Multiple power ground references, how to connect

[davegravy]

I'm interfacing a few components with different grounds and confused how to connect them.

I have a solar battery charging module: GV-5-MOD (datasheet) and it has a several relevant terminals:

* PANEL-
* BATTERY+/PANEL+ (shared)
* BATTERY-
* LOAD-
* LOAD+

Since my SBC and its peripheral sensors are my load, I am running it from LOAD+ and LOAD-,  so I expect LOAD- will be my main GND reference.

I have a fuel gauge IC I want to use to track battery state of charge: LTC2959 (datasheet). From the typical application diagram



 it should be powered from the charger (BATTERY+ terminal on the charger), have the battery's positive terminal connected to the SENSEN pin, and battery's negative terminal connected to the GND pin. But it says the battery's negative terminal and MCU negative should be one and the same in contradiction with the charger which has a separate BATTERY- and LOAD- terminal.

Options I see are:

1. tie BATTERY- and LOAD- together. I suspect this will bypass some low side MOSFET, AKA protection circuitry like LVD?
2. Use BATTERY- as the GND reference of the LTC2959 and LOAD- as the GND reference of the SBC. Won't these be at different potentials if there's a drop across RDS_on of the the charger's low side mosfet? Do I need level shifting/conversion here?
3. Use LOAD- at the GND pin of the LTC2959. I think this wouldn't affect charge counting or current measurements by the IC but would add a load-current dependent error to the IC's voltage readings.

What should I do?

[davegravy]

In case this helps visualize the problem:



I have other I2C sensors that are grounded to LOAD- but I think the fuel gauge needs to be grounded to BATTERY- to give accurate battery voltage readings.

[Alex Nikitin]

LOAD- is the best point of reference IMHO, if you have a significant voltage drop between BATTERY- and LOAD-, than it should be proportional to the load current, which you are measuring anyway, so you can easily apply a linear correction for the voltage readings to compensate for that drop.

Cheers

Alex

[BillyO]

You should NOT connect the grounds.  These solar controllers use low side regulation.  The grounds (negatives, actually) are not common and should not be connected together at any point.  To do what you want to do here pin 10 would go to the battery negative and you would need to provide some sort of optical isolation for SDA and SCL.  BTW, pin 11 (the exposed pad) on that device is not connected to anything.

[davegravy]

I've been looking at optical isolators and they seem to require 3.3V or 5V supply on both sides.

I have this on the SBC side, but not on the battery side since it's a 4 cell lifepo4. Do I need to add a voltage regulator to the battery just to drive this side of the isolator?

[Alex Nikitin]

One possible option is to drop the connection from the battery to Vdd pin altogether, use -LOAD as the ground reference and use an instrumentation amplifier to provide Vdd voltage referenced to ground and equal/proportional to the battery voltage (making essentially a "floating" voltage sensing).

Cheers

Alex

[davegravy]

BillyO I'm not clear if your reply was in response to Alex.

You should NOT connect the grounds.  These solar controllers use low side regulation.  The grounds (negatives, actually) are not common and should not be connected together at any point.

I don't see how using LOAD- for the IC ground would create a connection between LOAD- and BATTERY- (option #3 from my OP), so I'm assuming you weren't discouraging that approach and were referring specifically to option #1.

Option #3 is looking like the most simple solution assuming the potential difference between LOAD- and BATTERY- is deterministic (I have this question submitted to the solar charger's application engineer).

[BillyO]

BillyO I'm not clear if your reply was in response to Alex.

It was and my suggestion might be considered an option 4.

[Alex Nikitin]

And my suggestion (to use an instrumentation amp for Vdd supply of the fuel gauge IC) should be #5  .

Cheers

Alex

[davegravy]

Noticed this in the charger datasheet



My load current will be well under 5A but it sounds like it's ok to connect the two together if you don't mind losing reverse polarity protection? Does this change anything?