bq78PL114, bq78PL116, bq76PL102 Power Connection Sequence

Hi Jim,

I asked this question a while ago and the answer was the following:

<< Connection of VSS first then each higher cell is the preferred method.  We’ve done testing here with random connection and not seen a problem but, we generally fall back to the conservative recommendation of VSS => V1 => V2  etc… >>

Now it is a different answer? What did you find in the power sequencing that makes this new instruction important ?

I'm asking because we're still having many BMS that fail to start correctly. Their internal LDO give an output of 0.6V instead of the normal 2.5V (with a VLAN error).

The current power sequencing we're using is the following:

=> First, VSS with cell4 at the same time, the other cell connections are polarized between 2V and 4.5V with zener diodes (they are 5.6V, not 4.3V as indicated in the schematics)

=> Second, cell 1, 2, 3 at the same time

Hello Jim,

so in case of a 7s pack for example, the connection is following:

Vss, BATT-, Cell2, Cell1, Cell4, Cell3, Cell6, Cell5, Cell8, Cell7

Is this really true?

We do not have Zener-diodes within our circuit. In my experience, the bq-chips are destroyed if the connection is not done in strict incremental cell order!

That connection sequence is correct.  The bq76PL102 that connects to cell 7 should have its V2 and V1 shorted together on the PCB.

We use premade modules containing multiple series connected cells with integral balancing connectors. This hinder us from following the recommended connection sequence listed above.

We have not seen any problems yet but we plan on upgrading from bq78pl114 to bq78pl116. Is this new chip more likely to get problems from connecting cells in groups? We have implemented Zener diodes between all the cell connections.

The connection sequence also applies to the bq78PL116 as well. The Zeners will help clamp any high voltage into the device pins. The connection sequence ensures proper power sequence of  the bq78PL114 / bq78PL116.

Ben

I also found, for some reason, that to make sure you "inhibit safety rules" and un-  "wired" the pack.  Even with the connection sequence correct, without these settings you still don't have a good start-up, even thought the "114" doesn't get damaged.

Randy

I would also add to make sure that the cells are not fully charged or wildly out of balance.  This scenario can happen when removing cells from several packs and re-using them after they get mixed in a group. 

Hi,

When disconnection the cells, do I have to respect the inverse sequence? 

Thanks,