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BQ25798: Having issues with the overall design of a system with 4 solar panels and two battery strings.

Kipton Moravec
Expert 1545 points
Part Number: BQ25798

Tool/software:

I have 4 solar panels.  Let’s call them A, B, C & D.

A & B are 12W. C & D are 6 W. The open circuit voltage is around 15-16V.   
A, B, and C are facing the sun, D is facing away from the sun.

I have two independent 4S strings of 18650 Lithium batteries.

A charge discharge cycle is around 90 minutes.  The batteries are charged for around 50 minutes and discharged for around 40 minutes.  We are shooting for 29,000 charge discharge cycles, (5 years). We think we can do this by charging to 60% and discharging to 40%.

We do not have the exact wattage of the system, but it is about 20 to 22 W maximum.

I was going to roll my own MPPT controller, but when I got to layout, I did not have room.

I started looking at the BQ25798 I2C Controlled 1- to 4-Cell, 5A Buck Boost Battery Charger with Dual-Input Selector, MPPT for Solar Panels and Fast Backup Mode. And am trying to figure out if I could use it.

In my current design, I was having the MPPT output 19V and then had two charge controllers (and fuel gauges) take the 19V and drip that to charge the batteries gently.  I also have a 12V Buck Boost power supply, a 5V Power Supply, a 3.3V Power Supply, and a 1.2V power supply to power the rest of the system. 

While the batteries last the system will work all the time. When the batteries die, it will work only during the sunlight portion of the 90 minutes.

I am having issues with the system design.

One option is to keep the same architecture I have now and use the BQ25798 to only generate 19V for the system power bus. When the sunlight is not enough the BMS will switch over and the batteries will provide the system power.

Another option may be to use the BQ25798 with the 12W panel (A) to charge one battery string and power the system.
And another BQ25798 with the 12W panel (B) to charge the other battery string and power the system.
And have a BQ25798 on Panels C & D just putting out 19V to help power the system.

I did make a power MUX to allow me to switch which solar panels are attached to which MPPT in case of MPPT failure. (I have a redundancy requirement)

Any other suggestions for how to do this? I did not find any other MPPT chips that were supporting only providing power not also charging batteries. Can the BQ25978 do just that? I know I am throwing away a lot of its capability, but it is very integrated, and the package is small and only a few of these systems will be built. This is not for building tens, or thousands of units.  So cost is not much of an issue. It just has to work. 

Any ideas are welcome.

Thanks, in advance. 

  • 0
    TI__Guru**** 166446 points

    HI Kipton,

    In general, chargers like BQ25798 can provide a regulated system voltage and not charge batteries as their SYS output is the output of the converter.  The normal way to do this is to disable charge with /CE pin pulled high and have the host set the regulated SYS pin voltage using the MINSYS I2C register.  However, the max MINSYS setting is 16V.  If you want higher, you must place as big a capacitor at BAT as you can fit and set the charger to regulate V(BAT) up to its max of 18.8V, essentially fooling the charger into thinking there is a battery present. You will need to add a zener clamp because the abs max on BAT and SYS is 20V.

    A couple of things to note about BQ25798 MPPT feature:

    1. it is not default enabled and requires an I2C write to implement

    2. it auto turns off if V(BAT) < MINSYS so you want to set MINSYS as low as possible so MPPT runs as long as possible

    3. even if MPPT (periodic reset of VINDPM min input voltage threshold) is not on, charger sets VINDPM threshold at power up (sunrise) to input voltage minus 0.7V for low input voltages and 1.4V for high voltages.  However, if the sun goes behind a cloud and panel voltage drops below the VINDPM threshold, the charger's converter stops, SYS crashes and the charger enters HiZ mode.  Once in HiZ mode, the charger can only exit if the panel voltage drops further below input UVLO.  I am in the process of writing an app note to add an external circuit to prevent this.  

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Expert 1545 points

    I was setting my system voltage to be 19V because that would be higher than the battery charge voltage and I could use a BUCK charger. If the system voltage fell because the MPPT could not keep up, it would first cut off the chargers t say 16.8V and maybe losing that power load would be enough to make it recover to 19V. 

    And if it still fell then the batteries would kick in when it was below battery voltage. 

    The 12V Power supply was a buck-boost. So it would work above and below 12V. The rest were 5V or below and so a buck would work fine. 

    I could change the voltage thresholds without much consequences except having to use a buck boost for the real battery charger as opposed to the buck charger I am currently using.

    I have a 3.3V power supply that wakes up before the MPPT to control the microcontroller, which can wake up the MPPT chips, so #1 iot default enabled and requires an I2C write to implement is not an issue. 

    2. it auto turns off if V(BAT) < MINSYS so you want to set MINSYS as low as possible so MPPT runs as long as possible
    Minsys can be as low as the minimum voltage of the battery.  We hope to never hit that. So I do not see that is an issue, 

    #3 This is an application where I do not have to worry about clouds. I just have to worry about night and day.