AllinOne: Fuel Gauge, Charging and Protection

I would consider the bq40z50. It has special features that support LiFePO4 cells that the bq20z95 does not have. The bq34z and bq76920 are good options for applications that at more than 4S. They can support 3S and 4S applications, but the bq40z50 offers the gauge and AFE features in one package.

Thomas the bq40z50 says nothing in the data sheet about being designed for LiFePo4 cells so you are you saying its a good match for LiFePo4 cells simply because its easy to program the voltages to properly handle LiFePo4 cells? 

I also just found the new BQ76920 + Fuel Gauge chips and EVM boards and was wondering what you think is the best solution out of all of these. 

I have the Bq34z100 fuel gauge up and running and its working great but I would like a newer chip with battery protection built in if possible. 

So what would you say the big difference is between the new BQ76920 + Fuel Gauge companion vs the BQ40z50? I'm reading the data sheets but I know you know these products way better than I do. 

I build and sell portable battery systems that are all 4 cell , 12v, LiFePo4 , with the largest being 200Ah and the current PCM allows 250 amps of current. Can the BQ40z50 scale up to the 200Ah and 250A of current? 

Ryan, 

Gauging LiFePO4 chemistry cells is challenging for any device that uses the OCV in its gauging algorithm. The discharge profile for these cells is very flat, so predicting the capacity when resting in the flat zone suffers. Some features have been added to the bq40z50 that improve gauging for the these cells and you can find these in the device TRM. The bq76920 uses a CEDV gauging algorithm, so it is not as dependent on the OCV. It uses coulomb counting for gauging. Either device can be used for a 200Ah cell by scaling the currents and capacities. Both devices have a lot of nice features and should provide very good performance with these cells. 

Tom

Thanks Tom. 

Since the new BQ76920 chip + companion Fuel Gauge is not as reliant on OCV would you say its a more accurate at fuel gauging? If its even slightly better than the BQ40z50 then I would want to use it. 

I have clients who deploy our systems on top of Arctic mountains for weather monitoring and I'm looking for a battery management system that can best gauge capacity in -20C weather conditions. 40Ah CALB batteries which you have in your chem profiles already takes a decent hit in usable capacity at these -20C temps and I'm wondering which fuel gauge would best be able to handle fuel gauging at these temps. The fuel gauge data will be transmitted back remotely via Sat datalinks. 

I'm still having a hard time deciding on which fuel gauge is a better choice between Bq40z50 and the newest Bq76920. What is the main difference between the 2 other than the CEDV Gauging Algorithm? 

It took me awhile to get the code right to correctly pull fuel gauge data from the Bq34z100 chip so I can use it with my Micro Controller and LCD Display. I see these 2 new chips communicate via SMBus which is basically i2c , do you think I could use the same code I'm using with the Bq34z100 to pull status info from either of the 2 new fuel gauges or do you think I will need to start over from scratch? 

As always I appreciate your help on this. Trying to decide which EVM module to buy next. 

I see the Bq34z100 as the perfect solution for my 1 cell LiFePo4 battery applications. Its been pretty accurate during my testing over a few months now. 

Ryan,

I would go with a CEDV gauge for extremely low temperature applications. Some updates in IT gauges are limited when operating below 0 degC. The communications protocols are different for these devices as compared to the bq34z100, so some rework will be required. One of our other CEDV gauges should also work for the application, although you would have to scale the current to support a 40Ah configuration.

Please check your operating voltage range for using the bq34z100 with a 1S LiFePO4 cell. You may want to operate the cell to a lower voltage than what is required to power the device.

Tom

Thanks for your answer! I took a look for your suggestion, I will acquire it.

It is difficult to discover all the BM ICs that can work with LiFEPO4, because just a few specify the chemistry, even in search filters.

Tom I just received the  BQ76920 EVM module. Going to get it setup and running and begin testing it. 

Thanks for your help with making the decision on which solution was right for our applications. 

I'm sure I'll be back with questions in a new thread.