BQ27426: Question about Qmax update rule temperature conditions

#1: This depends on the ChemID accuracy. It's not possible to say unless one checks the OCV curve at temperatures higher than 40 deg.C. For accurate numbers, you'd have to characterize the cell at 45, 50, 60 and 70 deg.C and then compare how much the model deviates.

#2: FCC depends on accurate Qmax because Qmax is used to calculate DOD as a function of passed charge. As this is the result of a complex simulation, it's not feasible to give you a definitive number as in FCC error % = function (Qmax error %). What will happen inside the gauge, with inaccurate Qmax, is that the gauge will use incorrect OCV and incorrect Ra (both of which are temperature dependent), which will lead to incorrect estimated loaded voltage (which is based on estimated (predicted) current) and therefore incorrect total passed simulated charge, if DOD is incorrect due to incorrect Qmax.

#3: TI can analyze the DOD error based on the deviation of OCV at a specific temperature, if the customer provides an OCV(DOD) profile for the cell at a specific temperature.

hello Dominik,

We have seen examples of updating Qmax temperature rules in other sources.

1, Our application will exceed 40℃ because of the running environment, so can you help to modify the four FS files? They correspond to 45°C, 50°C, 60°C and 70°C, and the minimum temperature is still 10°C.

2, In addition, the reason for setting the Qmax temperature to 10℃-40℃ is still not quite understood, please help to explain in detail and provide the basis.

Thank you very much!

The algorithm uses voltage predictions for a discharge scenario. These predictions rely on correct Qmax (see the other E2E thread). Qmax in turn relies on correct OCV modeling. The gauge uses a mathematical model for OCV as a function of DOD and temperature. This model loses accuracy for higher temperatures. The TI algorithm team decided that 10deg.C to 40deg.C is a range where the model matches reality well enough for Qmax to be accurate enough.

The only way to reasonably change this is to characterize the OCV over DOD for a specific temperature. TI characterizes this at 0deg.C, 25 deg.C and 50deg.C so we do not have information for 60deg.C and 70deg.C.

Qmax will update only if the cell is relaxed, hence it's very unusual for cell temperature to be so high (70deg.C) unless the ambient temperature is consistently high. It's better to let the gauge update Qmax when temperature drops below 40deg.C (which usually happens quickly after end of discharge or charge when the cell is relaxing and ambient temperature is below 40deg.C) than to configure the gauge to ignore high temperature and risk a bad Qmax update.

Hello dominik，

1. In response to your reply, our understanding is that below 50℃ (including 50℃), the correspondence between DOD and OCV is already included in our chemID. Is the understanding correct?

Dominik Hartl11 said:

The only way to reasonably change this is to characterize the OCV over DOD for a specific temperature. TI characterizes this at 0deg.C, 25 deg.C and 50deg.C so we do not have information for 60deg.C and 70deg.C.

2、Please help to provide the FS file corresponding to 50℃, we need to do the verification to confirm whether the parameter needs to be modified.

3、Please tell us how much error in Qmax will be caused after the temperature range is modified to 10-50℃? What are the specific effects?

Because of risk of inaccurate Qmax updates, I cannot provide a file where the limit is set to 50deg.C unless you waive any expectations of accuracy and support. Please coordinate this with the TI FAE who you are already working with. We can discuss in our call which is already scheduled.