!

These are good news for me. At least we know that there definitely is a working way to configure and program the bq27541-G1. Then we will stick with the bq27541 for now and perhaps we even buy this outdated EVM for further testing...

It would be great to get some information from the TI-staff at this point!
Perhaps there is a new EVM for the bq27541-G1 planned? And to repeat the questions from my opening post: what is the recommended procedure for configuring the bq27541-G1 and which tools should be used? Can you give us some kind of roadmap for the Gauge Studio and the other programs (like EVSW, bqEASY, bqCONFIG,...)?

Can you at least give me a link for downloading the bqCONFIG version known working with the bq27541-G1?

Thanks!

Best regards,

Daniel

We have a working prototype with the bq27541-G1 now.

Configuration and chemistry selection was already done but we have a problem with the learning cycle:

our charger uses a fast charging current of 1A. As soon as the charging current is reduced to 200 mA the charger "tops-off" for another 15 seconds and then cuts off (charging current = 0 mA). With the default settings the bq27541-g1 does not detect the full charge condition (FC not set during learning cycle).

Can you please give us some hints on how to configure "Chg. Current Threshold" and "Taper Current" in this case? Should the Taper Current be set to well above the cut-off current of our charger, to 225 mA for example?

After the learning cycle failed during the charging phase, how should the gas gauge be reset? Do we have to disable IT first before we restart the learning cycle? Anything else we have to do?

Another question is related to the thermal influence on the gas gauging algorithm:

our design has to use the internal temperature sensor of the bq27541-g1 but the gauge is located next to the linear charger. During fast charge phase the charger IC naturally heats up and so does the gauge. The internal sensor shows temperatures > 50 °C during fast charge.

The cell on the other hand is only lightly loaded during device operation and also the charging current of 1A is well below the maximum fast charge current allowed. So the cell itself does not noticeably heat up during operation.

What do you suggest in this situation? Can the thermal influence in the gauging algorithm be deactivated? Does it affect gauging performance at all?

Thanks

Daniel

The gauge taper current needs to be higher than the charger taper current in order for valid termination to occur. A good rule of thumb is to set the gauge taper between 10% and 20% higher than the maximum charger taper. Chg Current Threshold can be left at default for this case.

There isn't a way to disable temperature influence on the gauging algorithm. While accuracy might be affected in this situation, it may not be that bad. The accuracy effects at higher temperature aren't as noticeable as they are at lower temperatures, so the usable capacity at 25C is often very similar to that at 50C.

Thank you Chase,

these are good news.

What about the "Safety Configuration Parameters" like OT Chg, OT Chg Recovery etc., are those relevant for the gauging algorithm or can we use default values here?

To recover from a failed learning cycle, are there any special actions required (like disabling IT / resetting RAM)?

best regards,

Daniel

Thank you, Chase.

We were able to complete a successful Learning Cycle and Gauging seems to work fine now!

best regards,
Daniel