BQ29706: BMS applications

Robert Dee38

Part Number: BQ29706

Hi,

I'm designing a LiFeP04 power wall. The inverter/charger will give me what ever voltage and current I spec into it.

What i want the bq297xx to do is over voltage and under voltage.

I see the bq29706 (the lowest OV cutoff device you show) cuts off at 3.85 volts but our batteries spec at 3.65 OV. The other parameters are fine. I plan on splitting the batteries up into 16s x 12p for 16 x bq29706's, less 'p' if I have to but the total draw on the 16s x 36p pack is under 15A and the max charge current is 80A over the entire pack so I think the chip should be fine using the CSD16406Q3 mosfets speced in the data sheet.

Is there anything I'm missing here or perhaps another chip with a lower OV cut off might fit this app better?

Thanks for your time and help,

Rob

over 5 years ago

0 Onyx Ahiakwo over 5 years ago

TI__Guru* 76230 points

HI Rob,
I am not sure i follow your explanation of using less cells in parallel to help with an overvoltage condition. it is the number of cells in series that determines the stack voltage.

thanks
Onyx

0 Robert Dee38 over 5 years ago in reply to Onyx Ahiakwo

Prodigy 40 points

Onyx,

Thanks for the reply.

I am using one 29706 on each parallel leg in the pack (16s, 48V system). My thinking is that reducing the number of parallel batteries in each leg I'll reduce the stress on the 29706. Or is it a function of the MOSFETs I choose for the 29706?

Rob

0 Onyx Ahiakwo over 5 years ago in reply to Robert Dee38

TI__Guru* 76230 points

Hi ROb
But you said so your self, your lowest Over voltage is 3.8 and but batteries are spec at 3.6. How will this device protect from OV condition for your applicaion?
'thanks
Onyx

0 Robert Dee38 over 5 years ago in reply to Onyx Ahiakwo

Prodigy 40 points

Onyx,

Peak voltage is 3.65 for LiFeP04 batteries, the lowest 297xx is the 29706 which is 3.85. I'm asking if TI did this deliberately to compensate for the RDSon of the MOSFETs. If each RDSon was 100mV then the 29706 would meet our criteria. Is it possible that TI expects us to find specific MOSFETs to meet these specs?

0 Onyx Ahiakwo over 5 years ago in reply to Robert Dee38

TI__Guru* 76230 points

Hi Rob,
yes that is the expectation. The device operates using the Rdson of the fets so you have to choose appropriate fets

thanks
Onyx

0 Robert Dee38 over 5 years ago in reply to Onyx Ahiakwo

Prodigy 40 points

Thanks for your help, I'll have to do some testing with the samples we just got.

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BQ29706: BMS applications