BQ24773: UPS based Multiple Battery Pack design

Hi All,

I'm designing a UPS system that will have an initial battery pack (4 serial Lithium 5.4Ah Cells). This will be the base UPS module.

I then want to add additional capacity to the UPS by adding more battery packs. ie additional board with another group of four series cells and a battery monitor.

The basic main UPS module BOM that I'm looking at will consists of the following:

1 * LTC4357 (Reverse polarity protection)

1 * LT4363 (Surge Suppressors High Voltage Surge Stopper with Current Limit. Auto-Retry after Fault)

1 * BQ24773 (Proposed 5A Battery Charger)

1 * BQ40z50-R2 (Li-Ion Battery Pack Manager)

I expect that the additional battery pack modules will have:

1 * BQ40z50-R2 (Li-Ion Battery Pack Manager)

I will have a micro that will be interfacing to the BQ24773 and BQ40z50-R2. Its power supply will need to come from either a) charger input AND/OR b) a charged battery pack input. Basically ORed together via diodes or such. It will control the charger and talk to the multiple BQ40z50's.

Basically, I will use the micro to switch the charger between various battery packs to either charge the pack from a low state or to perform top-up's. 

Question is, how do I connect the additional battery packs to the BQ24773? (I need make before break. So no back feeding of flat battery pack.)

At what point would you suggest placing a switching Mosfet?

Is the BQ24773 the best IC to use in this situation?

Thanks,

Neil