BQ29700 for 2 cells - possible?

I meant: two BQ29700 in serie for 2 cells. That means: two cells in serie and two BQ29700 in serie and connected in the "middle", so that each BQ29700 is parallel to one BQ29700.

Hi Pascal, 

I am not sure exactly what you are looking for but bq29209 is 2S protector. 

You can't put two bq29700 in series. 

Again, I am not clear what you are trying to do here. Bq29700 is primary protector and bq29209 is 2nd level. 

If you can explain your requirements then we can provide you a solution. 

Roger

Hi Roger

Thanks for your reply.

First, excuse me, there is an important typo in my second post in the second sentence:
"I meant: two BQ29700 in serie for 2 cells. That means: two cells in serie and two BQ29700 in serie and connected in the "middle", so that each BQ29700 is parallel to one cell."

 

Back to my essential question:

I need to have a two cell battery pack (two li-ion cells in series) and a protection for it (overcharge-protection, overdischarge-protection, and - if possible - cell balancing). 

I found bq29209 - looks good: overvoltage-protection and cell balancing. But no overdischarge-protection.

I also found bq29700 - looks good too: overvoltage-protection, overdischarge-protection, and much more, but just for one cell.

Cell balancing is not as important as voltage-protection. Isn't it possible to use one bq29700 for each cell and use them in series with a second pair of bq29700 and a cell (see picture)

 

Or is there another/better possibility to "build" a 2-cell battery pack with li-ion cells in series with at least over-/ undervoltage-protection, and - if possible - cell balancing?

 

Thanks

 

Hi Pascal , 

I understood your question yesterday. 

You can't do your block diagram.  You can not stack Bq29700.

There are difference between primary and 2nd level protector.

We don;t have a good 2S primary protection solution.

Review the typical application diagram for primary and 2nd level. You will see the difference between them.

Again, bq29700 can't support your block diagram.

. 

Hello Roger,

could you elaborate on why the BQ29700 part is inappropriate for cells in series as Pascal has drawn above? I understand that a protection function of a single cell would disconnect the whole string, but accepting that risk is there another reason why it is a bad idea?

I'm trying to do the same thing with 4 cells, Mehrdad was asking on my behalf here:
e2e.ti.com/.../520186

I am curious because I recently connected 3 of the BQ29700EVM in series as shown above, and the string functioned properly, charging, discharging, and protecting from short circuit. There may be some degradation over time that I'm not aware of but it seemed to work perfectly during my day of testing.

Thank you,
Leonard